Formations of Zn-histidine, -histamine, -histidine methylester, -histidylglycine, -glycylhistidine and -glycylglycine complexes in aqueous solution were examined by means of a potentiometric titration method. On the basis of the results obtained the possible chemical structures were given to the individual complexes. The main object of this investigation is to know what donor groups of histidine participate in the Zn-complex formation at a given pH value.
In neutral and alkaline solutions above pH 6, the imidazole and the a-NH2 groups of histidine coordinate to the Zn ion and form the Zn-histidine (1:2) complex. In acidic solutions, the a-COOH group also coordinates and forms the Zn-histidine (1:1) complex together with the above two groups. The complexes formed by the participation of the imidazole group of histidine are considered to be more stable than the Zn-glycine or -glycylglycine complex by reason of the difficulty in the precipitation of Zn(OH)2 during the titrations. This was also the case for the Zn-histidine-containing dipeptide complexes.