Influence of tyrosine on the dual electrode electrochemical detection of copper(II)-peptide complexes

The bluret reaction makes peptides electrochemically active. This is the basis of an electrochemical method for the detection of peptides following their liquid chromatographic separation. This paper discusses the influence of tyrosine, an electroactive amino acid, on the detection of Cu(II)-peptide (bluret) complexes containing it. The dual electrode detector has an upstream anode and a downstream cathode. Tyrosine-containing peptides yield anodic signals that are approximately the sum of the tyrosine signal and the signal from the bluret complex of a nonelectroactive model peptide, phenylalanylglycyl-glycine (FGG). The cathodic signal is depressed in comparison to FGG. This is traced to the presence of an intramolecular reaction between Cu(III) and a reaction product resulting from the oxidation of the tyrosinyl residue. The rate constant for the corresponding intermolecular reaction is significant (10(6)-10(7) M-1 s-1), but in practical analytical situations, the concentrations of the reactants will be small, so the reaction will not be a major factor. Sensitivities for several bioactive peptides are reported. The dependence of the signals on the position of tyrosine in a tripeptide is also studied.