Electrochemical Studies of L-Histidine
Authors: Clark Dexter C. , Cheek Graham T. 
Published: February 2020

Introduction
The naturally occurring amino acid L-histidine is involved in many important biochemical processes, particularly those involving complexation of metal ions by histidine. Such interactions are involved in the formation of “zinc finger” proteins (1) and in proteins containing copper(II) ions (2). The imidazole moiety in L-histidine is electrochemically active, although at rather highly positive potentials (3), and this behavior allows investigation of its properties and interactions with other substances. The research involved in this paper deals with the electrochemical investigation of L-histidine in buffered aqueous media by techniques such as cyclic and square-wave voltammetry. Further studies of the complexation of L-histidine with zinc(II) ions is also planned for presentation. These studies complement those carried out previously with L-cysteine in this laboratory (4, 5).
Experimental
L-Histidine and ZnSO4
. 7 H2O were obtained from Sigma-Aldrich Corporation. Electrochemical experiments were carried out under nitrogen using a Gamry Instruments Interface 1000 potentiostat and 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
The electroactive nature of the imidazole portion of L-histidine is illustrated in Figure 1, which presents the square-wave voltammogram (SWV) of 8 mM L-histidine in pH 7.4 phosphate buffer. Two partially resolved oxidation processes are evident, and the resolution was poorer using cyclic voltammetry. It was found that the SWV peak currents were nonlinear with L-histidine concentration. Addition of ZnSO4 to L-histidine (1:2 molar ratio) produced a positive 50 mV potential shift for the SWV L-histidine oxidation, consistent with complexation of Zn2+ with L-histidine. Similar results were observed for L-cysteine (5). Further work with Zn2+ complexation by L-histidine was then carried out in pH 7.4 MOPS buffer at glassy carbon. Initial addition of ZnSO4 produced a symmetrical SWV Zn2+ reduction peak upon scanning to more negative potentials, and addition of L-histidine caused a -100 mV potential shift in the reduction peak at the 1:2 ZnSO4 : L-histidine point. The shape of the square-wave voltammetric reduction peak was also found to become somewhat narrower upon addition of L-histidine. Investigations of L-histidine under other conditions (pH 11) and using other techniques (electrochemical impedance spectroscopy) are also intended for inclusion in this presentation.
References

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Figure 1. Square-wave voltammograms at glassy carbon in aqueous pH 7.4 phosphate buffer, ESW = 25 mV, ΔE = 2 mV, frequency = 10 Hz.

Solid black line : L-Histidine 8 mM

Dashed red line : pH 7.4 background

Figure 1