Tafel behaviour and competitive adsorption of amino acids and peptides with surface oxide formation at the Pt electrode

Abstract

The electrochemical oxidation reactions and competitive adsorption behaviour of glycine, the peptides glycyl-glycine and glycyl-glycyl-glycine, α- and β-alanine, and α-, β-, and γ-aminobutyric acid have been investigated by cyclic voltammetry at a platinum electrode in a pH 7.0 phosphate buffer solution at ambient temperature. Strong adsorption of all species at anodic potentials in neutral solution was accompanied by an efficient ability to block both the initial stages of OH monolayer deposition as well as the second stage involving an irreversible process of continued deposition of OH or O species. Glycine and the peptides blocked the initially and more reversibly deposited OH species in preference to those formed at higher potentials. Surface concentrations determined with the lower anodic end potentials (0.50 V) gave information on the orientation of the adsorbed molecules. Steady-state polarization measurements gave Tafel slopes in the range of 253–498 mV, considerably higher than that normally seen for oxygen evolution from a phosphate buffer solution. These results suggest that, in the pH 7.0 buffer, the negatively charged carboxylate group preferentially adsorbs under these anodic potentials in a mechanism similar to the Kolbe reaction, with the decarboxylation step being rate determining. Keywords: electrochemical oxidation, glycine, alanine, aminobutyric acid, peptides, platinum oxide.