An electron spin resonance study of coordination modes in the copper(II)-histamine and copper(II)-L-histidine systems in fluid aqueous solution

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Abstract

Copper(II)-histamine and copper(II)-L-histidine equilibrium systems were studied in fluid aqueous solution by ESR spectroscopy. Eighty-seven spectra taken in a circulating system at various ligand-to-metal concentration ratios and pH were analysed. The experimental curves were decomposed to one to four component spectra which were built up from the hyperfine lines of 63Cu and 65Cu, and a maximum of four non-equivalent 14N nuclei. The isotropic ESR parameters (g-factors, hyperfine coupling constants and relaxation parameters) and the relative concentrations of the different species were optimized. New, pH-potentiometrically non-identified species were also considered in the equilibrium models. In th copper(II)-histamine system the complex [CuLH-2] was added to the species [CuLH]3+, [CuL]2+, [CuLH-1]+, [Cu2L2H-2], [CuL2H]3+ and [CuL2]2+. In the copper(II)-L-histidine system, in addition to the complexes [CuLH]2+, [CuL]+, [CuLH-1], [Cu2L2H-2], [CuL2H2]2+, [CuL2H]+ and [CuL2], the new species [CuLH2]3+ and [CuLH-2]- were found. The relative concentrations obtained from the ESR spectra are in good accordance with the concentrations calculated from the literature pH-potentiometric formation constants. The two ligands in their 'LH' states are bound differently: the histamine by the imidazole, and the L-histidine through the amino and the carboxylate groups in equatorial positions (complexes [CuLH], [CuL2H] and [CuL2H2]). For the [CuL], [CuL2H] and [CuL2] complexes of both histamine and L-histidine, the first 'L' ligand is coordinated equatorially by the amino and imidazole nitrogens. The second 'L' ligand in the [CuL2] complexes is either bound in the former way, or its imidazole group occupies an axial site. The carboxylate group of L-histidine is coordinated to the metal ion in each complex, in either an equatorial or an axial position. The deprotonation of the [CuL] complex takes place from the imidazole ring, which is followed by the proton loss of the equatorial water molecule in the highly alkaline region. (C) 2000 Elsevier Science Ltd.

Original languageEnglish
Pages (from-to)1123-1131
Number of pages9
JournalPolyhedron
Volume19
Issue number9
DOIs
Publication statusPublished - May 15 2000

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Keywords

  • Coordination modes
  • Copper(II) complexes
  • Electron spin resonance
  • Histamine
  • Isomerism
  • L-Histidine

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry
  • Materials Chemistry

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