Interactive adsorption behavior of NAD+ at a gold electrode surface

The adsorption of an oxidized form of nicotinamide adenine dinucleotide, NAD(+), on a polycrystalline gold electrode surface and the subsequent surface conformation of the molecule were investigated over a wide temperature and potential range, using electrochemical differential capacitance and PM-IRRAS techniques. The adsorption process was described by the Langmuir adsorption isotherm. The corresponding thermodynamic parameters were determined: the Gibbs energy, enthalpy, and entropy of adsorption. The large negative Gibbs energy of adsorption (-43 +/- 4 kJ mol(-1) and -39 +/- 2 kJ mol(-1) on a positively and negatively charged surface, respectively) confirms that the NAD(+) adsorption process is highly spontaneous, while the large entropy gain (285 J K-1 mol(-1) and 127 J K-1 mol(-1) on a positively and negatively charged surface, respectively) was found to represent the adsorption driving force. It was demonstrated that the energetics of the adsorption process is surface-charge controlled, while its kinetics is both mass-transport and surface-charge controlled. A surface-charge dependent conformation model for the adsorbed NAD(+) molecule is proposed. These findings suggest that the origin of the NAD(+) reduction overpotential is related to the surface conformation of the adsorbed NAD(+) molecule, rather than to the electrode Fermi level position.