TAFEL BEHAVIOR AND COMPETITIVE ADSORPTION OF AMINO-ACIDS AND PEPTIDESWITH SURFACE OXIDE FORMATION AT THE PT ELECTRODE

The electrochemical oxidation reactions and competitive adsorption beh aviour of glycine, the peptides glycyl-glycine and glycyl-glycyl-glyci ne, alpha- and beta-alanine, and alpha-, beta-, and gamma-aminobutyric acid have been investigated by cyclic voltammetry at a platinum elect rode in a pH 7.0 phosphate buffer solution at ambient temperature. Str ong adsorption of all species at anodic potentials in neutral solution was accompanied by an efficient ability to block both the initial sta ges of OH monolayer deposition as well as the second stage involving a n irreversible process of continued deposition of OH or O species. Gly cine and the peptides blocked the initially and more reversibly deposi ted OH species in preference to those formed at higher potentials. Sur face 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 prefe rentially adsorbs under these anodic potentials in a mechanism similar to the Kolbe reaction, with the decarboxylation step being rate deter mining.