Raman Scattering Study on Coordination Structures of Cu(II)-L-Histidine(1:2) in Aqueous Solutions

Article abstract of DOI:10.1246/bcsj.53.3131

The pH and pD dependences of the Raman spectra of Cu(II)-L-histidine (1:2) and Cu(II)-L-histidine-d_3-5 (1:2) were analyzed based on the pH titration data.Several ring vibrations of the imidazolyl group with cationic, neutral and anionic forms, the out-of-plane bending vibration of the bound carboxylate, and the Cu(II)-N stretching vibrations of the bound amino groups are used to study the chelation structures of the species taken by the solutions.From the results it was concluded that MHA²⁺, the major species below pH 3, assumes a glycine-like chelation structure with an unbound imidazolium cation.The results also indicate that the species MHA2⁺, which is prominent in the pH region 4.0-4.5, takes a mixed-type structure, in which one histidine molecule coordinates to Cu(II) as a terdentate ligand and another binds as a substituted glycine.MA2, the major species around pH 7, is determined to have the chelation structure in which two nitrogen atoms of the amino groups and two nitrogen atoms of the imidazole rings are bound to the Cu(II) ion in the square-planar trans position and at least one carboxyl group in the axial position.