Electrocatalytic behavior of substituted cobalt phthalocyanines towards the oxidation of cysteine

Ring substituted cobalt phthalocyanine complexes of the form CoPc(X)(4), (w here X = NH2, NO2, C(CH3)(3), SO3H and COOH) are compared for their catalyt ic activities towards the oxidation of cysteine. The potential for the elec trocatalytic oxidation of cysteine is closely related to the Co-III/Co-II c ouple of the CoPc(X)(4) complexes in acidic media and to the Co-II/Co-I cou ple in basic media. The catalytic currents and the oxidation potential for cysteine are dependent on the pH of the solution, the potential becoming le ss positive as the pH increases and the catalytic currents decreasing with increase in pH, for the same concentration of cysteine. The magnitudes of t he catalytic currents (after ten scans) for cysteine oxidation at pH 8.3 an d on electrodes modified with the CoPc(X)(4) complexes increased with the n ature of the ring substituent as follows: NO2 < NH2 < SO3H < C(CH3)(3) < CO OH. (C) 2000 Elsevier Science B.V. All rights reserved.