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On electrolysis of NAD+ in aqueous solution at a potential corresponding to the initial one-electron reduction of NAD+ to a free radical, a greenish-yellow color appears which fades when electrolysis is complete. Literature ultraviolet absorption data for the resulting dimer show considerable variation. When the electrolysis is conducted in darkness, the colored product has epsilon 340 of approx. 5700 M-1 . cm-1 and epsilon 259 of approx. 31000 M-1 . cm-1. On ultraviolet and visible illumination, the color disappears, the 340-nm peak decreases and the 259-nm peak increases. On only visible illumination, the color disappears, both peaks increase, the dimer's polarographic oxidation wave decreases and the wave due to 1-substituted nicotinamide reduction increases. The data suggest that the dimer decomposes to NAD+ and 1,4-NADH.

The biologically important redox couple, &mcotinarmde adenine dmucIeot~de/l.4.&d~hydromcotinamide 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) dimenzation of an intcrmtiate free radical product, (b) protonation-deprotonation of an intermediate product, (c) other chermcal reactions, (d) adsorption of reactant, mtermcdlate and product specxes, and (e) medlatlon 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 lo the second electron-transfer; a first-order chemical reaction coupled lo the latter may involve rearrangement of an initial diiydro product to l&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 lo 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 2 neutral radical, NAD-. which may participate in 2 second heterogeneous electron transfer (ECE mccharusm) or inay react with NADH+* (disproportionation mechanism DISP 1 or half-regeneration mechanism) to yield NAD+ * Invited lecture read at the International Symposium on Bioelectrochennstry and Bioenergetics, June 28th-July 3rd 1981, Kuyat Anavim, Israel.