Copper(II) Complexes of Histidine and Its Related Compounds in Aqueous Solutions

Abstract The copper(II) complexes of histidine and its related compounds in aqueous solution were investigated by potentiometric, spectrophotometric, and magnetic methods. The presence of a diolated binuclear copper(II) complex was indicated in the 1 : 1 histamine-copper(II) system by quantitative analysis of the titration data and the antiferromagnetic character of the complex. The titration data could not be quantitatively interpreted owing to precipitation, but the formation of a similar diolated copper(II) complex was also shown in the 1 : 1 histidinecopper(II) system by the magnetic properties of the complex. From a comparison of the spectral properties of the pertinent complexes, it was shown for the neutral 2 : 1 histidine-copper(II) complex that histidine binds to copper(II) through the amino and imidazole groups in the chelate plane and through the carboxyl group in the apical position. In the copper(II) complexes of histidine and histamine, it was shown that ionization of the pyrrole nitrogen of the imidazole ring is promoted, causing a spectral blue shift.