524-nad/nadh as a Model Redox System: Mechanism, Mediation, Modification by the Environment *

The biologically important redox couple, /I-nicotinamide adenine dinucleotide/l,4,/%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 esteps 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,CNADH (and some 1,6-NADH). In the apparently single stage 2 eanodic 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 NADH+’ (disproportionation mechanism DISP 1 or half-regeneration mechanism) to yield NAD+.