| Abstract
| - The adsorption of an oxidized form of nicotinamide adenine dinucleotide, NAD+, on a polycrystalline gold electrodesurface and the subsequent surface conformation of the molecule were investigated over a wide temperature andpotential range, using electrochemical differential capacitance and PM-IRRAS techniques. The adsorption processwas 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 kJmol-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 ona positively and negatively charged surface, respectively) was found to represent the adsorption driving force. It wasdemonstrated that the energetics of the adsorption process is surface-charge controlled, while its kinetics is bothmass-transport and surface-charge controlled. A surface-charge dependent conformation model for the adsorbedNAD+ molecule is proposed. These findings suggest that the origin of the NAD+ reduction overpotential is relatedto the surface conformation of the adsorbed NAD+ molecule, rather than to the electrode Fermi level position.
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