524--NAD/NADH as a model redox system: Mechanism, mediation, modification by the environment

Abstract

The biologically important redox couple, [beta]-nicotinamide adenine dinucleotide/1,4,[beta]-dihydronicotinamide adenine dinucleotide, provides a grossly reversible prototype system for an overall electrode reaction consisting of two successive one-electron (1 e-) transfer steps coupled with (a) dimerization of an intermediate free radical product, (b) protonation--deprotonation of an intermediate product, (c) other chemical reactions, (d) adsorption of reactant, intermediate and product species, and (e) mediation by electrode surface species. Cathodic reduction of NAD+ proceeds through two 1 e- steps well separated in potential; protonation of the free radical produced on the first step occurs prior to the second electron-transfer; a first-order chemical reaction coupled to the latter may involve rearrangement of an initial dihydro product to 1,4-NADH (and some 1,6-NADH). In the apparently single stage 2 e- anodic oxidation of NADH, the initial step is an irreversible heterogeneous electron transfer, which proceeds to at least some extent through mediator redox systems located close to the electrode surface; the resulting cation radical, NADH+-, loses a proton (first order reaction) to form a neutral radical, NAD-, which may participate in a second heterogeneous electron transfer (ECE mechanism) or may react with NAD+- (disproportionation mechanism DISP 1 or half-regeneration mechanism) to yield NAD+.