Electrochemical detection of dipeptides and dipeptide amides

A postcolumn reagent is used to create electroactive species from non-electroactive peptides. The reagent, based on the classical biuret reagent, consists of Cu(II), tartrate, bicarbonate and base. The detection is by dual electrode electrochemical detection. N-Acetylated dipeptides are oxidized at pH 12 and low potential. Dipeptides and carboxy terminal dipeptide amides give useful signals at lower pH. The dipeptide amides react with Cu(II) as do the tripeptides to yield the electronic spectrum and electrochemistry of the biuret complex. The complexes formed from the dipeptides are reversibly oxidized at potentials greater than 0.85 V vs. Ag/AgCl, 3 M NaCl. Analytically useful signals are obtained for the dipeptide amides at sensitivities equivalent to the sensitivities for longer peptides, 3-5 nA/microM, while the sensitivities for the dipeptides are about an order of magnitude lower.