The amino acid L-cysteine has been studied because of its role in several biochemical pathways (1). Electrochemical studies are possible due to the redox characteristics of the sulfhydryl group, so that the electrochemical characteristics of L-cysteine can be determined by voltammetric methods (2). In this study, the biochemical importance of zinc(II)/cysteine complexes in proteins (“zinc fingers”) has prompted an investigation of the interaction of ZnSO4with L-cysteine in various media.
Experimental
L-Cysteine was obtained from Sigma-Aldrich Company, and ZnSO4.7 H2O was obtained from ThermoFisher Scientific. A Gamry Interface 1000 potentiostat with Gamry Framework software was used to carry out electrochemical experiments. Potentials are referenced against a Ag/AgCl reference electrode. Platinum and glassy carbon electrodes were purchased from BASi. Gold electrodes were obtained from eDAQ.
Results and Discussion
In a study of the complexation of Zn2+ by L-cysteine in pH 7.4 phosphate buffer, it was found that addition of ZnSO4 . 7 H2O to the buffer solution produced a cloudy appearance due to the low solubility of the resulting Zn3(PO4)2 . Upon addition of L-cysteine, the system was still cloudy at the 1:1 L-cysteine : Zn2+ point; however, at the 2:1 L-cysteine : Zn2+ point, the solution became clear. This behavior evidently occurs because the interaction of Zn2+ with L-cysteine is stronger than is its interaction with phosphate. These results are consistent with Zn(cys)2 complexation reported in the literature (3). The corresponding voltammograms at glassy carbon are shown in Figure 1. For ZnSO4 addition, the scan showed only a very small zinc stripping peak at the negative end of the potential range. At the 1:1 L-cysteine : Zn2+ point, processes are clearly evident for zinc deposition/stripping, and oxidation of complexed L-cysteine is observed at +0.7 V vs Ag/AgCl. This oxidation process is slightly altered compared to that observed for only L-cysteine (no added Zn2+) (2). Similar behavior for the oxidation processes was also observed at gold. Further additions of L-cysteine gave larger currents for the L-cysteine oxidation process. These results support the uptake of zinc ion into the solution as L-cysteine interacts with Zn2+, initially added as ZnSO4 . 7 H2O in pH 7.4 phosphate buffer.
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.
S. Foley and M. Enescu, Vibrational Spectroscopy, 2007, 44, 256-265.
Figure 1. Cyclic voltammograms at glassy carbon (3 mm diameter) in aqueous pH 7.4 phosphate buffer, 100 mV/s scan rate.
[ Dashed red line : ZnSO4 . 7 H2O, 2.1 mM nominal concentration ]
[ Solid black line : Addition of 2.4 mM L-cysteine to previous solution ]