ABSTRACT
Most amino acids are electroactive by being susceptible to oxidation. However, their oxidation potentials at carbon or bare metal electrodes are too high (>+1.0 V vs. Ag/AgCl [3 M NaCl]) to be useful for trace determinations, and their electrochemistry will not be described here. Despite this, several di-and tripeptides without tryptophan, tyrosine, cysteine, and cystine can still be oxidized at potentials suitable for trace determinations (<+1.0 V vs. Ag/AgCl [3 M NaCl]). For instance, several dipeptides and tripeptides containing alanine, arginine, aspartic acid, glutamic acid, glycine, leucine, lysine, and valine have been oxidized at a glassy carbon electrode used as detector in an HPLC system [110, 111]. The electrochemistry is, however, unclear. Improved sensitivity and selectivity for α-dipeptides (over β-and γ-dipeptides and amino acids) can be obtained by anodic pretreatment of the glassy carbon electrode at +1.2 to +1.4 V versus Ag/AgCl (3 M NaCl) in a solution containing Cu(II) and tartrate [110, 111]. The changed electrochemical behavior of the glassy carbon electrode seems to be due to an increase in the oxygen to carbon ratio at the surface rather than formation of copper-containing species on the surface.
