(Invited) Electrochemical Studies of Zinc/Cysteine Interactions in Aqueous Buffers

Abstract

Introduction The naturally occurring amino acid L-cysteine is involved in many important biochemical processes, including the formation of “zinc finger” proteins (1). The electroactive thiol group in cysteine allows electrochemical investigation of its properties and interactions with other substances. The present contribution deals with the complexation of L-cysteine with zinc(II) ions in various buffered aqueous media and is a continuation of work previously presented (2). Experimental L-Cysteine was obtained from Sigma-Aldrich Corporation. Electrochemical experiments were carried out under nitrogen using a Gamry Instruments Interface 1000 potentiostat and Framework software. Working electrodes were obtained from BASi (Glassy carbon, 3.0 mm diameter; platinum 1.6 mm) and eDAQ (gold, 1.0 mm diameter). Potentials were measured with respect to a silver/silver chloride saturated KCl reference electrode (BASi). Results and Discussion Recent work from this laboratory has shown that the interaction of zinc(II) ions, added as ZnSO4 heptahydrate, with L-cysteine can be followed by cyclic voltammetry (Figure 1). This work is intended as a model for the formation of “zinc finger” proteins, which involve interactions between zinc(II) and cysteine and histidine units in the protein structure (1). Figure 1 (a) shows the voltammogram for 8.6 mM L-cysteine in 0.1 M pH 7.4 phosphate buffer at glassy carbon. Upon addition of ZnSO4 to L-cysteine in a 1:2 Zn2+: Cysteine mole ratio [Figure 1(b)], the broad oxidation process underwent a significant shift to a more positive potential. This observation is consistent with the complexation of Zn(II) by L-cysteine. Upon sweeping to more negative potentials, reduction of the complex was found to occur near the solvent reduction limit, followed by zinc stripping at approximately -1.0 V vs Ag/AgCl. Interesting comparisons with results at gold electrodes (2) can also be made. Further details for the complexation process are also planned for presentation. Other investigations have involved electrochemical impedance spectroscopy (EIS) to determine the extent to which L-cysteine interacts with various electrode surfaces. References C.K. Mathews, K.E. Van Holde, D.R. Appling, and S. J. Anthony-Cahill, Biochemistry, 4th Edition, Pearson Canada, Toronto, 2013. G. T. Cheek and M. A. Worosz, ECS Transactions, 2016, 72(27), 1-8. Figure 1. Cyclic voltammograms at glassy carbon in aqueous pH 7.4 phosphate buffer, 100 mV/s scan rate. Solid black line : L-Cysteine 8.6 mM, initial positive-going sweep Dashed red line : ZnSO4 . 7 H2O 4.3 mM addition, to previous solution, initial positive-going sweep Figure 1