Electrochemical Studies of Amino Acid: Metal Ion Interactions
Authors: Choi Pyung O. , Clark Dexter C. , Cheek Graham T. 
Published: February 2020

Introduction
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). Subsequent work has involved similar investigations of L-histidine (5), and the work has now been extended to L-tryptophan. Clear evidence for the formation of Zn(II) complexes with L-cysteine and L-histidine has been obtained by cyclic voltammetric peak potential shifts for zinc ion reduction (2, 5). In the present study, we have extended this work to L-tryptophan by means of voltammetric studies of the amino acid and by investigation of its interactions with metals such as zinc and copper. Results for the interaction of bismuth(III) with L-cysteine and with glutathione, prompted by the significance of such interactions in human biochemistry (6-8), are also planned for presentation.
Experimental
L-Cysteine, L-glutathione, L-histidine, and MOPS (3-(N-morpholino)propanesulfonic acid) were obtained from Sigma-Aldrich Corporation, and L-tryptophan was obtained from Nutritional Biochemicals Corporation. 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
Electrochemical studies of L-tryptophan in pH 7.4 phosphate buffer have shown that this amino acid undergoes oxidation at +0.82 V vs Ag/AgCl at 100 mV/s, which is a considerably less positive potential than that observed for L-histidine (5). The effects of L-tryptophan additions to ZnSO4 added to pH 7.4 MOPS buffer were found to be generally similar to those for L-histidine (5). For the case of interactions of L-cysteine with Bi(III) in pH 7.4 MOPS buffer, Bi(NO3)3 was added to pH 7.4 MOPS buffer resulting in the formation of slightly soluble hydroxyl bismuth complexes. 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(III) 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 (10) 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.
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