Free and bound histidine in reactions at mercury electrode

•Histidine (His) electrochemical behaviour was studied at Hg electrode.•Free His and its residues bound in peptides and proteins were investigated.•His can be chemisorbed to Hg-anode via deprotonated imidazole group.•Oxidatively formed His mercuric imidazolate undergoes reversible reduction in an adsorbed state.•His-containing neuropeptides and protein α-synuclein along with its His-tagged version were studied.