The reaction of hydroxylamine with 4-thiouridine

Abstract

Hydroxylamine reacts with 4-thiouridine very easily in neutral aqueous solutions to give N 4- hydroxycytidine . A quantitative conversion of 4-thiouridine into the product was obtained by 21-h incubation in 0.025 M hydroxylamine at pH 7.0 and 37 °C. The reaction was not affected by addition of EDTA or hydroquinone. The optimum pH of the reaction was about 7.2, indicating the mechanism to be a nucleophilic attack of hydroxylamine free base towards C-4 of the undissociated 4-thiouridine. The apparent pseudo-first order rate constant was proportional to the concentration of hydroxylamine, the apparent rate constant in 1 M hydroxylamine at pH 7.0 and 37 °C being 0.292 min −1. The alteration of 4-thiouridine was much faster than the known reactions of cytidine and uridine with hydroxylamine both at pH 7.0 and at pH 5.5. Bis-(4-thiouridine) disulfide also reacts with hydroxylamine to give N 4- hydroxycytidine as a final and sole product. The conversion was complete within 15 min at pH 7.0 and 37 °C, using 1 M hydroxylamine solution. Methoxyamine also reacts with 4-thiouridine in a similar manner, the apparent rate constant at pH 7.0 and 37 °C in 1 M methoxyamine being 0.009 min −1.