L-Cysteine is an important amino acid because of its role in various biochemical pathways (1).  Considering that L-cysteine is one of the few amino acids with significant redox characteristics, it was considered worthwhile to investigate its electrochemical behavior in various media and electrode surfaces.

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.

Results and Discussion

    At a platinum electrode, L-cysteine in aqueous 0.10 M KNO₃  produced a cyclic voltammogram with two merged oxidation processes (Figure 1), which were much more clearly resolved using differential pulse voltammetry. Preparative electrolysis at the potential of the first oxidation process (+0.80 V) resulted in the precipitation of cystine, which is the coupling product formed as a result of one-electron L-cysteine oxidation.  The current for the reduction peak at -0.60 V (dotted line in Figure 1) increased during the electrolysis, suggesting that this reduction process is related to cystine formation.  Similar results, including cystine formation, were also obtained in aqueous phosphate buffer at pH 7.  It is likely that the platinum oxide layer on the electrode surface plays a role in L-cysteine oxidation under these conditions (2).  The results of additional studies of L-cysteine oxidation in other media, particularly dimethylsulfoxide (DMSO), are also planned for presentation.

References

1. C. K. Mathews, K. E. Van Holde, D. R. Appling, and S. J. Anthony-Cahill, Biochemistry, 4^th Edition, Pearson Canada, Toronto, 2013.
2. J. Koryta and J.  Pradac, J. Electroanal. Chem., 17, 185-189 (1968).

Figure 1.  Cyclic voltammogram of 10 mM L-cysteine at platinum in aqueous 0.10 M  KNO₃. 100 mV/s.

                First sweep (solid line),  second sweep (dotted line).