Molecular orientation and dynamics of ferricyanide ion-bearing copoly(ionic liquid) modified glassy carbon electrode towards selective mediated oxidation reaction of cysteine versus ascorbic acid: A biomimicking enzyme functionality

Abstract

Understanding the electron-transfer (ET) functionality of enzymes in relation to their molecular orientation and dynamics is a cutting-edge research interest in the interdisciplinary areas of chemistry and biology. In this work, we demonstrate how the molecular structure and orientation of a redox polymer influence the ET behavior and specific catalytic functionality to target substance, ascorbic acid (AA) or cysteine (CySH) in a physiological condition. In the literature, Fe(CN)63−, a well-known benchmarking redox system, based chemically modified electrodes (CME) prepared by the ion-exchange method, has been widely used as an electrocatalyst for ascorbic acid oxidation reaction without the interference of CySH. In this work, a Fe(CN)63− bearing copoly(ionic liquid)-ethanol solution modified glassy carbon electrode, designated as GCE@{Fe(CN)63−}-coPIL, prepared as a homogenous solution followed by CME formation, has shown a unique and selective CySH oxidation reaction, rather than expected AA mediation, similar to the Thiol Oxidase enzyme-based biomimetic functionality. Andrieux and Saveant and Michaelis-Menten kinetic models were adopted to explain the reaction mechanism. The polymer network orientation and dynamics are found to influence strongly on the electrocatalytic functionality of the new CME. It has been revealed that underlying surface, functional groups, and solvent nature impact the molecular architectures of the {Fe(CN)63−}-caped polymer back-bone (insulating) network as a fully (H2O and CHCl3) or partially (C2H5-OH and CH3CN) shielded structures and decide its electron-transfer and selective mediated oxidation reaction functionalities. This observation is similar to the protein folding of enzymes at various external stimuli conditions and its unfolded structures for direct electron transfer and selective mediated oxidation/reduction reaction. As a proof of concept, electroanalytical application of thiol functional group (CySH) oxidation to disulfide bond (CyS-SyC) catalyzed by the GCE@{Fe(CN)63−}-coPIL in neutral pH solution has been demonstrated.