Surface ECE mechanism in protein film voltammetry—a theoretical study under conditions of square-wave voltammetry

Abstract

For the first time, the features of a surface electron transfer–chemical reaction–electron transfer (ECE) mechanism, relevant to protein-film set-up, have been studied theoretically under conditions of square-wave voltammetry. The considered surface ECE mechanism is presented by following reaction scheme:$A_{\left( {{\text{adsorbed}}} \right)} + ne^ - \rightleftarrows B_{\left( {{\text{adsorbed}}} \right)} + Y\xrightarrow{{k_{\text{f}} }}C_{\left( {{\text{adsorbed}}} \right)} + ne^ - \rightleftarrows D_{\left( {{\text{adsorbed}}} \right)}$. The mathematical solutions of this complex redox mechanism are given in form of integral equations, and they can be applied to any chronoamperometric technique. Attention is given to two frequently met situations: (a) case where the energy for the reduction in the second electron transfer step is lower or equal to that of the first reduction step and (b) case where the energy for the reduction of the second electron transfer step is much higher than that of the first reduction step. The theoretical square-wave voltammograms feature various shapes, depending mainly on the energy difference between the two electron transfer steps, but they also depend on the kinetics of the first and the second electron transfer, as well as on the rate of the chemical reaction. Hints are given for qualitative recognition of the surface ECE mechanism and for its distinguishing from similar surface redox systems. Reliable methods are proposed for the estimation of kinetic parameters of the electron transfer steps and that of the chemical reaction. Since many biological compounds undergo this redox mechanism, the theoretical results presented in this work can be of help for the people dealing with organic electrochemistry or protein-film voltammetry.