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Nitric oxide

Name Nitrogen monoxide
Chemical formula NO
Appearance Colourless gas
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
ΔfH0gas 90 kJ/mol
ΔfH0liquid 87.7 kJ/mol
S0gas, 1 bar 211 J/mol·K
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 (http://ull.chemistry.uakron.edu/erd/chemicals/7/6847)
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 NO. It is an important signalling molecule in the body of mammals including humans, one of the few gaseous signalling molecules known. It is also a toxic air pollutant produced by car engines and power plants.

Nitric oxide (NO) should not be confused with nitrous oxide (N2O) or with nitrogen dioxide (NO2).

The nitric oxide molecule has 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 (as in combustion engines), molecular nitrogen and oxygen combine to form nitric oxide. 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 the ozone layer depletion.

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

Biological functions

In the body, nitric oxide is created from arginine and oxygen by the enzyme nitric oxide synthase[?].

The cells of the endothelium[?] (inner wall) of blood vessels use nitric oxide to signal the surrounding smooth muscle[?] to relax, thus increasing blood flow. This is part of the process of erection of the penis, and explains the mechanism of Viagra. It also underlies the action of nitroglycerin and amyl nitrate[?] 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 blood vessels around the heart. 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 cause trouble: fulminant infection (sepsis) causes the macrophages to produce so much nitric oxide that many blood vessels dilate and the blood pressure drops to dangerously low levels.

Nitric oxide is also used by nerve cells as a neurotransmitter. Unlike other neurotransmitters which 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 the formation of memory.

See also the section on nitric oxide in the article on signal transduction.

The discovery of the biological functions of nitric oxide in the 1980s came as a complete surprise and created quite a furor. 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 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.

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