Electrocatalysis of metalloporphyrins: Part 9. Catalytic electroreduction of cystine using water-soluble cobalt porphyrins

Abstract

The electrocatalytic reduction of cystine with water-soluble cobalt(II) tetrakis(4-trimethylammoniumphenyl)porphyrin (CoTTAPP) and cobalt(II) tetrakis(4-sulphophenyl)porphyrin (CoTSPP) in 0.2 M H 2SO 4 solution was investigated. The experimental results of cyclic voltammetry and the rotating disc electrode technique indicated that both CoTTAPP and CoTSPP obviously exhibit catalytic activity for cystine reduction in 0.2 M H 2SC 4 solution. Cystine reduction at a glassy carbon electrode in 0.2 M H 2SO 4 solution containing CoTTAPP or CoTSPP is an irreversible, simple charge-transfer reaction. The number of electrons involved in the rate-determining step is 1. The rate constants of cystine reduction in these systems were calculated to be 2.9 × 10 −3−7.3 × 10 −3 cm s −1 in the potential range of −0.80 to −0.90 V (vs. SCE). It is demonstrated that Co(II) complexing in the N 4 internal ring of porphyrin plays the main role in the electrocatalytic reduction of cystine. The reaction mechanism of cystine reduction electrocatalysed by CoTTAPP or CoTSPP in 0.2 M H 2SO 4 solution is discussed.