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style="background: #F8EABA; text-align: center;" colspan="2" Chemistry
CAS number 10102-43-9
Molecular formula NO
Molar mass 30.0061
Melting point

−163.6°C (109.6 K)

NFPA 704

NFPA 704.svg

style="background: #F8EABA; text-align: center;" colspan="2" Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Nitric oxide or Nitrogen monoxide is a chemical compound with chemical formula NO. This gas is an important signaling molecule in the body of mammals including humans.

Nitric oxide (NO) should not be confused with nitrous oxide (N2O), a general anaesthetic, and with nitrogen dioxide (NO2) which is another poisonous air pollutant. The nitric oxide molecule is a free radical, which is relevant to understanding its high reactivity. It reacts with the ozone in air to form nitrogen dioxide, signalled by the appearance of the reddish-brown color.

Despite being a startingly simple molecule, NO is a fundamental player in the fields of neuroscience, physiology, and immunology, and was proclaimed “Molecule of the Year” in 1992[1]

Biological functions

Main article: Endothelium-derived relaxing factor

NO is one of the few gaseous signaling molecules known. It is a key vertebrate biological messenger, playing a role in a variety of biological processes. Nitric oxide, known as the 'endothelium-derived relaxing factor', or 'EDRF', is biosynthesised endogenously from arginine and oxygen by various nitric oxide synthase (NOS) enzymes and by reduction of inorganic nitrate. The endothelium (inner lining) of blood vessels use nitric oxide to signal the surrounding smooth muscle to relax, thus resulting in vasodilation and increasing blood flow. Nitric oxide is highly reactive (having a lifetime of a few seconds), yet diffuses freely across membranes. These attributes make nitric oxide ideal for a transient signal molecule between adjacent cells and within cells.[2] The production of nitric oxide is elevated in populations living at high-altitudes, which helps these people avoid hypoxia. Effects include blood vessel dilatation, neurotransmission (see Gasotransmitters), modulation of the hair cycle, and penile erections. Nitroglycerin and amyl nitrite serve as vasodilators because they are converted to nitric oxide in the body. Sildenafil, popularly known by the trade name Viagra, stimulates erections primarily by enhancing signaling through the nitric oxide pathway in the penis.

Nitric oxide (NO) contributes to vessel homeostasis by inhibiting vascular smooth muscle contraction and growth, platelet aggregation, and leukocyte adhesion to the endothelium. In humans, a high-salt intake was demonstrated to attenuate NO production. [1]

Nitric oxide is also generated by macrophages and neutrophils as part of the human immune response. Nitric oxide is toxic to bacteria and other human pathogens. In response, however, many bacterial pathogens have evolved mechanisms for nitric oxide resistance.[3]

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

A biologically important reaction of nitric oxide is S-nitrosylation, the conversion of thiol groups, including cysteine residues in proteins, to form S-nitrosothiols (RSNOs). S-Nitrosylation is a mechanism for dynamic, post-translational regulation of most or all major classes of protein.


When exposed to oxygen, NO is converted into NO2.

2NO + O2 → 2NO2

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.

2NO + Cl2 → 2NOCl

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


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 by reduction of nitric acid:

8HNO3 + 3Cu → 3Cu(NO3)2 + 4H2O + 2NO

or by the reduction of nitrous acid:

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

The iron(II) sulfate route is simple and has been used in undergraduate laboratory experiments.

Measurement of nitric oxide concentration

The concentration of nitric oxide can be determined using a simple chemiluminescent reaction involving ozone:[4] 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 with 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. The detection of NO radicals in biological tissues is particularly difficult due to the short lifetime and concentration of these radicals in tissues. One of the few practical methods is spin trapping of nitric oxide with iron-dithiocarbamate complexes and subsequent detection of the mono-nitrosyl-iron complex with Electron Paramagnetic Resonance (EPR).[5][6]

A group of fluorescent dye indicators exist that are also available in acetylated form for intracellular measurements. The most common compound is 4,5-diaminofluorescein (DAF-2).[1]


  1. 1.0 1.1 Elizabeth Culotta and Daniel E. Koshland Jr (December 1992). NO news is good news. (nitric oxide; includes information about other significant advances & discoveries of 1992) (Molecule of the Year).. Science 258 (5090): 1862-1864. Cite error: Invalid <ref> tag; name "undefined" defined multiple times with different content
  2. Stryer, Lubert (1995). Biochemistry, 4th Edition, pp. 732, W.H. Freeman and Company.
  3. C. A. Janeway, et al. (2005). Immunobiology: the immune system in health and disease, 6th ed., New York: Garland Science.
  4. Fontijn, A., A. J. Sabadell and R. J. Ronco (1970). "Homogeneous chemiluminescent measurement of nitric oxide with ozone." Analytical Chemistry 42(6): 575-579.
  5. Vanin A. F.; Huisman A.; van Faassen E.E.; Methods in Enzymology vol 359 (2002) 27 - 42
  6. Nagano T.; Yoshimura T.; "Bioimaging of nitric oxide", Chemical Reviews vol 102 (2002) 1235 - 1269.

Further reading

  • Butler A. and Nicholson R.; " Life, death and NO." Cambridge 2003. ISBN-13: 978-0-85404-686-7.
  • van Faassen, E. E.; Vanin, A. F. (eds); " Radicals for life: The various forms of Nitric Oxide." Elsevier, Amsterdam 2007. ISBN-13: 978-0-444-52236-8.
  • 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.
  • Pacher, P.; Beckman, J. S.; Liaudet, L.; “Nitric Oxide and Peroxynitrite: in Health and disease” Physiological Reviews 2007, volume 87(1), page 315-424. PMID 17237348.

External links

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