Nitric oxide (NO) should not be confused with nitrous oxide (N₂O), a general anaesthetic, or with nitrogen dioxide (NO₂) which is another poisonous air pollutant.

The nitric oxide molecule is a free radical, which makes it very reactive and unstable. In air, it quickly reacts with oxygen to form nitrogen dioxide, signalled by the appearance of the orange colour:

2 NO + O₂ → 2 NO₂

1 Production and environmental effects
2 Technical applications
- 2.1 Miscellaneous applications
3 Biological functions
4 Chemistry
5 Measurement of nitric oxide
6 References
7 Further reading
8 External links

[edit] Production and environmental effects

From the thermodynamic perspective of thermodynamics, NO is unstable with respect to O₂ and N₂, although this conversion is very slow at ambient temperatures in the absence of a catalyst. Because NO is an endothermic molecule, its synthesis from molecular nitrogen and oxygen would require elevated temperatures, >1000 °C. A major natural source is lightning. The use of internal combustion engines has drastically increased the presence of nitric oxide in the environment. One purpose of catalytic converters in cars is to minimize NO formation by catalytic reversion to O₂ and N₂.

Nitric oxide in the air may convert to nitric acid, which has been implicated in acid rain. Furthermore, both NO and NO₂ participate in ozone layer depletion.

[edit] Technical applications

Although NO has relatively few industrial uses, it is produced on a massive scale as an intermediate in the Ostwald process for the synthesis of nitric acid from ammonia. In 2005, the US alone produced 6M metric tonnes of nitric acid.<ref>“Production: Growth is the Norm” Chemical and Engineering News, July 1 0, 2006, p. 59.</ref> As a raw material it is used in the semiconductor industry for various processes. In one of its applications it is used along with nitrous oxide to form oxynitride gates in CMOS devices.

[edit] Miscellaneous applications

Nitric oxide can be used for detecting surface radicals on polymers. Quenching of surface radicals with nitric oxide results in incorporation of nitrogen, which can be quantified by means of X-ray photoelectron spectroscopy.

[edit] Biological functions

Main article: Endothelium-derived relaxing factor

See also: signal transduction

In the body, nitric oxide (the 'endothelium-derived relaxing factor', or 'EDRF') is synthesized from arginine and oxygen by various nitric oxide synthase (NOS) enzymes and by sequential reduction of inorganic nitrate. The endothelium (inner lining) of blood vessels use nitric oxide to signal the surrounding smooth muscle to relax, thus dilating the artery and increasing blood flow. The production of nitric oxide is noted to be increased in high-altitude populations for this affect, which helps to avoid hypoxia in thin air. Nitric oxide is a key biological messenger, playing a role in a variety of biological process. These include blood vessel dilatation, neurotransmission, modulation of the hair cycle, and penile erections. "Nitro" vasodilators such as nitroglyceric are converted to nitric oxide. Similarly, as the name implies, the vasodilating antihypertensive agent Minoxidil is a nitric oxide agonist [1]. Nitric oxide also plays a role in the modulation of hair growth and hair loss. Thus, Minoxidil is also a hair growth stimulator.

Nitric oxide is also generated by macrophages as part of the human immune response. Nitric oxide is toxic to bacteria and other human pathogens. Many bacterial pathogens have evolved mechanisms for nitric oxide resistance.

Nitric oxide can contribute to reperfusion injury when excessive nitric oxide produced during reperfusion (following a period of ischemia) reacts with superoxide to produce the damaging free radical peroxynitrite. Inhaled nitric oxide has been shown to help survival and recovery from paraquat poisoning, which produces lung tissue damaging superoxide and hinders NOS metabolism.

In plants, nitric oxide can be produced by nitric oxide synthase (as in animals), or by plasma membrane-bound nitrate reductase or by mitochondrial electron transport chain or by non enzymatic reactions. It is a signaling molecule, acts mainly against oxidative stress and also plays a role in plant pathogen interactions.

A biologically important reaction of nitric oxide is S-nitrosation (or S-nitrosylation), the covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine within proteins. S-nitrosylation has emerged as a mechanism for dynamic, post-translational regulation of most or all main classes of protein.

[edit] Chemistry

The chemistry of nitric oxide is extensive, as indicated by the following overview.

[edit] Preparation

As stated above, nitric oxide is produced industrially by the direct reaction of O₂ and N₂ at high temperatures. In the laboratory it is conveniently generated from nitric acid:

8HNO₃ + 3Cu → 3Cu(NO₃)₂ + 4H₂O + 2NO

or from the following aqueous reactions,

2 NaNO₂ + 2 NaI + 2 H₂SO₄ → I₂ + 4 NaHSO₄ + 2 NO

2 NaNO₂ + 2 FeSO₄ + 3 H₂SO₄ → Fe₂(SO₄)₃ + 2 NaHSO₄ + 2 H₂O + 2 NO

The iron(II) sulfate reaction is a simple method that has been used in undergraduate laboratory experiments. NO can be produced from the following non-aqueous reagents,

3 KNO₂(l) + KNO₃ (l) + Cr₂O₃(s) → 2 K₂CrO₄(s) + 4 NO

Commercially, NO is produced by the oxidation of ammonia at 750 to 900 °C in the presence of platinum. The uncatalyzed reaction of O₂ and N₂ has not been developed into a practical commercial synthesis.

[edit] Reactions

When exposed to oxygen, NO is converted into NO₂. This conversion has been speculated as occurring via the ONOONO intermediate. In water, NO react with oxygen and water to form HNO₂ or nitrous acid. The reaction is thought to proceed via the following stoichiometry:

4 NO + O₂ + 2 H₂O → 4 HNO₂

NO will react with fluorine, chlorine, and bromine to from the XNO species, known as the nitrosyl halides, such as nitrosyl chloride. Nitrosyl iodide can form but is an extremely short lived species and tends to reform I₂.

Nitroxyl (HNO) is the reduced form of nitric oxide.

[edit] Coordination Chemistry

NO can also serve as a ligand in transition metal complexes, such species are called metal nitrosyls. The most common bonding mode of NO is the terminal linear type (M-NO). The angle of the M-N-O group can vary from 160-180° but are still termed as "linear". In this case the NO group is formally considered a 3-electron donor. Alternatively, one can view such complexes as derived from NO⁺, which is isoelectronic with CO.

Nitric oxide can serve as a one-electron pseudohalide. In such complexes, the M-N-O group is characterized by an angle between 120-140°

The NO group can also bridge between metal centers through the nitrogen. The μ₂-symmetric or unsymmetric, μ₃ and μ₄ bonding modes are possible.

[edit] Measurement of nitric oxide

The concentration of nitric oxide can be determined using a simple chemiluminescent reaction involving ozone: A sample containing nitric oxide is mixed with a large quantity of ozone. The nitric oxide reacts with the ozone to produce oxygen and nitrogen dioxide. This reaction also produces light (chemiluminescence), which can be measured using a photodetector. The amount of light produced is proportional to the amount of nitric oxide in the sample.

NO + O₃ → NO₂ + O₂ + light

Other methods of testing include electroanalysis, where NO reacts with an electrode to induce a current or voltage change.

[edit] References

[edit] Further reading

F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann; Advanced Inorganic Chemistry, 6th ed. Wiley-Interscience, New York, 1999.
K.J. Gupta , M. Stoimenova, and W. M. Kaiser "In higher plants, only root mitochondria, but not leaf mitochondria reduce nitrite to NO, in vitro and in situ" Journal of Experimental Botany 2005 56(420):2601-2609.
E.Planchet, K.J. Gupta, M .Sonada & W.M.Kaiser (2005) "Nitric oxide emission from tobacco leaves and cell suspensions: rate limiting factors and evidence for the involvement of mitochondrial electron transport"The Plant Journal 41 (5), 732-743.
Christine Stöhr and Stefanie Stremlau "Formation and possible roles of nitric oxide in plant roots" Journal of Experimental Botany 2006 57(3):463-470