Free and bound histidine in reactions at mercury electrode

The intrinsic electrochemical activity of some amino acids as well as their residues in peptides and proteins at mercury electrodes can be utilized for fundamental research purposes and for the development of label-free reagentless analytical methods. Here we present a new concept of how to investigate free histidine (His) and how to detect its residues within the more complex structures of some peptides and proteins at a hanging mercury drop electrode. The approach is based on the ability of the deprotonated His imidazole group to oxidize the electrode mercury at potentials less positive than those of the electrolytic (anodic) dissolution of mercury. Using cyclic voltammetry, we found that in a weakly alkaline solution of free His or bound in His homohexapeptide, the oxidative formation and the reductive cleavage of the bond between the imidazole group and the electrode mercury are reversible and take place in an adsorbed state. In addition to free His and His homohexapeptide, also more complex heteropeptide samples such as angiotensin peptides, a modified fragment of amyloid beta peptide, and His-tagged mutant protein alpha-synuclein A53T were investigated by constantcurrent chronopotentiometry in neutral solution. The proposed methodology could be further applied for the investigation of the acid-base and metal chelating properties of His-containing peptides or proteins, and for the study of their structural changes and interactions with other substances.