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: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.

Catalytic mechanism in successive two-step protein-film voltammetry—Theoretical study in square-wave voltammetry

“…In Refs. [34,36] we offered free files containing all details in respect to the definition of the potential signal in SWV, as well as facts about the calculation procedure in MATHCAD.…”

“…All theoretical calculations have been performed by using the MATHCAD software. A detailed description of the simulation procedure of some representative redox mechanisms in MATHCAD program is provided in our previous works [34,36], and one can use those files for free. Fig.…”

“…As a result, the voltammetric response of EEC′ protein-film mechanism converges to that of a two-step surface EE reaction. The features of the protein-film EE reaction under conditions of SWV are described in more details elsewhere [17,30,34,38].…”

“…A quite distinctive feature that is typical for all catalytic mechanisms studied under voltammetric conditions is the notable rising of both branches of the forward and backward currents at more negative potentials. This phenomenon is due to the unremitting effect of the catalytic reaction that supplies permanently electroactive material B during the whole measuring time in SWV [30,34]. Another remarkable feature by this mechanism is seen in the backward branch of the current, which gets the same sign as the forward current in the potential regions after the faradaic voltammetric response appears (see the features of the branches of both forward and backward currents at the left-side of the voltammograms in Fig.…”

“…The redox transformations of the aforementioned proteins can only be described by theoretical models considering successive two-step electron transfer. In the last few years, we started theoretical considerations of various redox mechanisms relevant to protein-film voltammetry, and we published scores of theoretical and experimental works in the past decade [17,21,22,25,[27][28][29][30][31][32][33][34]. In this paper we consider the surface catalytic mechanism that follows the two-step surface EE (electron-electron) redox reaction, which can be described by the following reaction scheme: The common designation of this complex protein-film redox mechanism in the electrochemical terminology is a surface EEC′, or surface EECat [13,14].…”