Kinetics and mechanism of reaction of hydroxylamine with cytosine and its derivatives

The reactions of hydroxylamine with cytosine, and with 1-methyl-, 5,6-dihydro-, and 5,6-dihydro-1-methylcytosine in the pH range 4–7 involve terms both first- and second-order in amine and proceed predominantly via the protonated form of the pyrimidine bases. The sole products of reaction for the dihydrocytosines are the corresponding 5,6-dihydro-N4-hydroxycytosines. Cytosine and 1-methylcytosine are known to give two products each, namely the N4-hydroxy-(II) and the 5,6-dihydro-N4-hydroxy-6-hydroxyamino-cytosines (III). The ratio of these two products is shown to change with changes in pH and in hydroxylamine concentration. This behaviour and the relative rates of reaction of the various substrates supports a mechanism of formation of (IIIb) involving rate-limiting general acid-base catalysis of the Michael addition of hydroxylamine to position 6 of 1-methylcytosine (Ib).