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.
Damian
Alexis
A
Department of Chemical Engineering, McGill University, Montreal, Quebec H3A 2B2, Canada.
Omanovic
Sasha
S
D000327:Adsorption; D004563:Electrochemistry / Q000379:methods*; D004566:Electrodes; D006046:Gold / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D008968:Molecular Conformation; D009243:NAD / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D013499:Surface Properties; D013696:Temperature; D013816:Thermodynamics; D013997:Time Factors
D009243:NAD; D006046:Gold; D010100:Oxygen