Electrochemical Studies of Cysteine

Introduction L-Cysteine is an important amino acid because of its role in various biochemical pathways (1). The sulfhydryl functional group gives L-cysteine significant redox characteristics, prompting an investigation of its electrochemical behavior in various media and electrode surfaces. The interaction of protein cysteine residues with Zn2+ (“zinc fingers”) also suggested a voltammetric study of the effect of Zn2+ on L-cysteine electrochemistry. Experimental L-Cysteine was obtained from Sigma-Aldrich. Electrochemical experiments were carried out using a Gamry Interface 1000 potentiostat and Gamry Framework software. Potentials are referenced against a Ag/AgCl reference electrode. Platinum and glassy carbon electrodes were obtained from BASi. Gold electrodes were purchased from eDAQ. Results and Discussion The voltammetric behavior of 5 mM Zn2+ in 0.10 M KNO3 is shown in the figure. Zinc deposition at -1.0 V was not very distinct, and there was no well-defined stripping peak. Addition of equimolar L-cysteine produced a definite stripping peak at -0.8 V, evidently an effect of complexation of Zn2+ by L-cysteine. The L-cysteine voltammetry was also affected by the presence of Zn2+. At platinum and gold with no addition of Zn2+, oxidation of L-cysteine occurred as merged peaks at +0.8 and +1.0 V (2), with no oxidation feature at +0.6 V. Even with Zn2+ added to L-cysteine n solution, an initial positive-going scan gave no +0.6 V peak, and scanning through the zinc reduction/stripping region was necessary to produce the +0.6 V feature on the second sweep to more positive potentials as shown in the figure. The stripping of the zinc layer from the gold surface may produce a renewed gold surface which allows strong adsorption of L-cysteine on gold. In general, L-cysteine is known to interact strongly with gold surfaces (3). References C. K. Mathews, K. E. Van Holde, D. R. Appling, and S. J. Anthony-Cahill, Biochemistry, 4th Edition, Pearson Canada, Toronto, 2013. J. Koryta and J. Pradac, J. Electroanal. Chem., 1968, 17, 185-189. A. J. Tudos, P. J. Vandeberg, and D. C. Johnson, Analytical Chem., 1995, 67, 552-556. Figure 1. Cyclic voltammograms at gold (1 mm diameter) in aqueous 0.10 M KNO3. 100 mV/s. [ Dashed line : 5.0 mM ZnSO4 ] [ Solid line : 5.0 mM ZnSO4 + 5.3 mM L-cysteine ] Figure 1

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