Electrochemical Aspects of a NADH/FAD Model System

Abstract

Abstract A model system of the NADH/FAD redox couple has been studied by the use of electrochemical techniques. The system studied was composed of 1-benzyl-1,4-dihydronicotinamide (BNAH) and riboflavin (RF), which were used in place of NADH and FAD respectively, dissolved in a DMSO solution. The results obtained were as follows. RF is reduced either by BNAH or sodium borohydride, obeying a second-order rate law, presumably by the mechanism of hydride-ion transfer. The resultant product is inactive to the electrochemical reduction at a mercury-pool electrode. Entirely different products were produced when RF was reduced electrochemically. RF was reduced at a rate of 1 F/mol at the initial stage of controlled potential electrolysis to form, probably, a dimeric compound, but the latter product underwent further reduction and the total quantity of electricity necessary for the complete reduction exceeded 10 F/mol. Interestingly, the electrolytic reduction gives the same product as that obtained by BNAH when electrolysis is performed in the presence of a sufficient amount of BNAH, indicating that the initial product of electrochemical reduction receives the hydride ion rather than further electrons. 2RF+2e\ightleftarrows(RF)_2^2-\overset+H^-,+H^+→2RFH^- The Zn2+ added to the BNAH/RF system was found to act as the oxidizing agent of RFH− and regenerated the neutral RF. This is markedly different from the case of BNAH/organic carbonyl compounds, where the metal ions behave as catalysts to promote the reaction.