Previous studies (1-3) in this laboratory have involved electrochemical studies of the interaction of Zn(II) ions with L-cysteine in an attempt to more fully characterize the formation of “zinc finger” proteins (4). This interaction can be investigated both by the effect of Zn(II) addition upon the oxidation of the thiol group of L-cysteine, and by the effect of L-cysteine addition upon the reduction potential of Zn(II). In particular, the addition of L-cysteine to ZnSO₄ dissolved in pH 7.4 MOPS buffer results in a large negative shift in the reduction potential for Zn(II) ions (2). This behavior is clear evidence for the formation of a Zn²⁺ : Cysteine complex. In the present study, we have extended this work to the interaction of bismuth(III) with L-cysteine and with glutathione, prompted by the significance of such interactions in human biochemistry (5-7).

Experimental

L-Cysteine, L-glutathione, and MOPS (3-(N-morpholino)propanesulfonic acid) were obtained from Sigma-Aldrich Corporation, and bismuth(III) nitrate was obtained from Baker. Electrochemical experiments were carried out under nitrogen using a Gamry Instruments Interface 1000 potentiostat and Gamry 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

It has been found that Bi(NO₃)₃ is only slightly soluble in pH 7.4 MOPS buffer due to probable formation of hydroxyl complexes (8). It is, however, possible to observe small currents for deposition and subsequent stripping of bismuth at gold and glassy carbon in this solution. Upon addition of 1:1 bismuth(III):L-cysteine, the solution appearance changed from cloudy to clear, and voltammetric currents increased substantially, indicating a strong interaction between bismuth(III) and L-cysteine. In addition, a negative voltammetric shift for the bismuth reduction potential was observed, as was the case for the zinc(II):L-cysteine interaction (2). Further additions of L-cysteine were found to produce additional negative shifts for bismuth(III) reduction. Similar behavior for L-glutathione was observed. MALDI-TOF spectra for the 1:2 Bi(III):L-glutathione and similar complexes in aqueous solutions have been reported (9) and provide supporting evidence for the electrochemical results in the present study. A similar spectrum for the 1:2 Bi(III):L-glutathione complex in pH 7.4 MOPS buffer has been obtained in the present work.

References

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