Nitroso refers to a functional group in organic chemistry which has the NO group attached to an organic moiety. As such, various nitroso groups can be categorized as C-nitroso compounds, S-nitroso compounds, N-nitroso compounds, and O-nitroso compounds.
Nitrosoarenes typically participate in a monomer-dimer equilibrium. The dimers, which are often pale-yellow, are often favored in the solid state, whereas the deep-green monomers are favored in dilute solution or at higher temperatures. They exist as cis- and trans-isomers. Due to the stability of the nitric oxidefree radical, nitroso organyls tend to have very low C–N bond dissociation energies: nitrosoalkanes have BDEs on the order of 30 to 40 kcal/mol, while nitrosoarenes have BDEs on the order of 50 to 60 kcal/mol. As a consequence, they are generally heat- and light-sensitive. Compounds containing O– or N– bonds generally have even lower bond dissociation energies. For instance, N-nitrosodiphenylamine, Ph2N–N=O, has a N–N bond dissociation energy of only 23 kcal/mol. Organonitroso compounds serve as a ligands for transition metals.
can enter two kinds of reaction, depending on the physico-chemical environment.
Nitrosylation is adding a nitrosyl ion NO− to a metal or a thiol, leading to nitrosyl iron Fe–NO or S-nitrosothiols.
Nitrosation is adding a nitrosonium ion NO+ to an amine –NH2 leading to a nitrosamine. This conversion occurs at acidic pH, particularly in the stomach, as shown in the equation for the formation of N-phenylnitrosamine:
: + H+ HONO
: HONO + H+ H2O + NO+
: C6H5NH2 + NO+ → C6H5NNO + H+
Many primary alkylN-nitroso compounds, such as CH3NNO, tend to be unstable with respect to hydrolysis to the alcohol. Those derived from secondary amines are more robust. It is these N-nitrosamines that are carcinogens in rodents.
Nitrosyls are non-organic compounds containing the NO group, for example directly bound to the metal via the N atom, giving a metal–NO moiety. Alternatively, a nonmetal example is the common reagent nitrosyl chloride. Nitric oxide is a stable radical, having an unpaired electron. Reduction of nitric oxide gives the nitrosyl anion, NO−: Oxidation of NO yields the nitrosoniumcation, NO+: Nitric oxide can serve as a ligand forming metal nitrosyl complexes or just metal nitrosyls. These complexes can be viewed as adducts of NO+, NO−, or some intermediate case.
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 . Cured meat is also added with sodium ascorbate. As demonstrated by S. Mirvish, ascorbate inhibits nitrosation of amines to nitrosamine, because ascorbate reacts with to form NO. Ascorbate and pH 5 thus favor nitrosylation of heme iron, forming nitrosylheme, 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: nitrosylheme is thus considered a benefit for the meat industry and for consumers.
In the stomach: secreted hydrogen chloride makes an acidic environment and ingested nitrite leads to nitrosation of amines, that yields nitrosamines. Nitrosation is low if amine concentration is low or if vitamin C concentration is high. Then S-nitrosothiols are formed, that are stable at pH 2.
In the colon: neutral pH does not favor nitrosation. No nitrosamine is formed in stools, even after addition of a secondary amine or nitrite. Neutral pH favors NO− release from S-nitrosothiols, and nitrosylation of iron. The previously called NOC measured by Bingham's team in stools from red meat-fed volunteers were, according to Bingham and Kuhnle, largely non-N-nitroso ATNC, e.g., S-nitrosothiols and nitrosyl iron.
