Chemical properties of higher nitric oxide. nitrogen oxides. Physiological regulation of NO production
At ordinary temperature, N 2 O is a colorless gas with a slight pleasant odor and a sweetish taste; has a narcotic effect, causing first convulsive laughter, then loss of consciousness.
How to get
1. Decomposition of ammonium nitrate with slight heating:
NH 4 NO 3 \u003d N 2 O + 2H 2 O
2. Action of HNO 3 on active metals
10HNO 3 (conc.) + 4Ca \u003d N 2 O + 4Ca (NO 3) 2 + 5H 2 O
Chemical properties
N 2 O does not exhibit either acidic or basic properties, that is, it does not interact with bases, with acids, with water (non-salt-forming oxide).
At T> 500 "C, it decomposes into simple substances. N 2 O is a very strong oxidizing agent. For example, it is capable of oxidizing sulfur dioxide to sulfuric acid in an aqueous solution:
N 2 O + SO 2 + H 2 O \u003d N 2 + H 2 SO 4
NO - nitric oxide (II), nitrogen monoxide.
At ordinary temperatures, NO is a colorless, odorless gas, slightly soluble in water, and very toxic (it changes the structure of hemoglobin in high concentrations).
How to get
1. Direct synthesis from simple substances can only be carried out at very high T:
N 2 + O 2 \u003d 2NO - Q
2. Obtaining in industry (1st stage of HNO 3 production).
4NH 3 + 5O 2 \u003d 4NO + 6H 2 O
3. Laboratory method - action razb. HNO 3 for heavy metals:
8HNO 3 + 3Cu \u003d 2NO + 3Cu (NO 3) 2 + 4H 2 O
Chemical properties
NO is a non-salt-forming oxide (like N 2 O). It has redox duality.
I. NO - oxidizing agent
2NO + SO 2 + H 2 O \u003d N 2 O + H 2 SO 4
2NO + 2H 2 \u003d N 2 + 2H 2 O (with an explosion)
II. NO - reducing agent
2NO + O 2 \u003d 2NO 2
10NO + 6KMnO 4 + 9H 2 SO 4 \u003d 10HNO 3 + 3K 2 SO 4 + 6MnSO 4 + 4H 2 O
NO 2 - nitric oxide (IV), nitrogen dioxide
At ordinary temperatures, NO 2 is a reddish-brown poisonous gas with a pungent odor. It is a mixture of NO 2 and its dimer N 2 O 4 in the ratio -1:4. Nitrogen dioxide is highly soluble in water.
How to get
I. Industrial - NO oxidation: 2NO + O 2 = 2NO 2
II. Laboratory:
action of conc. HNO 3 for heavy metals: 4HNO 3 + Cu \u003d 2NO 2 + Cu (NO 3) 2 + 2H 2 O
decomposition of nitrates: 2Pb (NO 3) 2 \u003d 4NO 2 + O 2 + 2РbО
Chemical properties
NO 2 - acid oxide, mixed anhydride of 2 acids
NO 2 interacts with water, basic oxides and alkalis. But the reactions do not proceed in the same way as with conventional oxides - they are always redox. This is explained by the fact that there is no acid with S.O. (N) \u003d +4, therefore, NO 2, when dissolved in water, disproportionates with the formation of 2 acids - nitric and nitrous:
2NO 2 + H 2 O \u003d HNO 3 + HNO 2
If dissolution occurs in the presence of O 2, then one acid is formed - nitric acid:
4NO 2 + 2H 2 O + O 2 \u003d 4HNO 3
NO 2 interacts with alkalis in a similar way:
in the absence of O 2: 2NO 2 + 2NaOH \u003d NaNO 3 + NaNO 2 + H 2 O
in the presence of O 2: 4NO 2 + 4NaOH + O 2 = 4NaNO 3 + 2H 2 O
NO 2 is a very strong oxidizing agent
The oxidizing power of NO 2 is superior to that of nitric acid. C, S, P, metals and some organic substances burn in its atmosphere. In this case, NO 2 is reduced to free nitrogen:
10NO 2 + 8P = 5N 2 + 4P 2 O 5
2NO 2 + 8HI \u003d N 2 + 4I 2 + 4H 2 O (violet flame appears)
In the presence of Pt or Ni, nitrogen dioxide is reduced by hydrogen to ammonia:
2NO 2 + 7H 2 \u003d 2NH 3 + 4H 2 O
As an oxidizing agent, NO 2 is used in rocket fuels. When it interacts with hydrazine and its derivatives, a large amount of energy is released:
2NO 2 + 2N 2 H 4 \u003d 3N 2 + 4H 2 O + Q
N 2 O 3 and N 2 O 5 - unstable substances
Both oxides have a pronounced acidic character; they are, respectively, anhydrides of nitrous and nitric acids.
N 2 O 3 as an individual substance exists only in the solid state below T pl. (-10 0 C).
Decomposes with increasing temperature: N 2 O 3 → NO + NO 2
N 2 O 5 at room temperature and especially in the light decomposes so vigorously that it sometimes explodes spontaneously.
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Dina Kamilevna Gainullina— Candidate of Biological Sciences, Researcher at the Department of Human and Animal Physiology, Faculty of Biology, Moscow state university them. M. V. Lomonosov, a specialist in the field of physiology of blood circulation. The area of scientific interests is the peculiarities of the regulation of the vascular system in early postnatal ontogenesis. |
Svetlana Ivanovna Sofronova– Postgraduate student of the same department, deals with the problems of hormonal regulation of the synthesis of endothelial nitric oxide. |
Olga Sergeevna Tarasova- Doctor of Biological Sciences, Professor of the same Department and Leading Researcher of the Laboratory of Physiology of Muscular Activity of the State Scientific Center of the Russian Federation "Institute of Medical and Biological Problems of the Russian Academy of Sciences", a specialist in the field of blood circulation and the autonomic nervous system. The area of scientific interests is the interaction of systemic and local mechanisms of regulation of the cardiovascular system. |
The tone of blood vessels and the level of blood pressure in the body are regulated by the coordinated work of many systems and mechanisms, among which the vascular endothelium plays an important role. The secretion of nitric oxide (NO) is one of the key functions of endothelial cells, and physicians often associate their dysfunction in various diseases with a decrease in NO production. What are the current ideas about the operation of this system? We will try to answer this question in our article.
Background
The layer of cells lining all the blood and lymphatic vessels, as well as the heart cavities, was first described in 1847 by T. Schwann as a "distinctly distinguishable membrane", which 18 years later V. Gies called the endothelium. In relatively large vessels (arteries and veins), this layer serves as a barrier between blood and smooth muscle cells, and the walls of the smallest vessels, capillaries, are entirely built from endothelial cells. Their total number is very large: in the body of an adult, the total mass exceeds 1 kg!
In the 50-60s of the XX century. scientists, armed with an electron microscope, described in detail the structure of the endothelium, but its role in the regulation of the functions of the cardiovascular system remained unclear. Until 1980, the endothelium was considered only a selectively permeable barrier between the blood and the vascular wall, although already at that time it was known that it was capable of releasing substances that prevent blood clotting.
The beginning of modern ideas about the functions of the endothelium was laid in 1980, when R. Farchgott and J. Zawadsky drew attention to its role in the regulation of vascular tone. In elegant experiments, the researchers proved that a substance such as acetylcholine causes relaxation of aortic preparations isolated from the body of a rabbit only in the presence of endothelium. This observation turned out to be so important that Farchgott subsequently became one of the laureates. Nobel Prize(1998). In our time, endothelial-dependent vascular response to acetylcholine and other substances has been described in large numbers. scientific works performed on a wide variety of arterial vessels - not only large, but also small ones that regulate the blood supply to organs (Fig. 1).
By 1986, it became clear that relaxation of vascular smooth muscle is caused precisely by nitric oxide (NO), which is released from the endothelium under the action of acetylcholine. How, in such a short time (only six years), was it possible to isolate NO from a long series of other contenders for the role of an intermediary between the endothelium and vascular smooth muscle? The fact is that even 10 years before the famous work of Farchgott and Zawadzki, the vasodilating effect of NO was studied. Indeed, by that time, nitroglycerin (it serves as a source of NO molecules) had already been treated for 100 years for angina pectoris, which occurs due to spasms of the heart vessels. The identity of the endothelial relaxing factor and NO was also established by such indicators as extreme instability (especially in the presence of reactive oxygen species), inactivation when interacting with hemoglobin and related proteins, as well as the ability to cause similar biochemical changes in vascular smooth muscle cells.
In humans and animals, nitric oxide is one of the key endogenous regulators of the cardiovascular and other systems. In 1992, it was named Molecule of the Year, and the annual number of publications on its functions in the body today is several thousand. The endothelium can be called a giant endocrine organ, in which the cells are not collected together, as in the endocrine glands, but are dispersed in the vessels that permeate all the organs and tissues of our body. Under normal physiological conditions, the endothelium is activated mainly mechanically: by the shear stress created by the blood flow, or by the expansion of the vessel under its pressure. In addition, endothelial cells can be activated by regulatory molecules, such as purine compounds (ATP and ADP), peptides (bradykinin, calcitonin-related peptide, substance P, etc.).
In addition to nitric oxide, endothelial cells synthesize other substances that affect vascular tone, tissue blood supply and blood pressure. For example, prostacyclin (prostaglandin I 2 ) and endothelial hyperpolarizing factor can be helpful in relaxing blood vessels. The proportion of their participation depends on the sex and type of animal, the type of vascular bed and the size of the vessel. For example, the effect of NO is stronger in relatively large vessels, and the effect of the hyperpolarizing factor is stronger in smaller ones.
In the endothelium, not only vasodilators are formed, but also vasoconstrictors: some prostaglandins, thromboxane, peptides endothelin-1 and angiotensin II, superoxide anion. In a healthy body, the secretory activity of the endothelium is directed to the production of vasodilating factors. But in various diseases (systemic or pulmonary hypertension, myocardial ischemia, diabetes mellitus, etc.) or in a healthy body with aging, the endothelial secretory phenotype can change towards vasoconstrictive effects.
Despite the variety of regulatory mechanisms dependent on the endothelium, its normal function is most often associated with the ability to secrete NO. When the endothelium changes its properties in diseases, doctors call this condition endothelial dysfunction, implying a decrease in NO production. In connection with the importance of NO, we will consider modern ideas about its regulatory role, first in the norm, and then in some forms of vascular pathology.
Synthesis and regulation of NO in the endothelium
In nature, the synthesis of nitric oxide can proceed through various pathways. So, in the troposphere, it is formed from O 2 and N 2 under the action of lightning discharges, in plants - due to the photochemical reaction between NO 2 and carotenoids, and in animals - when nitrites and nitrates interact with proteins containing metal atoms (for example, with hemoglobin ). All of these reactions take place without the participation of biological catalysts - protein enzymes, so it is relatively difficult to control the rate. However, in the body of animals, the main amount of NO as a regulator physiological processes is formed under the action of special enzymes NO-synthases (NOS), and the source of the nitrogen atom is the amino acid L-arginine [,].
There are several varieties (isoforms) of NO synthases encoded by different genes. In 1990, the neuronal form of the enzyme (nNOS) was isolated from the rat brain. A little later, inducible NOS (iNOS) was found in the cells of the immune system (macrophages), and endothelial NOS (eNOS) was found in the endothelium. Another isoform of NOS is localized in mitochondria; it regulates the processes of cellular respiration. Since a large number of cofactors are involved in NO synthesis, all enzyme isoforms have specific binding sites for them. Each NOS molecule consists of two identical halves. Their association into a dimer requires the cofactor tetrahydrobiopterin. With its deficiency, eNOS switches to the production of reactive oxygen species (superoxide anion and H 2 O 2), which can lead to damage to the endothelium and other cells of the vascular wall.
Two isoforms of the enzyme, eNOS and nNOS, are called constitutive because they are always present in cells and synthesize NO in relatively small (compared to iNOS) amounts, and the activity of these isoforms is regulated by physiological stimuli. In contrast to them, iNOS is constantly synthesized only in some cells, for example, in macrophages, and in endothelial, nervous, and many others, it appears only in response to external, mainly inflammatory, stimuli (for example, elements of bacterial cell walls - bacterial lipopolysaccharides). Active iNOS produces NO 1000 times faster than eNOS and nNOS. Macrophages use these large amounts of NO to kill pathogens before destroying them.
Thus, the main NO synthase in the vascular wall is eNOS, and it is contained mainly in the endothelium. Transcription of the eNOS gene in smooth muscle cells is prevented by special mechanisms, such as methylation of the "starting" site. Synthase binds to the outer membrane of the endothelial cell in special invaginations, caveolae, where a large number of regulatory molecules (various ion channels and receptors) are concentrated. This "fixation" of the enzyme provides its functional connection with receptors and channels, which facilitates the regulation of eNOS activity. The caveolin localizes the protein caveolin, which inhibits the activity of the enzyme in the absence of stimuli.
The functional role of endothelial NO synthase depends on the number of molecules in the cell (eNOS gene expression level) and on its activity. It should be noted that the synthesis of new protein molecules is relatively long, so it is used to provide long-term changes in NO production, for example, when the vascular system adapts to physical activity or to high-mountain hypoxia. To quickly control NO synthesis, other mechanisms are used, primarily a change in the intracellular concentration of Ca 2+ , a universal regulator cellular functions. It should be noted right away that such physiological regulation is characteristic only of eNOS and nNOS, while for iNOS (Ca2+-independent enzyme) it occurs mainly at the level of gene expression.
An increase in Ca 2+ concentration to a certain threshold level is an indispensable condition for the cleavage of endothelial NO synthase from caveolin and its transition to the active state. In addition to Ca 2+ great importance to regulate the activity of eNOS, it has phosphorylation, i.e., the covalent attachment of a phosphoric acid residue, carried out by intracellular enzymes - protein kinases. Phosphorylation alters the ability of eNOS to be activated by calcium (Fig. 2). Protein kinases attach phosphoric acid residues to strictly defined amino acid residues of the eNOS molecule, among which the most important are serine at position 1177 (Ser1177) and threonine at position 495 (Thr495). The Ser1177 site is considered the main site of eNOS activation. It is known that the degree of its phosphorylation increases rapidly under the influence of important regulatory factors: shear stress, bradykinin, vascular endothelial growth factor and estradiol. The main enzyme that performs this process is Akt (another name is protein kinase B), but other kinases are also known that can activate eNOS (we will talk about them later).
Phosphorylation at the Thr495 site reduces the activity of the enzyme. Such Negative influence may increase under certain pathological conditions - oxidative stress, diabetes mellitus, etc. On the contrary, under some normal physiological effects, phosphate is removed (i.e., Thr495 is dephosphorylated), due to which the affinity of eNOS to Ca 2+ increases and, consequently, its activity increases . Thus, the intensity of eNOS activity in endothelial cells can be dynamically regulated by Ca 2+ levels and phosphorylation/dephosphorylation under the action of various protein kinases. This ultimately provides fine regulation of nitric oxide synthesis and, accordingly, its physiological effects on the cardiovascular system.
Relaxation mechanisms of smooth muscle cells
How does NO secreted by endothelial cells cause vasodilation? The contraction of all types of muscle cells is provided by the interaction of two proteins - actin and myosin, and the motor activity of the latter in smooth muscle cells is manifested only after its phosphorylation. This implies the presence of a large number of regulatory mechanisms that affect the contractile activity of the smooth muscle cell, including nitric oxide.
NO molecules are lipophilic; therefore, they freely penetrate from endothelial cells into smooth muscle cells. In them, the main NO acceptor is the enzyme guanylate cyclase, located in the cytosol and therefore called soluble (that is, not associated with cell membranes). Guanylate cyclase, activated by nitric oxide, synthesizes cyclic guanosine monophosphate (cGMP), which serves as a powerful activator of another enzyme, protein kinase G. Its targets in smooth muscle cells are numerous proteins involved in the regulation of cytoplasmic Ca 2+ concentration.
Protein kinase G activates some types of potassium channels, which causes hyperpolarization (a shift in the membrane potential towards more negative values) of smooth muscle cells, closes the potential-controlled calcium channels of the outer membrane and thereby reduces the entry of Ca 2+ into the cell. In addition, this enzyme in its active state suppresses the release of Ca 2+ from intracellular depots, and also promotes its removal from the cytoplasm. This also reduces the concentration of Ca 2+ and relaxes the smooth muscles.
In addition to influencing Ca 2+ homeostasis, protein kinase G regulates the Ca 2+ sensitivity of the contractile apparatus of smooth muscle cells, i.e., reduces its ability to be activated when Ca 2+ increases. It is known that the activation of protein kinase G (with the participation of mediators) reduces the level of phosphorylation of smooth muscle myosin, as a result, it interacts worse with actin, which promotes relaxation. The combination of the described events leads to vasodilation, an increase in blood flow in the organs and a decrease in blood pressure.
Physiological regulation of NO production
The ability to produce NO serves as a marker of the normal functional state of the endothelium: the elimination of the effects of NO in a healthy body (for example, by pharmacological blockade of eNOS) leads to vasoconstriction and an increase in systemic arterial pressure. As a result of the action of almost all normal physiological stimuli, the content of NO-synthase in the endothelium (and/or its activity) increases. The key factor regulating NO production is blood flow. When it moves through the vessel, shear stress occurs on the surface of the endothelium. This stimulus is transmitted to intracellular endothelial NO synthase through activation of mechanosensing channels and Ca 2+ entry. Another variant of transmission is through membrane enzymes if the activity of Akt protein kinase increases and eNOS is phosphorylated (at the Ser1177 site). The blood flow ensures the constant secretion of small amounts of NO by the endothelium (Fig. 3).
The glycocalyx plays an important role in the sensitivity of the endothelium to shear stress. This is a layer of polymer molecules of a carbohydrate nature covering the cells, the thickness of which can be several micrometers and even exceed the thickness of the endothelium itself. Since the "bushes" of glycoproteins grow inside the lumen of the vessel, they are the first to experience the action of the blood flow. When deformed, glycocalyx fibers transmit a signal to membrane proteins and further to eNOS. Although this mechanism has been little studied so far, its importance is evidenced by the fact that the violation of the response of vessels to shear stress in various diseases (atherosclerosis, diabetes mellitus, etc.) is associated with “baldness” of the endothelium, i.e. with a decrease in thickness and a change in the structure glycocalyx.
An increase in blood flow velocity leads to the activation of endothelial NO synthase and to vasodilation, and such prolonged or repeated exposures increase the content of this enzyme in the endothelium. This is the basis of the beneficial effects of physical exercise: it is known that with the help of training it is possible to significantly improve the functioning of the endothelium without the use of drugs! However, it should be noted that not all exercises have such a beneficial effect. Firstly, the load should be accompanied by an increase in the blood flow velocity in the working muscles, as happens during fast walking, running or cycling, and strength exercises with kettlebells do not have such an effect. Secondly, you can’t train through strength: with excessive loads, the secretion of the main stress hormone, cortisol, sharply increases, which reduces the activity of eNOS.
Additional activation of endothelial NO-synthase during exercise is provided by protein kinase activated by adenosine monophosphate (AMP), which is found in almost all cells of our body, including endothelial cells. This enzyme is called the "sensor of the energy status of cells" because it is activated when the ratio of AMP / ATP in the cytoplasm of the cell rises, i.e., energy consumption begins to exceed its production. In the endothelium of arteries located inside intensely contracting skeletal muscles, this can occur as a result of hypoxia - muscle cells consume a lot of O 2, and the vascular endothelium lacks it. In addition, it has recently been shown that the activation of this protein kinase in endothelial cells is possible with an increase in shear stress, i.e., with an increase in blood flow to working muscles. Activated protein kinase phosphorylates eNOS at the Ser1177 site, NO production increases, and blood vessels dilate.
Cardiologists are well aware that through regular physical training, endothelial function can be improved not only in skeletal muscles and the heart, which are intensively supplied with blood during work, but also in organs that are not directly involved in training - in the brain, skin, etc. e. This suggests that in addition to the effect of blood flow on the endothelium, there are other mechanisms of regulation of endothelial NO synthase. Among them, the leading role belongs to hormones that are produced by the endocrine glands, transported by the blood and recognize target cells in various organs by the presence of special receptor proteins.
Of the hormones that can affect endothelial function during exercise, we note the growth hormone (somatotropic hormone), which is secreted by the pituitary gland. Both by itself and through its intermediaries, insulin-like growth factors, growth hormone increases endothelial NO synthase production and activity.
Most famous example hormonal regulation of endothelial functions is the influence of female sex hormones, estrogens. Initially, this idea was formed due to epidemiological observations, when it turned out that, for some reason, women of childbearing age, compared to men, suffer less from vascular disorders associated with endothelial dysfunction. Moreover, in women, its ability to produce NO changes during the menstrual cycle, and in the first half, when the concentration of estrogen in the blood is high, endothelial-dependent vasodilation is more pronounced. These observations have prompted numerous animal experiments. Thus, ovary removal in female rats reduced the content and activity of endothelial NO synthase in the arteries of various organs (brain, heart, skeletal muscles, kidneys, intestines, etc.), and the administration of estrogen to such females contributed to the normalization of impaired function. The effect of estrogens on eNOS activity is associated with the activation of the Akt protein kinase, and the increase in eNOS synthesis is associated with their effect on the genome of endothelial cells.
Interestingly, a violation of the reactions of the arteries of the brain was also found in experiments with the removal of the gonads in males, although the testes secrete not estrogens, but androgens, male sex hormones. This paradox became clear when aromatase, an enzyme that converts androgens into estrogens, was discovered in the endothelium of the arteries of the brain. Thus, the protective effect of estrogen on the vascular endothelium may also occur in males. However, in this case, we should talk about local regulation, which is provided by estrogens formed directly in the vascular wall.
In conclusion, let us consider the regulation of endothelial NO synthase by thyroid hormones. It is known that the intensity of NO synthesis in the vascular endothelium changes when its work is disturbed: it increases in hyperthyroidism, and decreases in hypothyroidism. This effect is mainly due to changes in the content of NO synthase in endothelial cells. However, data have recently appeared on the existence of another mechanism of action of these hormones on vascular endothelial cells. Thus, the Ca 2+ -dependent activity of eNOS and the degree of its phosphorylation at the Ser1177 site in the arteries of rats with experimental hyperthyroidism turned out to be significantly higher than in rats with hypothyroidism.
Thyroid hormones are known to play a key role in tissue differentiation in the developing organism. But their influence is not limited to accelerating or slowing down ongoing processes, but often has a programming character. This means that with a lack of thyroid hormones at a certain critical age, cells will not be able to turn into fully functioning ones, even if hormones are administered at later stages of life (in humans, hormone therapy is effective only during the first months after birth). The mechanisms of the programming influence of thyroid hormones have been studied in detail only for the nervous system, and for other systems - much worse. However, it is well known that maternal hypothyroidism during pregnancy is, among other things, a risk factor for the development of cardiovascular disease in the child. Interestingly, in the arteries of rat pups, in the first weeks after birth, elevated levels of thyroid hormone receptors, as well as the enzyme deiodinase, which converts thyroxine (tetraiodothyronine) into the more active triiodothyronine, are detected. Based on these observations, it is tempting to suggest that thyroid hormones can also have a programming effect on the vascular endothelium. How true this is, future research will show.
Mechanisms of impaired NO secretion by the endothelium
Unfortunately, the possibilities of the endothelium of our vessels to provide NO production are not unlimited. The activity of the body's regulatory systems is high at a young and mature age, but decreases with aging under the influence of a number of factors. First, only a few older people can try on the saying of the ancient Greek philosopher Aristotle: "Life requires movement." Secondly, with age, the activity of many hormonal systems fades: the secretion of growth hormone and sex hormones decreases, the thyroid gland “falls asleep”. Thirdly, there are changes in the metabolism of all cells. In particular, the energy stations of the cell, mitochondria, begin to produce reactive oxygen species in large quantities, which inactivate NO, and also suppress activity and reduce the content of endothelial NO synthase. Apparently, age-related changes in the endothelium cannot be prevented, but they can be slowed down by increasing mobility, limiting the intake of high-calorie foods (this also increases the activity of AMP-activated protein kinase), using hormone replacement therapy (for example, in women after menopause), or antioxidants, which have been developed and remains a priority area of gerontology.
Why is NO synthesis in the vascular endothelium impaired in various pathologies? Two types of changes are possible here: fast (decrease in the activity of NO synthase in the endothelium), and long-term - a decrease in its content in cells. We will not consider various diseases separately, but list the common mechanisms for them to adversely affect the operation of eNOS. A decrease in the activity of this enzyme in diseases is usually associated with an increase in its phosphorylation at the Thr495 site, due to an increase in the activity of protein kinase C. Its powerful activator is diacylglycerol. Normally, it is a secondary messenger in signal transmission from many membrane receptors, but its excessive accumulation in endothelial cells leads to pathology.
A striking example of such changes can be a disease such as diabetes mellitus, in which a violation of the synthesis or action of insulin on cells leads to an increased concentration of glucose in the blood. Since the transport of glucose into the endothelium is not regulated by insulin (unlike cells of skeletal muscles, heart, adipose tissue, and some others), sugar accumulates there and becomes a substrate for the synthesis of diacylglycerol, which activates protein kinase C.
The already mentioned oxidative stress serves as a marker of many cardiovascular pathologies. Increased formation of reactive oxygen species is also characteristic of diabetes mellitus, and for atherosclerosis, and for many forms of arterial hypertension. Under these conditions, a high activity of the renin-angiotensin system is often observed, and angiotensin II is a powerful provocateur of oxidative stress, which, on the one hand, reduces the activity of eNOS (for example, oxidized low-density lipoproteins can activate protein kinase C), and on the other hand, reduces gene expression eNOS, which also reduces NO production. The use of antioxidants or substances that interfere with the formation or action of angiotensin II (angiotensin-converting enzyme inhibitors or angiotensin II blockers) almost always increases the formation of NO. It must be said that the decrease in the production of nitric oxide in diseases can be associated not only with a direct effect on eNOS. Thus, the effect of glucocorticoids on the endothelium reduces the content of not only the enzyme itself, but also its cofactor, tetrahydrobiopterin.
Disruption of the work of endothelial NO-synthase may be associated with a lack of its main substrate - L-arginine. As a rule, this amino acid enters the body with food in sufficient quantities and, in addition, can be directly synthesized in the adult body. However, arginine, in addition to NO synthases, serves as a substrate for many other enzymes, in particular arginase, which is located in various types cells, including those in the vascular endothelium. In diabetes mellitus, oxidative stress, as well as inflammatory processes under the action of cytokines secreted by cells of the immune system (tumor necrosis factor, etc.), the content of arginase in the endothelium increases.
Finally, inhibitors of endothelial NO synthase, such as dimethylarginine, may appear in humans and other animals. This "false substrate" of endothelial NO synthase competes with the true substrate, L-arginine, for the enzyme's active site. Normally, dimethylarginine is formed in the body only in small amounts (in an adult, ~60 mg / day), however, with a variety of circulatory pathologies (arterial hypertension, atherosclerosis, coronary insufficiency, etc.), its production increases significantly, and the activity of endothelial NO -synthase, respectively, decreases.
So, nitric oxide is an important regulatory factor through which the endothelium has a relaxing effect on neighboring smooth muscle cells, causing vasodilation and smoothing out unwanted increases in blood pressure at the system level. As long as the endothelium retains the ability to secrete NO in an amount sufficient to solve these problems, there is no need to worry about the state of the vascular system.
This work was supported by the Russian Foundation for Basic Research. Project NK 14-04-31377 mol.
Literature
.
Furchgott R. F., Zawadzki J. V. The obligatory role of endothelial cells in the relaxation of arterial smooth muscle by acetylcholine // Nature. 1980. V. 288. P. 373–376.
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Melkumyants A. M., Balashov S. A. Mechanosensitivity of the arterial endothelium. Tver, 2005.
Calcitonin Gene Related Peptide) is formed from the same gene as calcitonin by alternative mRNA splicing in the brain and peripheral nervous system.
Residue numbers are given according to the location in the human eNOS molecule.
Oxides and hydroxides of nitrogenTHE MYSTERY OF OXIDATION STATES
Nitrogen forms a number of oxides that formally correspond to all possible oxidation states from +1 to +5: N 2 O, NO, N 2 O 3, NO 2, N 2 O 5, however, only two of them are nitric oxide (II) and oxide nitrogen(IV) are not only stable under normal conditions, but are also actively involved in the natural and industrial nitrogen cycles. Therefore, we will study precisely their properties (in comparison). Let's start, as usual, with the structure of molecules.
The structure of nitrogen oxide molecules
Molecule NO. The structure is quite simple to assume: oxygen has two unpaired electrons, nitrogen has three - a double bond is formed and one unpaired electron in the remainder ... It is not easy to answer the question why such a “non-standard” molecule is stable. By the way, it is worth noting that stable free radicals - molecules with unpaired electrons - are quite rare in nature. It can be assumed that NO molecules will pair and form a doubled, or dimeric, ONNO molecule. Thus it is possible to solve the problem of the unpaired electron.
Molecule NO2. It would seem, what is simpler - an oxygen atom has joined the NO molecule along an unpaired electron. (In fact, it is not an atom that is attached, but a molecule, and not to NO, but to the ONNO dimer. That is why the rate of attachment decreases with increasing temperature - the dimer falls apart into halves.) And now oxygen has an unpaired electron - the nitric oxide molecule (IV) is also a free radical. However, it is known that when two NO 2 molecules are combined and an N 2 O 4 molecule is formed, the bond is carried out through nitrogen atoms, which means that nitrogen should have this very unpaired electron. How can this be done?
The answer is unconventional, but quite in the "character" of nitrogen - a donor-acceptor bond. Using logic, consider the electrons that the nitrogen atom has in the NO molecule. This is an unpaired electron, a free pair of electrons, and two more electrons bonded to oxygen - a total of five. And the oxygen atom, "coming into contact", has six electrons in four orbitals. If you arrange them two by one, then one orbital will remain free. That's it and it is occupied by a pair of electrons of the nitrogen atom, and the unpaired electron in this connection is completely irrelevant (Fig. 1, 2).
It is worth mentioning one more point - since a pair of electrons located on s-orbitals, "went to bond", it was simply obliged to undergo hybridization - it is very difficult to offer the second atom for general use a pair of electrons evenly distributed over the surface of the first atom. The question arises: what type of hybridization does the atom use? Answer: three electron orbitals nitrogen are in a state sp 2-hybridization. The NO 2 molecule is angular, the angle is 134° (the angle is greater than 120° because one electron repels bond electrons from itself weaker than a pair of electrons) (Fig. 3–5).
Physical properties of nitrogen oxides
Nitric oxide(II) NO. The crystal lattice is molecular; the molecule is light, weakly polar (the electronegativity of oxygen is higher than that of nitrogen, but not by much). It can be assumed that the melting and boiling points will be low, but higher than those of nitrogen, since some kind of polarity of the molecule makes it possible to connect electrostatic forces of attraction to simple intermolecular forces. The formation of a dimer also contributes to an increase in the boiling point, making the molecule heavier. The structure of the molecule also suggests a low solubility in water, a solvent that is noticeably more polar than NO. Separately, it is worth emphasizing that nitric oxide (II) has neither color nor odor.
Nitric oxide(IV) NO2. The crystal lattice is also molecular, however, since the molecule itself is heavier than NO and its tendency to dimerization is noticeably higher, this substance should melt and boil at noticeably higher temperatures. The boiling point is 21 ° C, therefore, under normal conditions - 20 ° C and 760 mm Hg. Art. – nitric oxide (IV) liquid.
Now let's take a look at solubility. Recall that the word "solubility" can also mean chemical reactions with water; the main thing is that the solvent absorbs the solute. When oxides react with water, as is known, hydroxides are obtained - formally, these are simply hydrated oxides, but reality often presents a lot of interesting and completely informal things. So this nitric oxide dissolves in water, simultaneously reacting with it, and in this case two acids are obtained at once!
Note that nitric oxide (IV) has both a characteristic pungent odor and a reddish-brown color, the shades of which differ from each other depending on the concentration. It is for this color that emissions of nitrogen oxides into the atmosphere are called “fox tails”.
You ask: where is the secret? The first part of the mystery of oxidation states is in front of you: why does an element of the fifth (odd) group have stable oxides with even oxidation states? (At the same time, free radicals too!) In the most general sense, the answer is obvious - since they are stable, it means that they are so profitable. Energetically. And why? Apparently, the point is in the specifics of the structure of nitrogen and oxygen atoms - they have too many electrons and too few orbitals. It is the "orbital opportunities" that dictate their own rules, establish such "energetic benefits". Then the numbers "two" and "four" become clear: two electrons are not enough for oxygen to reach eight, and both atoms have only four orbitals.
And you can also say that NO is just ... waiting for an oxygen molecule to turn into NO 2. Using a metaphor, we note that the "meaning of life" of many atoms is the desire to find a "life partner" - an atom or atoms of another element. Although there are, of course, "convinced bachelors" such as gold.
Chemical properties of nitrogen oxides
1. Reactions with metals. Since the nitrogen atom in positive oxidation states is an oxidizing agent, and the higher the oxidation state, the stronger the ability to take electrons from other atoms, then nitrogen oxides will react with metals - essentially reducing agents. In this case, the products can be completely different, depending on the reaction conditions and the metal itself. For example, all nitrogen oxides give off oxygen to hot copper, and they themselves turn into a simple nitrogen substance:
By the amount of copper oxide and nitrogen formed, it is possible to determine which of the nitrogen oxides reacted with copper.
2. Reactions with non-metals. Let us first consider reactions with oxygen. Here, there is a difference between the oxides, and a very significant one.
Oxide NO reacts with oxygen to form nitric oxide(IV). The reaction is reversible. Moreover, with increasing temperature, the rate of this reaction decreases:
2NO + O 2 \u003d 2NO 2.
Oxide NO 2 does not react with oxygen at all.
Ozone converts both oxides into nitric oxide (V).
Nitric oxide(II) NO adds ozone completely:
2NO + O 3 \u003d N 2 O 5.
Nitric oxide (IV) NO 2 in reaction with ozone also releases oxygen:
2NO 2 + O 3 \u003d N 2 O 5 + O 2.
3. Reactions with water. NO oxide does not react with water. NO 2 oxide forms two acids with water - nitric (nitrogen oxidation state +5) and nitrous (nitrogen oxidation state +3). In the presence of oxygen, NO 2 oxide is completely converted into nitric acid:
2NO 2 + H 2 O \u003d HNO 3 + HNO 2,
4NO 2 + O 2 + 2H 2 O \u003d 4HNO 3.
4. Reactions with acids. None of the oxides - NO or NO 2 - reacts with acids.
5. Reactions with alkalis. Both nitric oxides react with alkalis.
Oxide NO forms with alkali a salt of nitrous acid, nitric oxide (I) and nitrogen:
10NO + 6NaOH \u003d 6NaNO 2 + N 2 O + N 2 + 3H 2 O.
Oxide NO 2 forms with alkali salts of two acids - nitric and nitrous:
2NO 2 + 2NaOH \u003d NaNO 3 + NaNO 2 + H 2 O.
Let's get back to our mystery of oxidation states. With the transition of oxygen compounds of nitrogen from the state of "gas", where you can move freely, to the state of "water solution", where there is more crush, where collectivism flourishes, where polar water molecules exist and actively operate, no one will allow a molecule, atom or ion to be alone, there is a "change of orientation." Just odd oxidation states become stable, as it should be for an element from an odd group. (Stable, however, relatively. Nitrous acid, for example, can exist only in solution, otherwise it decomposes. But acids formally corresponding to nitrogen oxides (II) and (IV) do not exist at all. Everything is known in comparison.)
It is interesting that not only the clearly acidic oxide NO 2 reacts with alkalis, but also NO - non-acidic in properties and degree of oxidation, while compounds of other oxidation states are obtained - odd! Secret? Quite!
The structure of the molecule of nitrogen (V) hydroxide - nitric acid
Of nitrogen hydroxides, we will consider one, but the most multi-tonnage - nitric acid.
Molecule nitric acid polar (primarily due to the different electronegativity of oxygen and hydrogen, because nitrogen is, as it were, hidden inside the molecule) and asymmetric. All three angles present in it between the bonds of nitrogen and oxygen are different. The formal oxidation state of nitrogen is the highest, i.e. +5. But at the same time, only four bonds have a nitrogen atom with other atoms - the valence of nitrogen is four. Another secret.
It is clear how it can be that atom valence numerically more than its oxidation state. To do this, it is enough to form a bond between identical atoms in a molecule. For example, in hydrogen peroxide, oxygen has a valence of two, and the oxidation state is only -1. Oxygen managed to pull the common electron pair of bonds with hydrogen closer to itself, and the pair of bonds of two oxygen atoms is still strictly in the middle. But how to make atom valence was less oxidation state?
Let's think: what is the general structure of the nitric acid molecule? The structure of a molecule is easier to understand if we consider the process of obtaining it. Nitric acid is obtained by the reaction of nitric oxide (IV) with water (in the presence of oxygen): two NO 2 molecules simultaneously "attack" the water molecule with their unpaired electrons, as a result, the bond between hydrogen and oxygen is not broken as usual (a pair of electrons in oxygen and "naked proton”), and “to be honest” - one NO 2 molecule gets hydrogen with its electron, the other - the OH radical (Fig. 6). Two acids are formed: both acids are strong, both quickly donate their proton to the nearest water molecules and ultimately remain in the form of and ions. The ion is unstable, two HNO 2 molecules decompose into water, NO 2 and NO. NO oxide reacts with oxygen, turning into NO 2, and so on until only nitric acid is obtained.
Formally, it turns out that the nitrogen atom is bound to one oxygen atom by a double bond, and to the other by an ordinary single bond (this oxygen atom is also bound to the hydrogen atom). Nitrogen in HNO 3 is linked to the third oxygen atom by a donor-acceptor bond, with the nitrogen atom acting as a donor. The hybridization of the nitrogen atom in this case should be sp 2 due to the presence of a double bond, which determines the structure - a flat triangle. In reality, it turns out that indeed a fragment of a nitrogen atom and three oxygen atoms is a flat triangle, only in a nitric acid molecule this triangle is incorrect - all three ONO angles are different, therefore, different sides of the triangle. When the molecule dissociates, the triangle becomes regular, equilateral. This means that the oxygen atoms in it become equivalent! All bonds become the same (a double bond is shorter than a single bond). How?
Let's reason. sp 2-Hybridization of the nitrogen atom forces oxygen atoms to the same type of hybridization. A flat structure is obtained, across which the p-orbitals, which are not involved in hybridization, are located, which are present in all four atoms.
Now let's deal with the total number of valence electrons: the ion contains five nitrogen electrons, six each for three oxygen atoms, and one more, which gives charge to the ion as a whole, for a total of twenty-four. Of these, six electrons are required to form three single bonds, twelve electrons are located along the perimeter of the molecule in hybrid orbitals (two electron pairs for each oxygen atom), six electrons remain for four of the same R-orbitals not involved in hybridization. The only reasonable explanation possible in this case is the socialization of all atoms of their electrons into a single electron cloud (Fig. 7). This is facilitated by small atomic radii and small interatomic distances. And symmetry is usually energetically favorable and therefore increases the stability of the structure as a whole. This is not the only case of the socialization of electrons by several atoms; a similar "collective electronic economy" is found in organic chemistry, for example in aromatic compounds.
Let us return, however, to the predictions of the properties of nitric acid, based on ideas about the structure of the molecule. The obvious advantage of being in the form of an ion explains not only the high degree of acid dissociation in an aqueous solution, but also the possibility of anhydrous acid dissociation. And it is dissociation that determines the physical properties of this substance.
Physical properties of nitric acid
An ionized compound, even if only partially, is difficult to convert into a gas. Thus, the boiling point should be high enough, but with such a small molecular weight (and due to high mobility), the melting point should not be high. Therefore, the state of aggregation at 20 °C is liquid.
With regard to solubility in water, like many other polar liquids, nitric acid is easily miscible with water in any ratio.
Pure nitric acid is colorless and odorless. However, due to decomposition into oxygen and nitric oxide (IV), which dissolves in it, we can say that ordinary concentrated nitric acid has a yellow-brown color and a pungent odor characteristic of NO 2.
Let's see how the structure of the nitric acid molecule affects its chemical properties.
Chemical properties of nitric acid
The main thing we should note is the presence the highest degree oxidation of the nitrogen atom limits the properties of nitric acid, it does not react with oxidizing agents. But with reducing agents, primarily with metals, it reacts in an unconventional and diverse way.
1. Reactions with metals. With metals, nitric acid reacts as a strong oxidizing agent even in dilute solutions (unlike sulfuric acid, which exhibits its oxidizing properties only in concentrated form). Metal nitrate is usually formed, but instead of hydrogen, gaseous nitrogen compounds are released: NO 2, NO, N 2 O, N 2 or ammonia, which in acidic environment immediately converted to the ammonium ion. In principle, when a metal reacts with nitric acid, this entire “bouquet” of gases is formed, but depending on the metal and the concentration of the acid, one or another component will prevail.
So, in laboratory conditions, nitric oxide (II) is usually obtained by the interaction of copper shavings with nitric acid with a density of 1.2 g / cm 3, i.e., when copper is treated with dilute acid, this oxide clearly prevails in the gaseous reaction products:
But when nitric acid of the same density (and, consequently, concentration) reacts with iron, the content of nitric oxide (II) in the mixture is only 40% - less than half, and the remaining 60% is evenly distributed between ammonium nitrate, nitrogen, nitric oxide (I ) and nitric oxide(IV) (Fig. 8).
It should be noted such an interesting and vital fact that neither iron nor aluminum reacts with 100% nitric acid (hence, it can be stored and transported in tanks and other containers made of these metals). The fact is that these metals are covered with strong films of oxides that are insoluble in pure acid. Acid properties require that the acid dissociate appreciably, and this, in turn, requires water.
2. Reactions with non-metals. Nitric acid does not react with oxygen and ozone.
3. Reaction with water does not occur. Water only contributes to the dissociation of the acid.
4. Reactions with acids. Nitric acid does not react with other acids in the form of exchange or compound reactions. However, it is quite capable of reacting as a strong oxidizing agent. In a mixture of concentrated nitric and hydrochloric acids, reversible reactions occur, the essence of which can be summarized by the equation:
The resulting atomic chlorine is very active and easily takes away electrons from metal atoms, and the chloride ion present “at the side” forms stable complex ions with the resulting metal ions. All this allows even gold to be transferred into solution. For the reason that gold is the "king of metals", a mixture of concentrated nitric and hydrochloric acids is called aqua regia.
Concentrated sulfuric acid, as a strong dehydrating agent, promotes the decomposition of nitric acid into nitric oxide (IV) and oxygen.
5. Reactions with bases and basic oxides. Nitric acid is one of the strong inorganic acids and naturally reacts with alkalis. It also reacts with insoluble hydroxides and basic oxides. These reactions are also facilitated by the fact that all salts of nitric acid have good solubility in water, therefore, the reaction products will not interfere with its course.
Physical properties of NO, NO 2 and HNO 3 compounds in numbers
Nitric oxide(II) NO. Molar mass 30 g/mol. The melting point is -164 °C, the boiling point is -154 °C. The density of gaseous NO under normal conditions (0 °C, 1 atm) is 1.3402 g/l. The solubility at atmospheric pressure and 20°C is 4.7 ml of NO gas per 100 g of water.
Nitric oxide(IV) NO 2 . Molar mass 46 g/mol. Melting point -11 °C, boiling point 21 °C. Density of gaseous NO 2 at n. y. 1.491 g/l. Solubility - provided that this oxide first reacts with water in air, and then also dissolves in the resulting nitric acid - can be considered unlimited (up to the formation of 60% HNO 3).
Since nitric oxide (IV) actively dimerizes (at 140 °C it is entirely in the form of the NO 2 monomer, however, at 40 °C, about 30% of the monomer remains, and at 20 °C, almost all of it passes into the N 2 O 4 dimer) , then the physical properties refer to the dimer rather than the monomer. This can explain the rather high boiling point (N 2 O 4 is a fairly heavy molecule). The degree of dimerization can be judged by the color: the monomer is intensely colored, while the dimer is colorless.
Nitric acid HNO3. Molar mass 63 g/mol. Melting point -41.6 °C, boiling point 83 °C. The density of liquid 100% acid is 1.513 g/cm 3 . Solubility is unlimited, in other words, acid and water are mixed in any ratio. It should be noted that nitric acid solutions boil at temperatures above the boiling points of pure water and acid. At the maximum temperature (122 °C), a 68.4% solution boils, while the percentage composition of the solution and the vapor is the same.
Mixtures of substances for which the composition of the vapor during boiling corresponds to the composition of the liquid are called azeotropic or inseparable boiling. (The word "azeotrope" comes from the Greek - boil, - change, - negative prefix.) For lower acid concentrations, an increase in the amount of water in the vapor compared to the solution is characteristic, which leads to the concentration of the solution. At higher concentrations, on the contrary, the vapor composition is enriched with acid.
Chemical properties of nitrogen compounds (addition)
Like any other substances containing an atom with an intermediate oxidation state, nitrogen (II) and (IV) oxides, unlike nitric acid, can act both as oxidizing agents and as reducing agents, depending on the reaction partner. However, many of these reactions are "irrelevant" and, accordingly, poorly studied.
Of the "actual" reactions, it is worth mentioning the reaction of nitric oxide (IV) with sulfur oxide (IV) in the presence of water:
This reaction is relevant because the addition of oxygen to sulfur(IV) oxide proceeds only at high temperatures and in the presence of a catalyst, while the addition of oxygen to nitric oxide(II) occurs under normal conditions. Thus, nitric oxide (IV) simply helps sulfur oxide to attach oxygen. This reaction proceeds under normal conditions (additional pressure in the mixture and heating is not required).
Nitric oxide(II) also reacts with sulfur oxide (IV), but under completely different conditions: either at a pressure of 500 atmospheres (!), then sulfur oxide (VI) and nitrogen are obtained, or in an aqueous solution, then sulfuric acid and nitric oxide (I ).
Nitric oxide(I). It has a slight pleasant smell and a sweetish taste. It does not react with oxygen, water, solutions of acids and alkalis. It decomposes into elements at temperatures above 500 ° C, in other words, it is quite stable.
The structure of the molecule is interesting: a linear molecule O=N=N, in which the central nitrogen atom is tetravalent. It forms two double bonds: one with oxygen according to the typical scheme for creating a covalent bond (two nitrogen electrons, two oxygen electrons), the other with a nitrogen atom (which pairs two of its three unpaired electrons and forms an empty orbital due to this), one of the bonds - covalent, the second - donor-acceptor (Fig. 9).
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Rice. nine. |
Nitric oxide(III). Consists of NO and NO 2 paired with their unpaired electrons. It begins to decompose into the corresponding gases already during melting (–101 °C).
Nitric oxide(V). Consists of two NO 2 groups connected through oxygen. A somewhat more stable compound than nitric oxide(III) begins to decompose at room temperature. Some of the bonds in it, of course, are donor-acceptor. And no "pentavalent nitrogens."
TO chemical properties nitric acid should be added that it reacts well with non-metals that it can oxidize. So, concentrated nitric acid reacts with sulfur, and with phosphorus, and with coal, forming sulfuric, phosphoric and carbonic acids, respectively.
The reactions of nitric acid with organic substances are interesting and important. For example, when three hydrogen atoms in toluene are replaced by NO 2 groups, trinitrotoluene (or simply tol) is formed - an explosive.
Ecological properties of nitrogen oxides and nitric acid
Nitric oxide(I) relatively inert, and therefore "environmentally neutral". However, it has a narcotic effect on a person, ranging from just fun (for which he was nicknamed "laughing gas") and ending with deep sleep, which has found its application in medicine. Interestingly, it is harmless, and for medical anesthesia, a mixture of nitric oxide (I) with oxygen is used in the same ratio as the ratio of nitrogen and oxygen in the air. The narcotic effect is removed immediately after the cessation of inhalation of this gas.
The other two stable nitric oxides easily transform one into the other, then into acids, and then into anions and. Thus, these substances are natural mineral fertilizers, however, if they are in natural quantities. In "unnatural" quantities, these gases rarely enter the atmosphere alone. As a rule, a whole "bouquet" of toxic compounds is formed that act in a complex way.
For example, only one nitrogen fertilizer plant emits into the air, in addition to nitrogen oxides, nitric acid, ammonia and dust from fertilizers, sulfur oxides, fluorine compounds, some organic compounds. Scientists find out the resistance of various herbs, bushes and trees to such "bouquets". It is already known that, unfortunately, spruce and pine are unstable and die quickly, but white locust, Canadian poplar, willow and some other plants can exist in such conditions, moreover, they contribute to the removal of these substances from the air.
Severe poisoning with nitrogen oxides can be obtained mainly in case of accidents at the relevant industries. The response of the body will be different due to differences in the properties of these gases. "Caustic" NO 2 primarily acts on the mucous membranes of the nasopharynx, eyes, causes pulmonary edema; NO, as a poorly soluble in water and non-caustic substance, passes through the lungs and enters the bloodstream, causing disturbances in the central and peripheral nervous systems. Both oxides react with blood hemoglobin, although in different ways, but with the same result - hemoglobin ceases to carry oxygen.
The ecological properties of nitric acid are made up of two "halves". As a strong acid, it has a destructive effect not only on living tissues (human skin, plant leaves), but also on the soil, which is quite important - acid (due to the presence of nitrogen and sulfur oxides) rains, alas, are not uncommon. When acid gets on the skin, a chemical burn occurs, which is more painful and heals much longer than thermal. These were the main ecological properties hydrogen cation.
Let's move on to study anion. Under the action of a strong acid, it is the acidic properties that come to the fore, so it is better to consider the properties of the anion using the example of salts.
Interaction of nitrate ion with fauna and flora. The fact is that the nitrate ion is an integral part of the nitrogen cycle in nature, it is always present in it. Under normal conditions and in dilute solutions, it is stable, weakly exhibits oxidizing properties, does not precipitate metal cations, thereby facilitating the transport of these ions with a solution in soil, plants, etc.
The nitrate ion becomes poisonous only in large quantities, disturbing the balance of other substances. For example, with an excess of nitrates in plants, the amount of ascorbic acid decreases. (It is worth recalling that a living organism is so finely organized that any substance in large quantities disturbs the balance and, therefore, becomes poisonous.)
Plants and bacteria use nitrates to build proteins and other essential organic compounds. To do this, it is necessary to convert the nitrate ion into an ammonium ion. This reaction is catalyzed by enzymes containing metal ions (copper, iron, manganese, etc.). Due to the much greater toxicity of ammonia and the ammonium ion in plants, the reverse reaction of the transfer of the ammonium ion to nitrate is also well developed.
Animals do not know how to build all the organic compounds they need from inorganic ones - there are no corresponding enzymes. However, microorganisms living in the stomach and intestines possess these enzymes and can convert the nitrate ion into the nitrite ion. It is the nitrite ion that acts as a poisoner, converting iron in hemoglobin from Fe 2+ to Fe 3+.
A compound containing Fe 3+ and called methemoglobin binds the oxygen of the air too strongly, therefore, it cannot give it to the tissues. As a result, the body suffers from a lack of oxygen, while there are disturbances in the functioning of the brain, heart and other organs.
Usually, the nitrite ion is formed not in the stomach, but in the intestines and does not have time to pass into the blood and produce all these destructions. Therefore, nitrate poisoning is quite rare. True, there is another danger: in our body there are many substances in which the hydrogen atoms of ammonia are replaced by organic radicals. Such compounds are called amines. When amines react with nitrite ions, nitrosamines are formed - carcinogenic substances:
They act on the liver, contribute to the formation of tumors in the lungs and kidneys. Interestingly, ascorbic acid, which has long been familiar to us, is an active inhibitor of the formation of nitrosamines.
O.R. VALEDINSKAYA
(MGU, Moscow)
Nitrogen forms a series of oxides with oxygen; they can all be obtained from nitric acid or its salts.
Nitric oxide(I), or nitrous oxide, N 2 O is obtained by heating ammonium nitrate:
Nitric oxide (1) is a colorless gas with a slight odor and a sweetish taste. It is sparingly soluble in water: one volume of water at 20 ° C dissolves 0.63 volumes of N 2 O.
Nitric oxide (I) is a thermodynamically unstable compound. The standard Gibbs energy of its formation is positive (DS°b p =
104 kJ/mol). However, due to the high strength of bonds in the N 2 O molecule, the activation energies of reactions occurring with the participation of this substance are high. In particular, the activation energy of N 2 O decomposition is high. Therefore, nitric oxide (I) is stable at room temperature. However, at elevated temperatures, it decomposes into nitrogen and oxygen; decomposition proceeds faster, the higher the temperature.
Nitric oxide (1) does not react with water, or with acids, or with alkali.
The electronic structure of the N 2 O molecule is discussed in § 41.
Inhalation of small amounts of nitric oxide (I) leads to a dulling of pain sensitivity, as a result of which this gas is sometimes used in a mixture with oxygen for anesthesia. Large amounts of nitric oxide (I) have an exciting effect on the nervous system; that is why it used to be called "laughing gas".
Nitric oxide(ii), or nitric oxide, NO is a colorless gas that is difficult to liquefy. Liquid nitric oxide (II) boils at -151.7°C and solidifies at -163.7°C. It is slightly soluble in water: 1 volume of water dissolves only 0.07 volumes of NO at 0°C.
According to its chemical properties, nitric oxide (II) is one of the indifferent oxides, since it does not form any acid.
Like N 2 O, nitric oxide (II) is thermodynamically unstable - the standard Gibbs energy of its formation is positive (AGo 6p = 86.6 kJ/mol). But, again, like N 2 O, NO does not decompose at room temperature, because its molecules are strong enough. Only at temperatures above 1000 0 C does its decomposition into nitrogen and oxygen begin to proceed at a noticeable rate. At very high temperatures, for the reasons discussed in § 65, the decomposition of NO does not go to the end - an equilibrium is established in the NO-N 2 -O 2 system. Thanks to this, nitric oxide (II) can be obtained from simple substances at electric arc temperatures (3000-4000 ° C).
In the laboratory, nitric oxide (II) is usually obtained by reacting 30-35% nitric acid with copper:
In industry, it is an intermediate in the production of nitric acid (see § 143).
Nitric oxide (II) is characterized by redox duality. Under the action of strong oxidizing agents, it is oxidized, and in the presence of strong reducing agents, it is reduced. For example, it is easily oxidized by atmospheric oxygen to nitrogen dioxide: 
At the same time, a mixture of equal volumes of NO and H 2 explodes when heated:
The electronic structure of the NO molecule is best described by the MO method. On fig. 116 shows the filling scheme of MO in the NO molecule (ep. with similar schemes for N 2 and CO molecules - see Figs. 51 and 53). The NO molecule has one more electron than the N 2 and CO molecules: this electron is in the loosening orbital l res 2 R. Thus, the number of bonding electrons here exceeds the number of loosening ones by five. This corresponds to a bond multiplicity of 2.5 (5:2 = 2.5). Indeed, the dissociation energy of the NO molecule into atoms (632 kJ/mol) has an intermediate value compared to the corresponding values for the O 2 molecule (498 kJ/mol), in which the bond multiplicity is two, and the N 2 molecule (945 kJ/mol) , where the bond is triple. At the same time, in terms of dissociation energy, the NO molecule is close to the molecular oxygen ion O 2 (644 kJ/mol), in which the bond multiplicity is also equal to 2.5.
When one electron is detached from the NO molecule, an NO + ion is formed, which does not contain loosening electrons; the multiplicity of bonds between atoms increases in this case to three (six bonding electrons). Therefore, the dissociation energy of the NO + ion (1050 kJ/mol) is higher than the dissociation energy of the NO molecule and is close to the corresponding value for the CO molecule (1076 kJ/mol), in which the bond multiplicity is three.
Rice. 116.
Dioxide(or nitrogen dioxide NO 2 is a brown poisonous gas with a characteristic odor. It easily thickens into a reddish liquid (bp 21 0 C), which gradually brightens when cooled and freezes at -11.2 °C, forming a colorless crystalline mass. When gaseous nitrogen dioxide is heated, its color, on the contrary, intensifies, and at 140 ° C it becomes almost black. The change in the color of nitrogen dioxide with increasing temperature is also accompanied by a change in its molecular weight. At low temperature, the vapor density approximately corresponds to twice the formula N 2 O 4 . With an increase in temperature, the vapor density decreases and at 140 ° C it corresponds to the formula NO 2. Colorless crystals, existing at -11.2 0 C and below, consist of N 2 O 4 molecules. As the N 2 O 4 molecules are heated, they dissociate to form molecules of dark brown nitrogen dioxide; complete dissociation occurs at 140 0 C. Thus, at temperatures from -11.2 to 140 ° C, NO 2 and N 2 O 4 molecules are in equilibrium with each other:
Above 140 °C, the dissociation of NO 2 into NO and oxygen begins.
Nitrogen dioxide is a very energetic oxidizing agent. Many substances can burn in an atmosphere of NO 2, taking away oxygen from it. Sulfur dioxide is oxidized by it into trioxide, on which the nitrous method for obtaining sulfuric acid is based (see § 131).
Vapors of NO 2 are poisonous. Inhalation causes severe irritation of the respiratory tract and can lead to serious poisoning.
When dissolved in water, NO 2 reacts with water, forming nitric and nitrous acids:
But nitrous acid is very unstable and quickly decomposes:
Therefore, in practice, the interaction of nitrogen dioxide with water, especially hot water, proceeds according to the equation
which can be obtained by adding the two previous equations, if you first multiply the first of them by three.
In the presence of air, the resulting nitric oxide is immediately oxidized to nitrogen dioxide, so that in this case NO 2 is eventually completely converted to nitric acid:
This reaction is used in modern ways obtaining nitric acid.
If nitrogen dioxide is dissolved in alkalis, then a mixture of salts of nitric and nitrous acids is formed, for example:
Nitric oxide(III), or nitrous anhydride, N 2 O 3 is a dark blue liquid that decomposes into NO and NO 2 even at low temperatures. A mixture of equal volumes of NO and NO 2 upon cooling again forms N 2 O 3:
Nitric oxide (III) corresponds to nitrous acid HNO 2 .
Nitric oxide(V), or nitric anhydride, N 2 O 5 - white crystals, already at room temperature, gradually decomposing into NO 2 and O 2. It can be obtained by the action of phosphoric anhydride on nitric acid:
Nitric oxide (V) is a very strong oxidizing agent. Many organic substances ignite on contact with it. In water, nitric oxide (V) dissolves well with the formation of nitric acid.
In the solid state, N 2 O 5 is formed by the nitrate ion NO 3 and the ion
nitronium NO2. The latter contains the same number of electrons as the mo-
molecule CO 2 and, like the latter, has a linear structure: O=N=O.
In vapors, the N 2 O 5 molecule is symmetrical; its structure can be represented by the following valence diagram, in which three-center bonds are shown by dotted lines (compare with the valence diagram of the nitric acid molecule).
Due to the fact that nitrogen exhibits different valences in its compounds, several oxides are characteristic of this element: dinitrogen oxide, mono-, tri-, di- and pentoxides of nitrogen. Let's consider each of them in more detail.
DEFINITION
dinitrogen oxide(laughing gas, nitrous oxide) is a colorless gas, thermally stable.
Poorly soluble in water. With strong cooling, the N 2 O × 5.75H 2 O clarate crystallizes from the solution.
DEFINITION
nitrogen monoxide It can exist both as a colorless gas and as a blue liquid.
In the solid state, it is completely dimerized (N 2 O 2), in the liquid state - partially (≈ 25% N 2 O 2), in the gas - to a very small extent. Extremely thermally stable. Poorly soluble in water.
DEFINITION
nitrogen trioxide is a thermally unstable blue liquid.
At room temperature, it decomposes by 90% into NO and NO 2 and turns brown (NO 2), does not have a boiling point (NO evaporates first). In the solid state, it is a white or bluish substance with an ionic structure - nitrosyl nitrite (NO +) (NO 2 -). In gas, it has the molecular structure ON-NO 2 .
DEFINITION
nitrogen dioxide(fox tail) is a brown gas.
At temperatures above 135 o With it is a monomer, at room temperature - a red-brown mixture of NO 2 and its dimer (nitrogen tetroxide) N 2 O 4 . The dimer is colorless in the liquid state and white in the solid state. It dissolves well in cold water (saturated solution - bright green), completely reacting with it.
DEFINITION
Nitrogen Pentoxide (Nitric Anhydride) is a white solid, colorless gas and liquid.
When heated, it sublimates and melts; at room temperature, it decomposes in 10 hours. In the solid state, it has an ionic structure (NO 2 +) (NO 3 -) - nitroyl nitrate.
Table 1. Physical properties nitrogen oxides.
Obtaining nitric oxide
Under laboratory conditions, dinitrogen oxide is obtained by gently heating dry ammonium nitrate (1) or by heating a mixture of sulfamic and nitric (73%) acids (2):
NH 4 NO 3 \u003d N 2 O + 2H 2 O (1);
NH 2 SO 2 OH + HNO 3 \u003d N 2 O + H 2 SO 4 + H 2 O (2).
Nitrogen monoxide is obtained by the interaction of simple substances nitrogen and oxygen at high temperatures (≈1300 o C):
N 2 + O 2 \u003d 2NO.
In addition, nitric oxide (II) is one of the products of the reaction of dissolving copper in dilute nitric acid:
3Cu + 8HNO 3 \u003d 3Cu (NO 3) 2 + 2NO + 4H 2 O.
When cooling a mixture of gases consisting of nitrogen oxides (II) and (IV) to -36 o C, nitrogen trioxide is formed:
NO + NO 2 \u003d N 2 O 3.
This compound can be obtained by the action of 50% nitric acid on arsenic (III) oxide (3) or starch (4):
2HNO 3 + As 2 O 3 = NO 2 + NO + 2HAsO 3 (3);
HNO 3 + (C 6 H 10 O 5) n = 6nNO + 6nNO 2 + 6nCO 2 + 11nH 2 O (4).
Thermal decomposition of lead (II) nitrate leads to the formation of nitrogen dioxide:
2Pb (NO 3) 2 \u003d 2PbO + 4NO 2 + O 2.
The same compound is formed when copper is dissolved in concentrated nitric acid:
Cu + 4HNO 3 \u003d Cu (NO 3) 2 + 2NO 2 + 2H 2 O.
Nitrogen pentoxide is obtained by passing dry chlorine over dry silver nitrate (5), as well as by the reaction of interaction between nitrogen oxide (IV) and ozone (6):
2Cl 2 + 4AgNO 3 = 2N 2 O 5 + 4AgCl + O 2 (5);
2NO 2 + O 3 = N 2 O 5 + O 2 (6).
Chemical properties of nitric oxide
Dianitrogen oxide is low reactive, does not react with dilute acids, alkalis, ammonia hydrate, oxygen. When heated, it reacts with concentrated sulfuric acid, hydrogen, metals, ammonia. Supports combustion of carbon and phosphorus. In OVR, it can exhibit the properties of both a weak oxidizing agent and a weak reducing agent.
Nitrogen monoxide does not react with water, dilute acids, alkalis, ammonia hydrate. Instantly adds oxygen. When heated, it reacts with halogens and other non-metals, strong oxidizing and reducing agents. Enters into complexation reactions.
Nitrogen trioxide exhibits acidic properties, reacts with water, alkalis, ammonia hydrate. Vigorously reacts with oxygen and ozone, oxidizes metals.
Nitrogen dioxide reacts with water and alkalis. In OVR, it exhibits the properties of a strong oxidizing agent. Causes corrosion of metals.
Nitrogen pentoxide exhibits acidic properties, reacts with water, alkalis, ammonia hydrate. It is a very strong oxidizing agent.
Application of nitric oxide
Dianitrogen oxide is used in the food industry (propellant in the manufacture of whipped cream), medicine (for inhalation anesthesia), and also as the main component of rocket fuel.
Nitrogen trioxide and dioxide are used in inorganic synthesis to produce nitric and sulfuric acids. Nitric oxide (IV) has also found use as one of the components of rocket fuel and mixed explosives.
Examples of problem solving
EXAMPLE 1
| The task | Nitric oxide contains 63.2% oxygen. What is the formula for oxide. |
| Solution | The mass fraction of the element X in the molecule of the HX composition is calculated by the following formula: ω (X) = n × Ar (X) / M (HX) × 100%. Let us calculate the mass fraction of nitrogen in the oxide: ω (N) \u003d 100% - ω (O) \u003d 100% - 63.2% \u003d 36.8%. Let us denote the number of moles of elements that make up the compound as "x" (nitrogen) and "y" (oxygen). Then, the molar ratio will look like this (the values of relative atomic masses taken from the Periodic Table of D.I. Mendeleev will be rounded to whole numbers): x:y = ω(N)/Ar(N) : ω(O)/Ar(O); x:y= 36.8/14: 63.2/16; x:y= 2.6: 3.95 = 1: 2. So the formula for the compound of nitrogen and oxygen will be NO 2. This is nitric oxide (IV). |
| Answer | NO 2 |
EXAMPLE 2
| The task | Which gases are heavier and which are lighter than air and how many times: carbon dioxide, nitrogen dioxide, carbon monoxide, chlorine, ammonia? |
| Solution | The ratio of the mass of a given gas to the mass of another gas taken in the same volume, at the same temperature and the same pressure, is called the relative density of the first gas over the second. This value shows how many times the first gas is heavier or lighter than the second gas. The relative molecular weight of air is taken equal to 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of "relative molecular weight of air" is used conditionally, since air is a mixture of gases. D air (CO 2) \u003d M r (CO 2) / M r (air); D air (CO 2) \u003d 44 / 29 \u003d 1.52. M r (CO 2) \u003d A r (C) + 2 × A r (O) \u003d 12 + 2 × 16 \u003d 12 + 32 \u003d 44. D air (NO 2) \u003d M r (NO 2) / M r (air); D air (NO 2) = 46/29 = 1.59. M r (NO 2) \u003d A r (N) + 2 × A r (O) \u003d 14 + 2 × 16 \u003d 14 + 32 \u003d 46. D air (CO) = M r (CO) / M r (air); D air (CO) \u003d 28 / 29 \u003d 0.97. M r (CO) = A r (C) + A r (O) = 12 + 16 = 28. D air (Cl 2) \u003d M r (Cl 2) / M r (air); D air (Cl 2) = 71/29 = 2.45. M r (Cl 2) = 2 × A r (Cl) = 2 × 35.5 = 71. D air (NH 3) \u003d M r (NH 3) / M r (air); D air (NH 3) \u003d 17/29 \u003d 0.57. M r (NH 3) \u003d A r (N) + 3 × A r (H) \u003d 14 + 3 × 1 \u003d 17. |
| Answer | Carbon dioxide, nitrogen dioxide and chlorine are heavier than air, respectively, by 1.52; 1.59 and 2.45 times, and carbon monoxide and ammonia are 0.97 and 0.57 times lighter. |
