The Role of Adsorption in the Initial One-Electron Electrochemical Reduction of Nicotinamide Adenine Dinucleotide (NAD+)

Article abstract of DOI:10.1021/ja00399a039

The title reduction was investigated at pH 9.1 in several base electrolytes of varying surface activity in order to elucidate the role of adsorption of NAD⁺, its free radical, and the resulting dimer at the aqueous solution/mercury electrode interface.In the presence of electrolytes of low activity, NAD⁺ is adsorbed at potentials positive to its 1-e reduction (ca. -0.9 V vs.SCE); the electrochemically generated dimer, (NAD)2, is adsorbed positive of -1.20 and -1.32 V in 0.06 and 0.4 M KCl solutions, respectively.NAD⁺ undergoes both diffusion- and adsorption-controlled reduction; the former predominates on slow time scale experiments and the latter on fast time scales.From low concentration surfactant (0.06 and 0.1 M tetraethylammonium (Tea⁺) chloride) solutions, NAD⁺ is only adsorbed positive of -0.65 V and an adsorption-controlled prewave appears, indicating that an adsorbed layer of NAD^. and/or (NAD)2 is formed on reduction of dissolved NAD⁺.From a high concentration (0.4 M) Tea⁺ solution, NAD⁺ is adsorbed positive of -0.66 V, but the adsorption-controlled prewave is suppressed and the reduction is entirely diffusion controlled.Under diffusion control, the heterogeneous rate constant for the title reduction is ca. 0.1 cm s^-1 and the rate constant for dimerization of NAD^. is ca. 3E6 M^-1 s^-1.