Theoretical Analysis of Two-step EEC′ Mechanism in Square-Wave Voltammetry: Application to Water Soluble Redox Systems with Inverted Potentials

Electrochemical systems with inverted redox potentials, where the second electron transfer requires less energy than the first one, often challenge conventional interpretations of sequential redox processes in biomolecules and related complexes. These systems commonly produce a single peak under voltammetric conditions, mimicking a concerted two-electron transfer, thereby obscuring the true stepwise nature of the redox transformation. Through theoretical analysis of a two-step electrode process coupled with a regenerative chemical reaction (the so-called EEC′ mechanism), we demonstrate that increasing the rate of the chemical regeneration step significantly alters the voltammetric response. Notably, this enhanced kinetics induces a negative shift in the potential of the second electron transfer process, eventually resolving the two electron transfer events that otherwise appear merged. Square-wave voltammetry simulations of a diffusional EEC′ mechanism reveal that altering the concentration of the regenerative agent "Y" is essential to achieve this resolution under inverted potential conditions. These findings highlight the critical role of chemical kinetics in shaping voltammetric behavior and provide a powerful framework for studying complex redox systems, including biologically relevant cofactors like iron–sulfur clusters, quinones and flavonoids.