Correlation study of metal ion geometry and Eu(Ⅲ) luminescence spectra: Relevance to calcium(Ⅱ) binding sites in biomolecules

Abstract

Binding sites of biomaterials e.g., protein or enzyme with Eu(Ⅲ) ion were Investigated using Eu(Ⅲ) excitation spectroscopy having useful property ($^7F_0 → ^5D_0$ transition). Eu(Ⅲ) excitation spectra of Eu(Ⅲ) ion as substitutional probe ion and various ligands complexes were obtained in order to investigate binding sites of biomaterials-Ca(Ⅱ) or Mg(Ⅱ) which it do not exhibit useful spectroscopic or magnetic probe properties. In addition, crystal structure of those complexes and Eu(Ⅲ) excitation spectra of these crystals were obtained and relationship between metal coordination structure and Eu(Ⅲ) excitation spectroscopy was studied. Eu(Ⅲ) excitation spectra of Eu(Ⅲ) with various ligand complexes were obatined for basic spectroscopic interpretation as a preliminary research for model ligand of biomaterials. Eu(Ⅲ) exciation spectra were obtained as a number of carbon in dicarboxylic acids. With dicarboxylic acid ligands $HO_2C(CH_2)_nCO_2H$(n=0~3), the complexes show less stable trend as n increased: oxalate ＞ malonate ≫succinate ~ glutarate. In case of oxalic acid, the three proposed structures were suggested from data obtained Eu(Ⅲ) excitation spectra, the hydration numbers, which are the number of coordinated water molecules, of Eu(Ⅲ) coordinated to oxalate were obtained form the lifetime measurements of luminescence decay, and element analysis. Peaks of Eu(Ⅲ) excitation spectra of Eu(Ⅲ) ion and malonate complexes showed blue shift which is abnormal shift compared with red shift shown those of Eu(Ⅲ) ion and other lignads complexes. However, according to X-ray crystal analysis, malonate seems to behave as chelate ligands to form ring with Eu(Ⅲ) ion. The Eu(Ⅲ) excitation spectroscopy studies were extended to single crystals of Eu(Ⅲ)-glutarate complexes formed in aqueous solution. Three identifiable peaks (579.01, 579.24, and 579.52 nm) seen in the Eu(Ⅲ) excitation spectra of Eu(Ⅲ)-glutaric acid system indicate the formation of at least three species in aqueous...