Electrochemical detection of dipeptides with selectivity against amino acids

Electrolysis of a basic mobile phase containing biuret reagent [Cu(II) and a tartrate salt] at high (> 1.2 V vs. Ag/AgCl) potentials modifies the glassy carbon electrode. This modified anode oxidizes dipeptides, yielding signals expected for a one-electron transfer, even at low (down to 0.7 V vs. Ag/AgCl) potentials and in the absence of intentionally added copper(II) ion in the reagent or mobile phase. The same modification demonstrates a selectivity to alpha-dipeptides over amino acids that is unprecedented. The product of the anodic reaction is reduced at a downstream cathode at low positive potentials. Sensitivities for several amino acids and dipeptides are reported under several conditions. Neither the anodic nor the cathodic signals for the buiret complex of the tripeptide Ala-Ala-Ala are significantly altered because of the modification.