Rotating ring-disk electrode study of copper(II) complexes of the model peptides triglycine, tetraglycine, and pentaglycine.

The reversible electrochemistry of the Cu(II)/Cu(III) couple was investigated for the copper(II) complexes of triglycine (G3), tetraglycine (G4), and pentaglycine (G5) in alkaline solution using a rotating ring-disk electrode (RRDE). The study was motivated by the need to elucidate electrochemical processes occurring in dual electrode postcolumn detection of peptides. The disk electrode served as the anode and the ring electrode as the cathode. The electrode was used in both linear sweep voltammetry and constant potential, varied rotation speed modes. Redox waves for two generic forms of the complexes, Cu(II)-NNNN and Cu(II)-NNNO, were identified, with respective E1/2 values of approximately 0.45 and 0.7 V. It was found that the G3 complex underwent an ECE-like process at the anode that magnified the anodic signal and suppressed the cathodic signal. The G4 and G5 complexes were subject to two CE processes, the C reactions being (i) deprotonation of Cu(II)-NNNO to Cu(II)-NNNN and (ii) loss of hydroxide ion by Cu(II)-NNN(OH-), a variant of Cu(II)-NNNO. An additional C reaction, dissociation of a carbonate variant of Cu(II)-NNNO, occurred in 0.2 M carbonate buffer. Visible absorbance measurements assisted in assignment of these forms. Measurements of diffusion coefficients of the complexes were performed by Taylor-Aris laminar flow axial dispersion measurements. The analytical implications for these findings are discussed.