Fernandez , Maria F. Sesamol attenuates oxidative stress—mediated experimental acute pancreatitis in rats. Dramatic increase of nitrite levels in hearts of anoxia-exposed crucian carp supporting a role in cardioprotection. The role of nitrogen oxides in human adaptation to hypoxia.
A novel parthenin analog exhibits anti-cancer activity: Activation of apoptotic signaling events through robust NO formation in human leukemia HL cells. Nitrite regulation of shock. Reactive oxygen species and small-conductance calcium-dependent potassium channels are key mediators of inflammation-induced hypotension and shock.
High-salt intake enhances superoxide activity in eNOS knockout mice leading to the development of salt sensitivity. Differential regulation of inducible and endothelial nitric oxide synthase by kinin B1 and B2 receptors. Antisickling property of fetal hemoglobin enhances nitric oxide bioavailability and ameliorates organ oxidative stress in transgenic-knockout sickle mice. Cocaine-induced status epilepticus and death generate oxidative stress in prefrontal cortex and striatum of mice. Determinants of Nitric Oxide Chemistry.
Antidiarrhoeal and intestinal modulatory activities of Wei-Chang-An-Wan extract. Beneficial effects of rutin and l -arginine coadministration in a rat model of liver ischemia-reperfusion injury. Nitric oxide synthase gene polymorphisms and prostate cancer risk. Therapeutic Antioxidant Medical Gas. Role of endothelial nitric oxide synthase-derived nitric oxide in activation and dysfunction of cerebrovascular endothelial cells during early onsets of sepsis.
Tempol diminishes cocaine-induced oxidative damage and attenuates the development and expression of behavioral sensitization. The chemical biology of nitric oxide: Implications in cellular signaling. The chemical biology of nitric oxide — an outsider's reflections about its role in osteoarthritis. Modulation of the myocardial redox state by vagal nerve stimulation after experimental myocardial infarction. Superoxide and its interaction with nitric oxide modulates renal function in prehypertensive Ren-2 transgenic rats.
Cole , Jitbanjong Tangpong , Daret K. Clair , and Terry D. Neuronal nitric oxide synthase protects neuroblastoma cells from oxidative stress mediated by garlic derivatives. Oxidative stress, chronic disease, and muscle wasting. Role of nitric oxide in neonatology. Localization of putative nitrergic neurons in peripheral chemosensory areas and the central nervous system ofAplysia californica. Nitric Oxide Synthase Inhibition in Sepsis? Lessons Learned from Large-Animal Studies. The increase in serum uric acid concentration caused by diuretics might be beneficial in heart failure.
The chemistry of nitroxyl HNO and implications in biology. Nitric Oxide and Axonal Pathophysiology. Gender differences in hepatic ischemic reperfusion injury in rats are associated with endothelial cell nitric oxide synthase-derived nitric oxide. Neuronal nitric oxide synthase negatively regulates xanthine oxidoreductase inhibition of cardiac excitation-contraction coupling.
Carboxypeptidase-mediated enhancement of nitric oxide production in rat lungs and microvascular endothelial cells. Cellular targets and mechanisms of nitros yl ation: An insight into their nature and kinetics in vivo. Immune cells: free radicals and antioxidants in sepsis. The chemistry of nitrosative stress induced by nitric oxide and reactive nitrogen oxide species.
Putting perspective on stressful biological situations. Blood-brain barrier disruption in multiple sclerosis. Orthogonal properties of the redox siblings nitroxyl and nitric oxide in the cardiovascular system: a novel redox paradigm. Nitroarginine methyl ester and canavanine lower intracellular reduced glutathione. Douglas D. Thomas , Katrina M. Miranda , Carol A. Ischaemia-reperfusion injury in photodynamic therapy-treated mouse tumours. Sexual dimorphism in reduced-size liver ischemia and reperfusion injury in mice: Role of endothelial cell nitric oxide synthase. Pharmaco-redox regulation of cytokine-related pathways: from receptor signaling to pharmacogenomics.
Focusing of nitric oxide mediated nitrosation and oxidative nitrosylation as a consequence of reaction with superoxide. Superoxide activates constitutive nitric oxide synthase in a brain particulate fraction. Nitric oxide synthase and hypertension. Volume 3 Issue 2 Apr Change Language. English Arabic. Important Links. Follow Us. App Download. US UK. Thank you for subscribing! Please check your email to confirm your subscription. Our Stores. Apply Filter Remove Filter Categories.
All the latest offers delivered right to your inbox! We Accept. Shipping Methods business days Minimum 10 business days. Other NONOates decompose much more slowly and so these compounds provide an admirable source of NO with controlled delivery. Oxadiazoles such as 5. Is there any evidence for the treatment of angina involving an NO-donor drug before the advent of modern medicine? In an amazing collection of manuscripts were found in a walled-up cave at a Buddhist shrine called Dunhuang, in central Asia, by a British explorer, Sir Aurel Stein. The Dunhuang find includes 19 medical manuscripts and these give insight into medical practice in China around AD These manuscripts have been subjected to the most rigorous scholarly scrutiny and have been translated.
One describes what is obviously a treatment for the pain associated with angina Figure 5. The patient is instructed to place nitre potassium nitrate under the tongue and leave it there while carefully conserving the saliva.
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This guarantees a cure. Potassium nitrate was well known to the Chinese because it is a component of gunpowder, but it is almost entirely without biological action. However, under the tongue are colonies of bacteria, some of which contain the enzyme nitrate reductase see Chapter 18 , which converts nitrate into nitrite.
Tissue starved of oxygen ischemic tissue , which is what happens in angina, is much more acidic than healthy tissue and so if nitrite is taken into the Drugs That Release N O 37 Figure 5. It may be that the therapeutic use of NO in China predates its use in the West by many centuries. But what of the future of NO-donor drugs? It is a natural area of research for many pharmaceutical companies.
Compounds that release NO are easy to find; compounds that do so within the relevant tissue and nowhere else are far harder to produce, but the search goes on. One interesting idea that has reached an advanced stage is a form of aspirin that also releases NO. When the blood vessels supplying the lungs have become constricted, the idea of giving inhaled NO to dilate them, thus enhancing the blood supply to the lungs, is an obvious one.
Inhaled NO has been used in intensive care units since However, the early work used substandard gas produced for industrial purposes, together with poor delivery systems. Inhalation of NO was, in general, a treatment of last resort, making medical outcomes difficult to evaluate. There are considerable dangers in the medical use of inhaled NO. On contact with oxygen 38 Chapter 5 it is, of course, readily converted into toxic NO,. NO2 also forms by the disproportionation of NO on storage see Chapter 8. All NO, must be removed before NO can be used clinically, but this is difficult to ensure. In recent years the use of inhaled NO for enhancing the blood supply to the lungs in adults has declined as several surveys have indicated outcomes not commensurate with the risks and difficulties involved.
The sudden withdrawal of inhaled NO can also have an adverse effect on some patients. Using inhaled NO for the treatment of newborn infants with respiratory failure has proved more successful, according to carefully controlled trials. It is generally given within 14 days of birth and reduces the need for the extreme treatment for this condition such as reoxygenation of the blood outside the body. The latter is a technique that is costly and difficult to use.
In inhaled NO was registered as a drug in the European Union for the treatment of newborn infants. Why inhaled NO works better with infants than with adults is not clear. The treatment of premature babies with NO has not proved successful, possibly because the lungs are not fully developed. Feelisch and J. Stamler eds , Wiley, Chichester, , Dagli ed , Torino, , Murrell, Nitro-glycerine as a remedy for angina pectoris. Lancet, , 18 Jan, Chen, J. Zhang and J. Stamler, Identification of the enzymatic mechanism of nitroglycerine bioactivation. USA, , 99, Butler and C.
Glidewell, Recent chemical studies of sodium nitroprusside relevant to its hypotensive action. Sorba, C. Medana, R. Fruttero, C. Cena, A. De Stilo, U. Galli and A. Gasco, Water soluble furoxan derivatives as NO prodrugs. Maragos, D. Morley, D. Wink, T. Dunams, J. Saavedra, A. Hoffman, A. Bove, L. Isaac, J. Hrabie and L. Vasorelaxant effects. Clark, T. Kueser, M. Walker, W. Southgait, J. Huckaby, J. Perez, B. Roy, M. Kessler and J. Kinsella, Low-dose nitric oxide therapy for persistent pulmonary hypertension of the newborn. New Eng. Butler and J. Lo and C. Cullen eds , Rutledge Curzon, in press.
Chapter 6 Discovering and Making NO It is generally stated that NO was discovered by the English natural philosopher a gracious but outdated term for a scientist and Unitarian minister Joseph Priestley , but this is not correct.
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As NO is readily made by the reaction of nitric acid HNOJ with any one of a number of commonly available metals see later , the discovery of NO is not very surprising once you have nitric acid. In fact, nitric acid was known in the thirteenth century and called aqua fortis, and a modern way of making it by the action of sulphuric acid on potassium nitrate was developed by Johann Glauber There is a curious coincidence here for Stephen Hales was also the first person to measure blood pressure, not in humans but in horses.
In his book Statical Essays: Containing Hemastaticks of he described an experiment in which he inserted a nine-foot glass tube into the artery of a horse and noted the height to which the blood rose. The experimental work is described lucidly in his book of , Experiments and Observations on Different Kinds of Airs Figure 6. In spite of some otherwise excellent experimental science, Priestley was one of the last natural philosophers to hold on to the theory of phlogiston, even after his move to the USA because of hostility in Birmingham to his support Chapter 6 42 of the French Revolution.
Even so, three of his observations are worth noting. NO when stored for some time in the presence of iron and moisture diminished in volume. Also, iron nitrosyls may have a biological role when NO acts as a cytotoxic agent see Chapter Although his observations are impeccably reported, Priestley had no way of knowing what elements made up nitric oxide.
It was the highly regarded, but also highly eccentric, scientist Henry Cavendish who showed that nitric oxide contained nitrogen and oxygen, and Sir Humphry Davy who proved the diatomic nature of the compound, one atom of nitrogen linked to one of oxygen. Priestley was able to characterize NO because he had devised ways of collecting gases in gas jars by the displacement of water or, for watersoluble gases, the displacement of mercury. However, this did not allow for the ready storage of gases because gas jars leak. The problem of the storage of NO was solved in an unexpected way by a French army pharmacist called Frangois-Zacherie Roussin Figure 6.
He lived at a time of great social upheaval in France and spent some time in prison as he was thought to be a counter-revolutionary. In his personal laboratory he studied the chemical processes involved in the production of artificial dyestuffs. Azo dyes, which had recently been discovered, are made from the products of the action of nitrous acid HNOJ on amines, a process known as diazotization. Roussin studied the action of nitrous acid on other substances and, probably by chance, obtained an intensely black, crystalline, ionic material by mixing iron I1 sulphate, ammonium sulphide and nitrous acid.
The anion was known to have the formula [Fe,S, NO ,]- but the exact structure was a mystery. Roussin noted two interesting chemical properties of the black salt. Although ionic, it is more soluble in organic solvents than in water. Also, addition of a copper salt, acting as an oxidizing agent, to a solution of the black salt results in the evolution of very pure NO.
In the laboratory, this provides a convenient way of producing the gas when required, whereas today we would use gas from a cylinder. A comprehensive survey of the incidence of different types of cancer in China during the s showed a very high incidence of oesophageal cancer in Linxian.
A team of Chinese scientists, sent there to investigate, decided it must be a matter of diet. It had never before been detected in a natural source. Some compounds containing an NO group such as secondary nitrosamines RHN-NO are known cancer-causing agents see Chapter 12 and so the red ester was immediately suspected as the cause of oesophageal cancer.
Its cancer-causing properties were examined and found to be slight. On the other hand NO itself does have a role in the formation of cancerous tissue and this is described further in Chapter In the years since its discovery chemists investigated the properties of NO but, apart from its role as a ligand described in Chapter 9, little of general interest was discovered. And then in January the modern NO story broke and it must now be one of the most investigated molecules of all time. That date is the start of a new era for many scientific disciplines.
Discovering and Making N O 45 Much has been written in this book about the way living systems make NO but it is also produced industrially and in laboratories. The use of NO from cylinders is not recommended for, as mentioned above, the gas is thermodynamically unstable and on storage becomes contaminated with N,O and NO2. It is produced industrially only on a small scale by the catalytic oxidation of ammonia but occurs on a large scale as an intermediate in the manufacture of nitric acid from ammonia, a process known as the Ostwald process Scheme 6.
Making pure NO in the laboratory is not easy. The reaction of metal with nitric acid appears to be simple and this is how the gas was discovered, but the NO obtained is rather impure. However, some of the impurities are not difficult to remove. The NO formed may be collected over water and this removes any NO, present. Care is needed in the choice of nitric acid when NO is prepared in this manner. As mentioned previously, textbooks of inorganic chemistry are careful to emphasise the word dilute or, more pedantically, moderately dilute when describing the nitric acid.
This is because, if concentrated nitric acid is used, NO2 is evolved instead of, or as well as, NO. If very concentrated nitric acid is used there is no reaction at all. Even when the nitric acid is at the right concentration to give NO, the presence of nitrous acid is required as a catalyst. Nitrous acid is usually produced spontaneously in nitric acid on storage.
When more dilute nitric acid reacts with copper turnings, some N,O is also formed Scheme 6. This is surprising for as poor a reducing agent as copper but is not so surprising in the reaction of zinc with dilute nitric acid. Handling NO in the laboratory needs to be carried out with care as contact with the atmosphere immediately gives NO,. For the preparation of very pure NO many methods have been proposed over the years and a brief review is given here.
More details are to be found in the Further Reading list. An intriguing method published by James Ogg and Richard Ray in used previously constructed pellets which, when heated, evolve Johnston and W. Giauque published an early method of producing extremely pure NO. Probably the most convenient laboratory preparation of pure NO is the reaction between ascorbic acid and nitrite. A reasonable mechanism is 0-nitrozation to give an ester that, by homolytic fission, gives NO 48 Chapter 6 and a semiquinone radical.
The latter is rapidly oxidized by another molecule of nitrous acid Scheme 6.
Ogg and J. Ray, The quantitative oxidation of gaseous ammonia to nitrate. Giauque, The heat capacity of nitric oxide from 14K to the boiling point and the heat of vaporization. Vapour pressure of solid and liquid phases. The entropy from spectroscopic data. Bunton, H. Dahn and L. Loewe, Oxidation of ascorbic acid and similar reductones by nitrous acid. Chapter 7 Making Smog - NO Becomes a Villain Photochemical smog, for which Los Angeles became famous in the s, forms when traces of volatile organic compounds are released into a warm atmosphere containing NO.
A complex series of chemical reactions occurs and the energy needed for these reactions is provided by sunlight. The strong Californian sun and the abundance of cars in the Los Angeles area provide conditions ideally suited to the generation of this type of smog. However, smog is not restricted to Los Angeles and is common in cities throughout the world Figure 7. Figure 7. In contrast the reactions leading to a photochemical smog take place, largely, in the gaseous phase and rarely involve charged species.
Instead the reactions involve radicals. A radical is a chemical species with an unpaired electron, indicated in formulae by means of an inconspicuous but vitally important superscript dot. A chemical radical is not outlandish in any way, but most radicals are highly reactive and one thing they often do is to react with another radical to give a stable molecule. For reasons we have tried to explain elsewhere NO is far less reactive than many other radicals. Only tiny amounts of NO form naturally in the atmosphere. The majority of it is produced in one of two ways: thermally or from fuel.
Thermal NO is formed by the reaction of nitrogen with oxygen. This reaction can be written simply as: but the truth, as is often the case, is not that simple.
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However, normally only small quantities of NOx are formed in this way. Petrol is a fairly complex mixture of substances and there are traces of nitrogen-containing compounds which, when petrol is burnt, form NO. This is known as fuel NO. The amount formed depends upon the conditions of combustion and is at a maximum under fuel-lean conditions.
The mixtures produced by the pollution source are thus termed thermal NOx and fuel NOx. In the UK, road vehicle emissions account for over half of the total NOx emissions but the NO, in these tailpipe emissions form only a small fraction of the total emitted NOx. Most of the NO, forms after the NOx has been released into the atmosphere.
More NO, could then form via continued reaction of NO with oxygen, but in practice this reaction is insignificant unless the concentration of NO rises beyond ppm a condition difficult to achieve even in the Los Angeles rush hour. The next stage in the production of photochemical smog appears somewhat regressive and almost perverse. NO2 can absorb high-energy sunlight and is broken apart to give NO and oxygen, but not ordinary molecular oxygen. One of the reasons that ozone is the culprit here, rather than NO,, is that ozone persists in the atmosphere far longer than N0,does.
The lifetime of ozone with respect to photolysis at midday levels in Europe is times that of NO,. Typical NO, photolysis rates at s-I. This midday are 6 x s-I, whereas those for ozone are 1 x equates to average lifetimes of 3 min for NO2 and 24 h for ozone. In 52 Chapter 7 bright sunlight, ozone splits into an oxygen molecule and an oxygen atom, but an oxygen atom in an especially excited state, known as 'singlet oxygen atom'. This is an oxygen atom in which there are no unpaired electrons, in contrast to the lowest energy state where two electrons are unpaired. It should not be confused with the term 'singlet oxygen', which is normally used to describe an oxygen molecule OJ, in which the two normally unpaired electrons are paired or paired with anti-parallel spins.
Most radicals react with oxygen but the hydroxyl radical does not and this enhances its lifetime in an oxygen-rich environment. This mechanism is very important in smog formation, because both alkyl and acyl radicals convert NO to NO, without consuming a molecule of ozone. However, this radically formed NO, may then be photolysed and begin the cycle again as NO. Each time this happens, another molecule of ozone is generated. This is how ozone is formed photochemically and builds up in a photochemical smog. The radicals listed in equations above all enter further reactions with the production of more radicals by radical propagation processes.
Of particular importance is the peroxyacetyl radical CH,C O '. It is generated from many different hydrocarbons, aldehydes and ketones, and is important because it reacts with NO, to give peroxyacetyl nitrate, CH3C 0 O2NO2 PAN , another major villain in atmospheric pollution. Although it can form naturally in small amounts, PAN was first detected in photochemical smog and causes eye and respiratory irritation, is damaging to proteins and more toxic to plants than ozone.
When NO, forms at any stage in the complex series of reactions occurring in a photochemical smog, it can react with HO' to give nitric acid, which dissolves in water droplets and contributes to acid rain. Also some of the products of reaction involving hydrocarbons are themselves irritants e. With increasingly sensitive monitoring equipment, undoubtedly more complexities in the chemistry of atmospheric pollution will be discovered.
Photochemical smog is characterized by a daily cycle due to the time taken for the reactions to proceed. The concentration of NOx is at its peak during the morning rush hour. As the sun gets stronger towards the middle of the day, the photochemical reactions increase in number and maximum ozone concentrations are reached about 5 h after the morning rush hour. The evening rush hour also produces a peak in NOx concentration, but in the absence of sunlight to initiate the photochemical reactions, far less smog is produced.
Burning of hydrocarbon fuel will not significantly decrease in the near future, but there are ways to reduce NOx emissions. There are six strategies: 1. Reduce the combustion temperature. Any tactic that wil reduce the temperature at the hottest part of the flame will reduce the amount of oxygen dissociated and thus the amount of thermal NOx released.
Measures include injection of steam, -e-injection of oxygen-depleted flue gases, fuel-rich conditions and fuel-lean conditions. Reduce the amount of time spent at peak temperature. The flame temperature is allowed to reach a maximum, but then air, fuel or steam is injected into the burner to reduce the temperature again very quickly, so that very little oxygen dissociates.
Remove the nitrogen, either by using ultra-low nitrogen fuel or by burning the fuel in oxygen instead of air. Burning the fuel in oxygen produces an intense flame that must be diluted and dilution with air generates some thermal NOx. Injection of the effluent gas with sorbents such as carbon, aluminium oxide, limestone or ammonia can remove pollutants such NOx and sulphur. The pollutants either absorb or adsorb on to the sorbent and can later be filtered out.
This is not possible for motor vehicles but can be used for large, stationary NOx sources. Chemical reduction of NOx to N2. Urea or ammonia is injected into the system to reduce the NOx on a catalyst surface. Currently this approach is not used for vehicles, only stationary combustion sources, although a similar principle is used in automobile threeway catalytic converters see Chapter In these catalytic systems, oxygen and fuel ratios are very carefully controlled so that unburned fuel acts as the reducing agent. Oxidation of NOx. Nitrogen oxides are most soluble at higher valencies, i.
NO, is more soluble than NO2. This technique uses injected ozone, hydrogen peroxide or a catalyst to oxidize the NOx before it is dissolved in water. The nitric acid can be collected or neutralized to a calcium or ammonium salt and then sold. The presence of man-made NO in the atmosphere is harmful. The small amount produced naturally from lightning, most of which ends Making Smog - NO Becomes a Villain 55 up as nitric acid, is an important part of the natural nitrogen cycle see Chapter IS , but the advent of the motor car and the growth of cities has created an enormous problem.
Legislation and technological innovation have gone some way to solving the problem in Los Angeles but the problem is growing in other cities of the world. NO may be a benign messenger necessary for cardiovascular health but in the wrong place, like the air above an industrialized conurbation, it can be the harbinger of death. Although the atmosphere inside city centre buildings is less polluted than that outside, the presence of pollutants indoors can still be a problem. This is particularly true for museums and art galleries. Even then the Trustees expressed concern about the possible effect of pollution on the paintings.
In the distinguished scientist Michael Faraday gave evidence to a Select Committee on the problem and recommended that the gallery should be moved westwards, where the prevailing winds would blow the pollution away. The legislation of the s on burning coal in open fires brought about dramatic improvements but pollution within the Gallery, and in all city buildings holding valuable artefacts, is still a major problem. Sophisticated monitoring equipment shows just how persistent that problem is and our enhanced sensitivity to preserving the past authentically has given the solution of the problem some urgency.
Sulphur dioxide and ozone are probably the most serious pollutants as far as museums and galleries are concerned. The dangers associated with high NOx levels are more difficult to assess but, nevertheless, are real. Most of the NOx within buildings comes from the same source as that outside: the burning of fossil fuels. However, there is an added source of NOx in certain museums and that is the degradation of nitrocellulose.
Where old nitrocellulose cinematographic film is stored this can give rise to high levels of NOx in the storage containers. The NOx released causes enhanced destruction of the film. Conservators also use nitrocellulose as a coating for bookbinding cloth and as an adhesive. As far as NOx in the general atmosphere is concerned the acceptable level is set at pg m The low level of lighting in most galleries and museums probably means that photochemical reactions of NOx are probably less important than those occurring over cities.
However, the presence of NOx is not without detrimental effects. It appears that NO alone causes little damage, but its ready conversion into NO, is a source of danger to artefacts. Hydrolysis of NO, produces nitric acid see Chapter S , which can have a damaging effect on calcareous stone, metal objects and textiles. NO, is also an oxidizing agent and can attack 56 Chapter 7 polymers, increasing brittleness. As an oxidant it can react with organic pigments and dyes, affecting significant colour change. NOx probably contains some N,O, which, as explained in Chapter 4, is a nitrosating agent.
It could nitrosate the amino group of organic dyes, giving a diazonium ion: p: This change from amino to hydroxyl group could have a considerable effect on the colour of a dye or pigment. NO, is said to have an effect on the arsenic pigments orpiment and realgar but it is difficult to see what the chemical process is. Locating museums and galleries in city centres is obviously good for the user but it is the location that causes pollution problems for the conservator. Pedestrianization of city centres is a surprisingly simple solution to the problem but it is not always popular.
Life,Death and Nitric Oxide
A more sophisticated solution is to filter the air entering the display rooms, but this process is costly and requires maintenance. If our treasures are to be preserved intact for future generations the problem of pollution has to be tackled, whatever the cost. Bailey, H. Clarke, J. Ferris, S. Krause and R. Saunders, Pollution and the National Gallery. National Gallery Technical Bulletin, , 21, Brimblecombe, The composition of museum atmosphere. Why is NO, although a radical, a reasonably stable molecule?
How can it react with both iron I1 and iron III? If NO is such a potent agent for reducing blood pressure, why is it not regularly given directly to patients? It is time to pause, and take a closer look at NO itself. The common name for the molecule, and the one used throughout this book, is nitric oxide, but IUPAC, the body struggling to make chemical nomenclature consistent, gives it the name nitrogen monoxide.
This name tells more about the molecule: mono indicates one, so it is made up of one nitrogen and one oxygen atom and has the chemical formula NO. NO is the second in a series of oxides of nitrogen in which the valency of the nitrogen increases, from the well-known N,O laughing gas to the unstable and therefore poorly characterized NO, or N,O,. The series is summarized in Table 8. Nitrogen may also access further valencies in other compounds, and for one non-metal atom to display so many ways of forming bonds with other atoms is extraordinary.
There are labile reaction pathways between all these states, hence the rich chemistry exhibited by nitrogen. An excellent example is the terrestrial nitrogen cycle, discussed in Chapter Three of the oxides of nitrogen are radicals, that is to say they contain an unpaired electron in their valence shell: NO, NO, and NO,.
NO has eleven valence electrons, five from the nitrogen atom and six from the oxygen. A ground state molecular orbital diagram is shown in Figure 8. There are three electrons in antibonding orbitals and eight in bonding orbitals - this means an excess of five bonding electrons i. Certain physical properties of NO reflect this. The bond length in NO is 1. Other physical properties of NO are listed in Table 8.
Back to that single electron in the antibonding n-orbital. Any chemical species with an unpaired electron is a radical. General discussions involving radicals are usually concerned with their reactions because the majority of radicals are so reactive that further reaction occurs rapidly once the free radical has formed. For this reason the reaction does not occur under normal circumstances at room temperature.
However, in the solid state, X-ray studies have shown that NO does exist as a dimer but the molecules are joined side-by-side rather than end-to-end 8. CO The small amount of dimer that persists in the gaseous phase has a longer N-N distance 2. This lack of dimerization is one example of the stability of NO compared with many common carbon-centred radicals. In carbon-centred radicals, such as the methyl radical, CH,', the unpaired electron is in a p-type orbital centred on the carbon atom 8.
It is possible to stabilize a carbon-centred radical by introducing substituents that allow interaction between the p-type singly occupied molecular orbital SOMO and the lowest unoccupied molecular orbital LUMO. The LUMO is delocalized over the central carbon atom and the surrounding atoms and the resulting delocalization of the single electron stabilizes the molecule. Delocalization of the single electron is an important factor in accounting for the lack of reactivity of NO. What happens to NO if you leave it alone?
The above discussion about the stability of NO might give the impression that a gas jar full of NO would, if left to stand, do nothing, but that is not the case. A reaction becomes possible if the energy of the products is lower than that of the reactants and the energy released as the reaction proceeds generally appears as heat. Occasionally students are advised to carry out a reaction with a supply of ice on hand to cool the reaction vessel if the temperature begins to rise out of control. This is thermodynamics in action and a simple example of First Law of Thermodynamics.
The Gibbs free energy function is a means of measuring the energy difference between products and reactants. Now examine the Gibbs free energy function for the decomposition or disproportionation reactions of NO shown in equations 2 to 5. In each case, the change in energy is negative, i. Why, then, is it possible to purchase a cylinder of NO? Does something prevent these reactions from taking place? Well, yes and no. What thermodynamics does not tell us is the size of the activation energy of any particular reaction and it is the activation energy that fixes how fast a reaction occurs.
For reactions the activation energies are high and the rates of reaction are so slow that the NO in a cylinder is still largely unchanged, even after some weeks of storage, but it will be contaminated with some N2, 02,N 2 0 and NO2. At high pressures the reactions are faster. The presence of contaminants is more than just a nuisance to chemists working with NO, because NO2 can initiate the explosive decomposition of hydrocarbons such as the waxes and greases found on vacuum lines.
It is also toxic to humans, limiting the use of NO as a medical inhalant Chapter 5. The overall reaction for the disproportionation of NO is shown in equation 5. Experimentally the rate of this reaction is dependent on the concentration of NO to the power of 3. In other words, the speed at which NO disappears is proportional to the concentration of NO cubed. In chemical notation this is written: This is an example of a third-order reaction and it suggests that the mechanism of the reaction is not as simple as it appears because the simultaneous collision of three molecules is a very rare event.
The rate constant for the reaction would be expected to increase with increasing temperature, i. But for this reaction the rate constant hardly varies when the reaction temperature is doubled.
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The reason for this is found in the mechanism of the reaction. This explains how we can have a third-order reaction without the statistically unlikely three-molecule collision. The fact that the rate of the reaction hardly varies with temperature may be explained by noting that although k increases with temperature, the value of K decreases and the product k K remains essentially unchanged. In order to begin to understand how NO can act as a signalling agent in this aqueous environment, we must consider how NO behaves in water.
Some other oxides of nitrogen react with water to form acids, i. N,O, is the anhydride of nitric acid equation 10 and N is the anhydride of the much weaker nitrous acid equation 11 : It was once thought that N O was the anhydride of the weak acid H2N,because the latter decomposes to give NO and H but H2Ndecomposes via an irreversible and complicated chain process and there is no acid-anhydride relationship between NO and H,N,O,. Another candidate for an acid-anhydride relationship with NO is the rather rare hydronitrous acid H2N02 because it could theoretically be formed by the reaction of NO with water.
However, isotope exchange studies have demonstrated that NO undergoes no hydrolysis at all in an aqueous environment. NO may indeed be described as the formal anhydride of hydronitrous acid, but the description is rather NO - a Not So Simple Little Molecule 63 meaningless. It can be generated in one cell, released into the aqueous environment between cells, and diffuse unaltered until it reaches a cell membrane provided that there is little oxygen around. Cell membranes are constructed of non-polar lipids, so the small, hydrophobic NO molecules can diffuse straight through and interact with enzymes in the new cell.
The description of NO as hydrophobic suggests that it is only sparingly soluble in water. This comes as no surprise since the NO molecule does not have much of a dipole moment, which is necessary for solution in a polar substrate such as water. Its solubility is approximately 1. This is similar to other non-polar diatomic molecules such as 02,N, and CO. The solubility of N,O, which has a small dipole, is 10 times greater. It is of particular interest that the solubility of NO in water is similar to that of O2as the best-known reaction of NO is that with 0,.
When a gas jar of NO is opened to the air, brown fumes of NO, are formed immediately. However, in solution the reaction takes a different course. NO, is still formed by the reaction of NO with oxygen, but then the NO, rapidly reacts with another molecule of NO to form the anhydride of nitrous acid: The reaction of NO, with NO is, in aqueous solution, faster than the hydrolysis of its dimer and so NO is quantitatively converted into nitrite. However, nitrite is readily oxidized to nitrate and so the nitrite may be contaminated with nitrate.
In some early experiments on the behaviour of NO in aqueous solution, oxygen was not rigorously excluded and what were thought to be the reactions of NO were those of NO, and N20,. When NO is present in an aqueous biological environment there is an additional complication. The rate of reaction between oxygen and NO in vitro is such that it would appear that the oxidation of NO is not significant in vivo because of the very low concentrations in living tissue of both species. However, the cellular environment is not pure water and there is good experimental evidence that in a heterogeneous environment containing both water and lipid, oxidation to N 2 0 3is much accelerated.
This observation has two consequences. Firstly, it is sensible to use nitrite levels in tissue as measures of NO activity Chapter 8 64 and, secondly, nitrosation of thiols by Ncan and will occur. The significance of the latter will be clear later. It has been suggested that part of the versatility of NO lies in its being both easily oxidized and easily reduced.
NO has a low ionization potential. The eleventh electron in the valence shell of NO is not strongly held by the molecule because it is in an antibonding orbital. Removal of this electron gives the nitrosonium ion, NO', which is isoelectronic with CO and, like CO, contains a strong triple bond. The nitrosonium ion forms two main types of compound. In water, these salts behave very differently from NO. This means that in the body, NO', if produced, would only have a transient existence.
However, there are instances of nitrosation reactions, where the NO' moiety is transferred from one molecule to another.