Study of the adsorption of a hydrophobic peptide onto carbon surfaces by capacitance measurements

Abstract

The adsorption of a hydrophobic peptide (β-CN (193–209)) onto glassy carbon and a carbon support of membrane was studied by double-layer capacitance measurements using electrochemical impedance spectroscopy (SIE). The kinetics of adsorption was investigated by recording the changes that occurred in the double-layer capacitance after adding the peptide. The change in capacitance was interpreted in terms of the number of close-contact areas between the peptide and the surface. A two-consecutive reaction model was used to describe peptide adsorption. The first step was attributed to the adsorption reaction itself and the second to a change in the conformation of the adsorbed peptide molecules. The corresponding theoretical equation of the variation of capacitance with time fits our results. Moreover, it allowed determination of the time constants for both reactions and three double-layer capacitances: the bare surface and the surfaces covered with monolayers of the peptide in states 1 and 2. For glassy carbon, the activation energies are 4.7 and 3.5 kJ mol−1 for the first and second step of the reaction and the reaction orders are 0.64 and 0.35, respectively. These values are different for each step. This confirms the model of two separate reactions to describe adsorption. The first step comprises two elementary mechanisms. The first may correspond to the adsorption of the peptide onto the surface. The second may be consistent with a modification of the adsorbed molecules induced by the adsorption of other molecules over them. The second step of the process requires the contribution of peptides of the solution for it to occur. The changes in the double-layer capacitance depend on the peptide bulk concentration. This suggests that the peptide adsorbs in a different conformation depending on its bulk concentration. Peptide polymerisation could be responsible for this phenomenon.