Kinetics and mechanism of the nitrosation of N-acetyltryptophan and of the denitrosation of N-acetyl-N1-nitrosotryptophan

Abstract

Both the formation and the denitrosation of N-acetyl-N¹-nitrosotryptophan have been studied kinetically in aqueous solution at 25 °C at acidities between 1 M-HClO₄ and pH 4. A value of 850 l mol^–1 has been obtained for the equilibrium constant for the formation of N-acetyl-N¹-nitrosotryptophan. At acidities ([H⁺]) greater than 0.1 M the rate constants for both nitrosation and denitrosation increase linearly with the concentration of acid, and the reaction rates are unaffected by the addition of nucleophiles (Br^– and SCN^–). The results are consistent with a mechanism for nitrosation where the rate-limiting step is the proton transfer from the protonated N-nitroso species to the medium. For denitrosation the corresponding protonation of the nitrosamine is rate-limiting. These conclusions are confirmed by the results obtained when the reactions are carried out in heavy water. However, in the pH range 1–4 the rates of both nitrosation and denitrosation are independent of the acidity of the medium and are again unaffected by the presence of nucleophiles or buffers. It is suggested that in this region nitrosation occurs at C-3. This is followed by deprotonation and an internal NO migration from C to N which is rate-limiting. This mechanism also accounts for earlier results on the denitrosation reaction at even lower acidities (pH 4–7), where acid catalysis and nucleophilic catalysis are found. Results of experiments in heavy water are compatible with this mechanism.