Electrochemical behaviour of proteins at graphite electrodes. I. Electrooxidation of proteins as a new probe of protein structure and reactions.

Electrochemical oxidation of proteins at a paraffin-wax-impregnated spectroscopic graphite electrode was studied by means of linear sweep and differential pulse voltametric techniques. It was found that proteins at the graphite electrode yield an oxidation current in the vicinity of potentials of 0.7-0.8 V in the range of neutral pH values. This current has the characteristics of adsorption currents. It was shown that irreversible electrooxidation of tyrosine or tryptophan residues is responsible for the occurrence of these protein currents. This previously undescribed electrochemical activity of proteins was exploited for the demonstration of the influence of urea on the accessibility of both tyrosine and tryptophan residues in protein molecules for interaction with their environment. It was also demonstrated that voltammetry at graphite electrodes permits an electrochemical analysis of proteins that do not contain cystine or cysteine residues.