Kinetics and mechanism of reaction of hydroxylamine with methylcytosines

Abstract

The reaction of hydroxylamine with 1,3-dimethylcytosine in the pH range 5–7 involves terms both first and second order in amine and proceeds entirely via the protonated form of the pyrimidine base (1a). The two products of the reaction, N(4)-hydroxy-1,3-dimethylcytosine (2a) and 5,6-dihydro-N(4)-hydroxy-6-hydroxyamino 1,3-dimethylcytosine (3a) are produced simultaneously in a ratio which varies with pH and with concentration of hydroxylamine. This behaviour, solvent kinetic isotope effects, and partial rates of reaction relative to those for cytosine (1c) and 1-methylcytosine (1b) establish a mechanism in which nucleophilic attack of nitrogen features in the rate-determining step for formation of (2) and is associated with general catalysis of proton transfer in the fomnation of (3).