Electrochemical characteristics of amino acids and peptides derivatized with naphthalene-2,3-dicarboxaldehyde: pH effects and differences in oxidation potentials

The electrochemical oxidations of the cyanobenz[f]isoindole (CBI) derivatives of 18 amino acids and 15 peptides, including enkephalins and several enkephalin fragments, were studied. Cyclic voltammetry indicated that the oxidation potentials of derivatized amino acids were virtually independent of pH. The utility of this pH-independence was demonstrated by controlling, through varying the pH, the selectivity with which CBI derivatives could be detected in the presence of phenolic compounds. In addition, hydrodynamic voltammograms of derivatized amino acids and peptides were constructed from chromatographic data and compared. The E1/2 values among the derivatized amino acids covered a range of 215 mV, with the derivatives of the basic amino acids being the easiest to oxidize and those of the acidic amino acids being the most difficult to oxidize. The E1/2 values of the derivatized peptides examined varied by 270 mV, with minor variations in structure capable of producing marked changes in oxidation potential. These results indicate that voltammetry can aid in identification or selective detection of CBI derivatives of amino acids or peptides.