Adsorption phenomena in the NAD+/NADH system at glassy carbon electrodes

Abstract

A variety of electrochemical approaches has been used to investigate the adsorption of NAD+, NADH and the NAD-NAD dimer from aqueous solution at glassy carbon electrodes (GCE) with supplementary studies of adsorption at pyrolytic graphite and platinum electrodes from aqueous media and at GCE from DMSO solution. The following hypotheses are advanced concerning the adsorption orientation: at carbon electrodes, on which NADH is not adsorbed, NAD+ produced by anodic oxidation of the NADH is first rapidly adsorbed in a planar configuration relative to the electrode surface, which is probably bound to the surface through the adenine moiety; there is the a relatively slow reorientation of the adsorbed NADH molecules to a perpendicular orientation relative to the electrode surface, which adsorbate is more tightly bound to the surface than the planar oriented adsorbate and which likely involves interaction between parallel adenine and pyridinium rings. Reduction (one-electron process) of NAD+ at the GCE produces the NAD-NAD dimer, which, at a clean electrode surface, involves a diffusion-controlled process and an adsorption-controlled process; the latter is due to formation of adsorbed dimer, which is more strongly adsorbed than NAD+. The dimer is oxidized at the GCE only if it is adsorbed. The factors controlling and involved in the adsorption processes have been examined with particular reference to the use of anodic voltammetry for the analytical determination of NADH.