Nitroso
Nitroso refers to a functional group in organic chemistry which has the general formula RNO. Nitroso compounds are a class of organic compounds containing the nitroso functional group, R−N=O.
Nitrosyls are molecules with the general formula RNO, where R represents an unspecified substituent.
Nitroso and bent Nitrosyl are synonyms; nitroso is used in organic chemistry, while bent nitrosyl is used in inorganic chemistry. Neither Nitroso nor bent Nitrosyl are identical to linear Nitrosil, which possesses sp hybridization. A common example of a nitroso compound is nitrosyl chloride, NOCl (although its structure is better represented ONCl).
Nitrosyl also refers to the discrete molecule nitric oxide, NO. Nitric oxide is a stable radical, having an unpaired electron.
Reduction of nitric oxide gives the hyponitrite anion, NO−:
- NO + e− → NO−
Oxidation of NO yields the nitrosonium cation, NO+:
- NO → NO+ + e−
Nitrosyl as a ligand
Main article:
Metal nitrosylNitric oxide can serve as a ligand in complexes. The resulting complexes are called metal nitrosyls, and can bond to a metal atom in two distinct modes: as NO+ and as NO−. NO+ coordinates linearly, the M−N−O angle being 180°, whereas NO− forms a bent geometry, with an M−N−O angle of approximately 120°.
Nitroso compounds
Nitroso compounds can be prepared by the reduction of nitro compounds or by the oxidation of hydroxylamines. A good example is (CH3)3CNO, known formally as 2-methyl-2-nitrosopropane, or t-BuNO, which is prepared by the following sequence:[1]
- (CH3)3CNH2 → (CH3)3CNO2
- (CH3)3CNO2 → (CH3)3CNHOH
- (CH3)3CNHOH → (CH3)3CNO
(CH3)3CNO is blue and exists in solution in equilibrium with its dimer, which is colorless, m.p. 80–81 °C.
In the Fischer-Hepp rearrangement aromatic 4-nitroso-anilines are prepared from the corresponding nitrosamines. Another named reaction involving a nitroso compound is the Barton reaction.
Nitrosation vs. nitrosylation
Nitrite can enter two kinds of reaction, depending on the physico-chemical environment.
- Nitrosylation is adding a nitrosyl ion NO− to a metal (e.g. iron) or a thiol, leading to nitrosyl iron Fe-NO (e.g., in nitrosylated heme = nitrosylheme) or S-nitrosothiols (RSNOs).
- Nitrosation is adding a nitrosonium ion NO+ to an amine -NH2 leading to a nitrosamine. This occurs at acidic pH, particularly in the stomach, as shown in the figure below. Primary amines R-NH2 lead to unstable nitrosamines (that turn to alcool), but secondary amines R1-NH-R2 lead to stable nitrosamines, most of which are carcinogens in rodents.
In food
In foodstuffs and in the gastro-intestinal tract, nitrosation and nitrosylation do not have the same consequences on consumer health.
- In cured meat: Meat processed by curing contains nitrite and has a pH of 5 approximately, where almost all nitrite is present as NO2− (99%). Cured meat is also added with sodium ascorbate (or erythorbate or Vitamin C). As demonstrated by S. Mirvish, ascorbate inhibits nitrosation of amines to nitrosamine, because ascorbate reacts with NO2− to form NO.[2][3] Ascorbate and pH 5 thus favor nitrosylation of heme iron, forming nitrosyl-heme, a red pigment when included inside myoglobin, and a pink pigment when it has been released by cooking. It participates to the "bacon flavor" of cured meat: nitrosyl-heme is thus considered a benefit for the meat industry and for consumers.[4]
- In the stomach: secreted HCl makes an acidic environment (pH=2) and ingested nitrite (with food or saliva) leads to nitrosation of amines, that yields nitrosamines (potential carcinogens). Nitrosation is low if amine concentration is low (e.g., low-protein diet, no fermented food) or if Vitamin C concentration is high (e.g., high fruit diet). Then S-nitrosothiols are formed, that are stable at pH 2.
- In the colon: neutral pH does not favour nitrosation. No nitrosamine is formed in stools, even after addition of a secondary amine or nitrite.[5] Neutral pH favors NO− release from S-nitrosothiols, and nitrosylation of iron. The previously called NOC (N-nitroso compounds) measured by Bingham's team in stools from red meat-fed volunteers[6] were, according to Bingham and Kuhnle, largely non-N-nitroso ATNC (Apparent Total Nitroso Compounds), e.g., S-nitrosothiols and nitrosyl iron (as nitrosyl heme).[7]
See also
References
- ^ A. Calder, A. R. Forrester, and S. P. Hepburn 2-Methyl-2-nitrosopropane and Its Dimer Organic Syntheses, Coll. Vol. 6, p.803; Vol. 52, p.77. Link
- ^ "Ascorbate-nitrite reaction: possible means of blocking the formation of carcinogenic N-nitroso compounds". Science 177 (4043): 65–8. July 1972. PMID 5041776.
- ^ "Effects of vitamins C and E on N-nitroso compound formation, carcinogenesis, and cancer". Cancer 58 (8 Suppl): 1842–50. October 1986. PMID 3756808.
- ^ Honikel, K.O., 2008, "The use an control of nitrate and nitrite for the processing of meat products", Meat Science, 78, 68-76. doi 10.1016/j.meatsci.2007.05.030
- ^ "Absence of volatile nitrosamines in human feces". Cancer Res. 41 (10): 3992–4. October 1981. PMID 7285009.
- ^ "Does increased endogenous formation of N-nitroso compounds in the human colon explain the association between red meat and colon cancer?". Carcinogenesis 17 (3): 515–23. March 1996. PMID 8631138.
- ^ . doi:10.1016/j.freeradbiomed.2007.03.011. PMID 17761300.