Investigation of the reduction mechanism of the NAD + model compound N'-methyl nicotinamide on mercury electrodes in strongly acidic solutions

Abstract

The electroreduction of the NAD + model compound N′-methyl nicotinamide has been studied in acidic media (5 M H 2SO 4 to pH 5) by dc and DP polarography and linear-sweep cyclic voltammetry. Tafel slopes and reaction orders were obtained from I- E curves traced at potentials corresponding to the foot of the polarographic wave. The protonation p K of the heterocyclic nitrogen was obtained from spectroscopic (UV) and potentiometric measurements, having a value of 3.50 ± 0.15. Controlled-potential electrolysis indicated that at pH < 2.5 two electrons are transferred per reactant molecule and that the carbonyl group is reduced to hydroxyl. The reduction pathway consists of the reversible transfer of an electron and an H + ion followed by an irreversible one-electron transfer, which is the rate-determining step, in agreement with what occurs in the reduction of NAD + coenzyme at these pH values.