Electrochemical Investigations of L-Tryptophan

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IntroductionRecent activity in this laboratory has involved the electrochemistry of amino acids such as L-cysteine and L-histidine in aqueous buffers (1-4). The effect of zinc ions, which are involved in determining the structures of “zinc finger” proteins (5), on the electrochemistry of amino acids has also been investigated. The present work has extended these studies to L-tryptophan and some of its derivatives. ExperimentalMOPS (3-(N-morpholino)propanesulfonic acid) was obtained from Sigma-Aldrich Corporation, and L-tryptophan was obtained from Nutritional Biochemical Systems. 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 (eDAQ). Results and Discussion Electrochemical studies of L-tryptophan have been carried out in pH 7.4 phosphate buffer, and we have found that L-tryptophan undergoes oxidation more easily than does L-histidine (4). Complexation studies of Zn(II) by L-tryptophan in pH 7.4 MOPS buffer have shown that the extent of complexation is generally similar to that of L-histidine (4). Similar experiments were also carried out on 5-fluoro-tryptophan and 6-fluoro-tryptophan, revealing that they underwent oxidation at somewhat more positive potentials than does L-tryptophan. This observation reflects the electron-withdrawing effect of the fluorine atom. Despite this positive potential shift, the fluorinated tryptophans were found to effectively complex zinc(II) in pH 7.4 MOPS buffer. Finally, initial experiments in the voltammetric behavior of L-tryptophan in the very polar solvent formamide have been initiated.