Electrochemical oxidation reactions of tyrosine, tryptophan and related dipeptides

The electrochemical oxidation reactions of tyrosine, tryptophan, tyrosyl-glycine and tryptophyl-glycine were investigated at a platinum electrode in a phosphate buffer (pH 7.0). Cyclic voltammetry was used to examine the adsorption behaviour of these compounds. Strong adsorption was displayed by all four species at the electrode surface, and it appears that at low anodic potentials the amino acids are arranged predominantly in a horizontal position, but rearrange to a vertical configuration to allow for tighter packing at the higher potentials. Steady-state polarization measurements gave Tafel slopes in the range of 303-359 mV above the inflection point, considerably higher than those 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. Cleavage of the dipeptide occurred in electrolysis experiments at potentials of 1.8-2.2V (E(SCE)), as determined by HPLC product analysis. However, quantitative measurements indicated that cleavage of the dipeptide was not the predominant pathway in the anodic oxidation process. (C) 1997 Elsevier Science Ltd. All rights reserved.