In organic chemistry, nitrosamines (or more formally N-nitrosamines) are organic compounds with the chemical structure , where R is usually an alkyl group. They feature a nitroso group () bonded to a deprotonated amine. Most nitrosamines are carcinogenic in nonhuman animals. A 2006 systematic review supports a "positive association between nitrite and nitrosamine intake and gastric cancer, between meat and processed meat intake and gastric cancer and oesophageal cancer, and between preserved fish, vegetable and smoked food intake and gastric cancer, but is not conclusive". Chemistry The organic chemistry of nitrosamines is well developed with regard to their syntheses, their structures, and their reactions. They usually are produced by the reaction of nitrous acid () and secondary amines. HONO + R2NH -> R2N-NO + H2O The nitrous acid usually arises from protonation of a nitrite. This synthesis method is relevant to the generation of nitrosamines under some biological conditions. With regards to structure, the core of nitrosamines is planar, as established by X-ray crystallography. The N-N and N-O distances are 132 and 126 pm, respectively in dimethylnitrosamine, one of the simplest members of a large class of N-nitrosamines Nitrosamines are not directly carcinogenic. Metabolic activation is required to convert them to the alkylating agents that modify bases in DNA, inducing mutations. The specific alkylating agents vary with the nitrosamine, but all are proposed to feature alkyldiazonium centers. History and occurrence In 1956, two British scientists, John Barnes and Peter Magee, reported that a simple member of the large class of N-nitrosamines, dimethylnitrosamine, produced liver tumours in rats. Subsequent studies showed that approximately 90% of the 300 nitrosamines tested were carcinogenic in a wide variety of animals. Tobacco exposure A common way ordinary consumers are exposed to nitrosamines is through tobacco use and cigarette smoke. Tobacco-specific nitrosamines also can be found in American dip snuff, chewing tobacco, and to a much lesser degree, snus (127.9 ppm for American dip snuff compared to 2.8 ppm in Swedish snuff or snus). Dietary exposure Nitrosamines are produced by the reaction of nitrites and secondary amines. Nitrites are used as food preservatives, e.g. cured meats. Secondary amines arise by the degradation of proteins (food). Nitrite and nitrosamine intake are associated with risk of gastric cancer and oesophageal cancer. Adverse reaction with dimethylamine During the 1970s, an elevated frequency of liver cancer was found in Norwegian farm animals after the farm animals had been fed on