The voltammetry of neuropeptides containing L-tyrosine

Neuropeptides containing an N-terminal tyrosine are irreversibly oxidized on glassy carbon electrodes using scan rates from 0.020 to 0.500 V/s. Despite variations in peptide length and amino acid composition from tyrosine to alpha-neoendorphin 1-10, peak potentials occurred in a relatively narrow range from +0.925 V to +0.971 V (vs. Ag/AgCl). An inverse linear relationship of E(p) vs. pH with a slope near -59 mV/pH unit suggests that the oxidation involves a one electron oxidation and a single proton transfer similar to the oxidation of simple para-substituted phenols. Linear slopes between 0.790 and 0.933 of log I-p vs. log upsilon suggest that surface processes contribute to the voltammetry on glassy carbon.