The Structure of Copper (II) -Histidine Chelate

The binding of Cu(II) by histidine in 1: 2 molar ratio has been investigated. The theoretical number of hydrogen ions displaced from each mole of bound llgand at several pH values was calculated considering all the possible interactions between Cu(II) and histidine involving (a) amino and carboxyl, (b) amino and imidazole, (c) imldazole and carboxyl, and (d) amino, imidazole, and carboxyl groups. When determined experimentally by discontinuous titration, the number of hydrogen ions coincided with the number consistent with the amino and carboxyl binding and strongly suggested that the imidazole moiety is not involved. Similar studies with Cu(II)-histamine, Cu(II)-alanine, and Cu(II)imidazole confirmed the accepted views regarding their metal binding sites. No resemblance could be seen in the ultraviolet spectra between Cu(II)-histidme and Cu(II)-imidazole. Samples of Cu(II)-histidine, Cu(II)-histamine, and Cu(II)-alanine showed absorption maxima between 235 and 242 rnp which strongly suggested that the X,., of Cu(II)histidine in the ultraviolet region is most likely due to Cu(I1) binding to amino and not to imidazole. Infrared spectrum of Cu(II)-histidme in DzO solution exhibited a strong shift in the antisymmetrlc carboxyl stretching frequency which is consistent with the coordination of the carboxyl group.