Denitrosation of N-acetyl-N1-nitrosotryptophan in acid solution

Abstract

The denitrosation of DL-N-acetyl-N¹-nitrosotryptophan has been studied kinetically in water in the acid range 4 × 10^–2–1M-H₂SO₄ and also at lower acidities in buffer solutions pH 2–6. The reaction gave DL-N-acetyltryptophan and nitrous acid quantitatively and was not significantly reversible under these conditions. First-order behaviour was found for both the nitrosamine and acid in the sulphuric acid reactions, and the reaction was also acid-catalysed in the pH region 2–6. The reaction rate constant was unaffected by the addition of N-acetyltryptophan. In 0.04M-H₂SO₄ the rate constant was unchanged by the addition of bromide ion, thiocyanate ion, iodide ion, and thiourea, and the kinetic solvent isotope effect k_H2O/k_D2O was 1.3 at 0.7M-H₂SO₄ and 1.1 at 0.1M- H₂SO₄. However at pH 6 catalysis was observed by chloride, bromide, thiocyanate, azide, and iodide ion with increasing efficiency along this sequence. As the concentration of the nucleophile was increased the reaction rate constant tended to become independent of the nucleophile concentration. Thus N-acetyl-N¹-nitrosotryptophan behaves as a typical nitrosamine at very low acidities around pH 6, whereas at higher acidities it shows a pattern of behaviour reminiscent of that shown by nitrosamides. The pH–rate profile for denitrosation shows clearly that there are two pathways associated with the acid-catalysed reaction, one predominant at around pH 4–7 and the other at acidities greater than pH 1. The former is associated with nucleophilic catalysis and the latter is not. The results are discussed in terms of two possible sites of protonation of the substrate (at O and C-3), and the changing effective rate limiting step, as the nucleophile concentration is changed. N-Acetyl-N¹-nitrosotryptophan can be used to nitrosate (or diazotise) other species such as 4-nitroaniline, but only in the absence of a nitrous acid trap, indicating that this nitrosamine is not a direct nitrosating agent towards amines under these conditions. A minor photochemical decomposition pathway has been established for N-acetyl-N¹-nitrosotryptophan at pH 6, but it was not examined in detail.