Nitric oxide

**Properties**
General
Name	Nitrogen monoxide

Chemical formula	N O
Appearance	Colorless gas
Physical
Formula weight	30.0 amu
Melting point	109 K (-164 °C)
Boiling point	121 K (-152 °C)
Density	1.3 ×10³ kg/m³ (liquid)
Solubility	0.0056 g in 100g water
Thermochemistry
Δ_fH⁰_gas	90 kJ/mol
Δ_fH⁰_liquid	87.7 kJ/mol
S⁰_{gas, 1 bar}	211 J/mol·K
Safety
Ingestion	Used for medicinal purposes but has side effects and dangerous in overdose
Inhalation	Dangerous, may be fatal.
Skin	Irritant.
Eyes	May cause irritation
More info	Hazardous Chemical Database
SI units were used where possible. Unless otherwise stated, standard conditions were used. Disclaimer and references

The chemical compound nitric oxide is a gas with chemical formula N O. It is an important signaling molecule in the body of mammals including humans, one of the few gaseous signaling molecules known. It is also a toxic air pollutant produced by automobile engines and power plants.

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 the poisonous nitrogen dioxide.

Production and environmental effects

At high temperatures, molecular nitrogen and oxygen can combine to form nitric oxide. A major natural source is lightning. Human activity has drastically increased the production of nitric oxide in combustion chambers. One purpose of catalytic converters in cars is to partially reverse this reaction.

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

Technical applications

Nitric oxide has some industrial uses. 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. It is an intermediate of the Ostwald process, which converts ammonia into nitric acid.

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.

Because of its production in allergic reactions, there is research on using levels of exhaled nitric oxide to optimize treatment of asthma.

Biological functions

See also: Endothelium-derived relaxing factor (EDRF) and signal transduction

In the body, nitric oxide 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; bodybuilders use this to achieve a more "ripped", vascular look. This underlies the action of nitroglycerin, amyl nitrate and other nitrate derivatives in the treatment of heart disease: The compounds are converted to nitric oxide (by a process that is not completely understood), which in turn dilates the coronary artery (blood vessels around the heart), thereby increasing its blood supply. Nitric oxide also plays a role in erection of the penis, and explains the mechanism of sildenafil (Viagra®). The effects of the recreational drugs known as poppers are also thought to be due to nitric oxide.

Macrophages, certain cells of the immune system, produce nitric oxide in order to kill invading bacteria. Under certain conditions, this can backfire: Fulminant infection (sepsis) causes excess production of nitric oxide by macrophages, leading to vasodilatation (widening of blood vessels), probably one of the main causes of hypotension (low blood pressure) in sepsis.

Nitric oxide also serves as a neurotransmitter between nerve cells. Unlike most other neurotransmitters that only transmit information from a presynaptic to a postsynaptic neuron, the small nitric oxide molecule can diffuse all over and can thereby act on several nearby neurons, even on those not connected by a synapse. It is conjectured that this process may be involved in memory through the maintenance of long-term potentiation. Nitric oxide is an important non-adrenergic, non-cholinergic (NANC) neurotransmitter in various parts of the gastrointestinal tract. It causes relaxation of the gastrointestinal smooth muscle. In the stomach it increases the capacity of the fundus to store food/fluids.

Production of NO also plays a role in development and maintenance of erection by stimulating the production of intracellular cGMP in the smooth muscle cells surrounding the blood vessels supplying the corpus cavernosum; through relaxation of these muscles, more blood can flow in. This is the biological basis of sildenafil (Viagra), which works to inhibit the enzyme PDE5 that lowers the cGMP concentration by converting it to GMP. The high levels of cGMP that result lead to vasodilation and hence erection.

Dietary nitrate is also an important source of nitric oxide in mammals. Green, leafy vegetables and some root vegetables (such as beetroot) have high concentrations of nitrate. When eaten and absorbed into the bloodstream nitrate is concentrated in saliva (about 10 fold) and is reduced to nitrite on the surface of the tongue by a biofilm of commensal facultative anaerobic bacteria. This nitrite is swallowed and reacts with acid and reducing substances in the stomach (such as ascorbate) to produce high concentrations of nitric oxide. The purpose of this mechanism to create NO is thought to be both sterilisation of swallowed food, to prevent food poisoning and to maintain gastric mucosal blood flow. A similar mechanism is thought to protect the skin from fungal infections, where nitrate in sweat is reduced to nitrite by skin commensal organisms and then to NO on the slightly acidic skin surface.

The discovery of the biological functions of nitric oxide in the 1980s came as a complete surprise and caused quite a stir. Nitric oxide was named "Molecule of the Year" in 1992 by the journal Science, a Nitric Oxide Society was founded, and a scientific journal devoted entirely to nitric oxide was created. The Nobel Prize in Physiology or Medicine in 1998 was awarded to Ferid Murad, Robert F. Furchgott, and Louis Ignarro for the discovery of the signalling properties of nitric oxide. It is estimated that yearly about 3,000 scientific articles about the biological roles of nitric oxide are published.

Chemistry

The chemistry of nitric oxide is very extensive, as indicated by this brief overview.

Preparation

As stated above, nitric oxide can be produced from the reaction of O₂ and N₂ at high temperatures. It can be produced 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.

Reactions

When exposed to oxygen, NO is converted into NO₂. This conversion has been speculated as occurring via the ONOONO intermediate. In water, NO will react with oxygen and water to form HNO₂. 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. Nitrosyl iodide can form but is an extremely short lived species and tends to reform I₂.

At 25 °C and 1 atm, NO is thermodynamically unstable with respect to disproportionation. In the 30-50 °C range, NO decomposes to N₂O and NO₂.

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.

Coordination Chemistry

NO can also serve as a ligand in transition metal complexes. 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 consider 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.

Reactions of NO ligands

The NO ligand has an extensive reactivity. Some reactions that can occur include:

Cp₂NbMe₂ + NO → Cp₂(Me)Nb(O)NMe + heat → Cp₂Nb(O)Me + ½MeN=NMe

(In the second part of this reaction the O is bonded both to the Nb atom and N atom, the N atom is bonded to Nb, O, and Me. This would be easier to understand if it was drawn out.)

(Ph₃P)₂(CO)ClOsNO + HCl → (Ph₃P)₂(CO)ClOsN(H)O

Generic oxidation can also occur

L_nMNO + ½O₂ → L_nMNO₂

This is just a sample of the reactions involving coordinated NO that can occur.

Measurement of nitric oxide

Nitric oxide can be measured 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

There are other methods of testing, including electrochemical methods, where nitric oxide in a test sample reacts to produce a current or voltage difference on a surface.

Reference

F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann; Advanced Inorganic Chemistry, 6th ed. Wiley-Interscience, New York, 1999.

External links

Oxides | Nitrogen compounds | Neurotransmitters | Nitrogen metabolism

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