Estimation of Coupling Rate Constant ofL-Cysteine Radical Studied by Concentration-Step Coulometry Using Porous Carbon Felt Electrode Impregnated with an Electrolyte

Abstract

Controlled potential coulometry using carbon felt electrode impregnated with electrolytic solution realizes very rapid complete electrolysis and can be used to measure the faster reaction rate constant than that using conventional electrolytic cell. In this research, concentration step method was adopted to investigate coupling reaction rate of L-cysteine radical. The coupling reaction rate of L-cysteine radical becomes much larger than further electrode reaction rate of L-cysteine radical at high L-cysteine concentration, because the coupling reaction rate is proportional to the second order of L-cysteine radical concentration although the further electrode reaction rate is proportional to the first order of L-cysteine radical concentration. At a low constant potential value, apparent number of electrons (napp) increased from 1 (L-cystine is produced) to 2 (L-cysteine sulfenic acid, RSOH, may be produced) according to decrease in concentration of L-cysteine to be electrolyzed. The second order rate constant of coupling reaction was estimated to be about 1200 dm3 mol−1 s−1 at 20 °C by curve fitting method for napp vs. logarithm of L-cysteine concentration. Apparent number of electrons (napp) consumed in the electrode oxidation of L-cysteine gradually increased as an applied potential increases, because the consecutive electrode reaction steps with different electrode reaction rates were involved in the electrode oxidation of L-cysteine. In the present method, the constant limited electrolytic current was observed at high electrode potential range, which suggests that electrode oxidation rate of L-cysteine is kinetically controlled.