ELECTROCHEMICAL OXIDATION REACTIONS OF TYROSINE, TRYPTOPHAN AND RELATED DIPEPTIDES

The electrochemical oxidation reactions of tyrosine, tryptophan, tyros yl-glycine and tryptophyl-glycine were investigated at a platinum elec trode in a phosphate buffer (pH 7.0). Cyclic voltammetry was used to e xamine the adsorption behaviour of these compounds. Strong adsorption was displayed by all four species at the electrode surface, and it app ears that at low anodic potentials the amino acids are arranged predom inantly in a horizontal position, but rearrange to a vertical configur ation to allow for tighter packing at the higher potentials. Steady-st ate polarization measurements gave Tafel slopes in the range of 303-35 9 mV above the inflection point, considerably higher than those normal ly seen for oxygen evolution from a phosphate buffer solution. These r esults suggest that in the pH 7.0 buffer, the negatively charged carbo xylate group preferentially adsorbs under these anodic potentials in a mechanism similar to the Kolbe reaction, with the decarboxylation ste p being rate determining. Cleavage of the dipeptide occurred in electr olysis experiments at potentials of 1.8-2.2V (E(SCE)), as determined b y 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 re served.