Electrochemical studies of the effect of temperature on the adsorption of yeast alcohol dehydrogenase at Pt

The interfacial behavior of yeast alcohol dehydrogenase (YADH) without and with the coenzyme nicotinamide adenine dinucleotide (NADH-YADH) at a Pt sur face was studied over the temperature range 273-353 K in a phosphate buffer solution of pH 7.0, using cyclic voltammetry and electrochemical impedance spectroscopy. It was shown that the surface charge density and correspondi ng polarization resistance, resulting from protein adsorption and its oxida tion, respectively, are directly proportional to the amount of adsorbed pro tein (surface concentration), indicating that adsorption at anodic potentia ls is accompanied by the transfer of charge, that is, chemisorption through carboxylate groups on the protein. The adsorption process for both protein s was described with the Langmuir adsorption isotherm, which revealed very high affinity of the proteins toward adsorption onto a Pt surface. From the calculated Gibbs energies of adsorption, it was concluded that both protei ns strongly adsorb onto the Pt surface via chemisorption. The adsorption pr ocess for YADH was found to be exothermic. However, the adsorption of NADH- YADH resulted in an endothermic adsorption process as a result of the prese nce of the coenzyme, nicotinamide adenine dinucleotide (NAD(+)/NADH), in NA DH-YADH which stabilizes the protein against denaturation. However, the ads orption of both proteins was found to be an entropically governed process, suggesting structural unfolding of the proteins at the electrode surface. T he maximum surface concentration values indicate that there is no significa nt difference in the amount of adsorbed proteins between YADH and NADH-YADH in the whole temperature range investigated.