An electrochemical study of NAD+ interaction with a polycrystalline gold surface /

The interaction of an oxidized form of nicotinamide adenine dinucleotide, NAD+, with a polycrystalline gold electrode surface was investigated using a range of electrochemical and spectroscopic techniques. It was shown that the reduction of NAD+ on gold is irreversible and occurs at cathodic overpotentials larger than -0.2 V. The NAD+ reduction reaction was found to be mass-transport controlled, and of first order with respect to NAD+. A set of kinetic parameters was calculated and verified independently using various experimental techniques: the apparent transfer and diffusion coefficients, the apparent formal heterogeneous electron-transfer rate constant, and the apparent Gibbs energy of activation.It was determined that the adsorption of NAD+ in the double layer region (-0.5 to 0.4 V) of the Au electrode is irreversible. The adsorption process of NAD+ was well described by the Langmuir adsorption isotherm. The corresponding thermodynamic parameters were determined: Gibbs free energy, enthalpy and entropy of adsorption. Their values indicate that the NAD+ adsorption process is spontaneous and endothermic under the conditions applied. The driving force for the adsorption represents a positive gain in entropy, which is related predominantly to the water displacement from the double layer, and also to conformational changes of NAD+ on the gold surface. The kinetics of NAD+ adsorption was found to be both surface charge and mass-transport controlled. Based on ex situ PM-IRRAS measurements, a surface-charge-dependent conformation model for the adsorbed NAD+molecule was proposed.Experiments done in a batch electrochemical reactor confirmed that an amount of enzymatically active NADH produced on a bare Au surface depends on the regeneration potential, and varies from 28 % at high overpotentials (higher conversion, faster kinetics) up to 78% at low overpotentials (low conversion, slow kinetics). In order to increase the amount of enzymatically active NADH formed at high (industrially relevant) overpotentials, the Au surface was modified with Pt nano-islands, which resulted in an increase in the amount of NADH from 28 % to 63 %. A reaction mechanism taking into account the influence of Pt nano-islands was proposed