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

Copper( II)-histamine and copper(II)-L-histidine equilibrium systems were s tudied in fluid aqueous solution by ESR spectroscopy. Eighty-seven spectra taken in a circulating system at various ligand-to-metal concentration rati os and pH were analysed. The experimental curves were decomposed to one to four component spectra which were built up fr om the hyperfine lines of Cu- 63 and Cu-65, and a maximum of four non-equivalent N-14 nuclei. The isotrop ic ESR parameters (g-factors, hyperfine coupling constants and relaxation p arameters) and the relative concentrations of the different species were op timized. New, pH-potentiometrically non-identified species were also consid ered in the equilibrium models. In the copper(II) -histalnine system the co mplex [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 literat ure pH-potentiometric formation constants. The two ligands in their 'LH' st ates are bound differently: the histamine by the imidazole, and the L-histi dine 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 coordi nated equatorially by the amino and imidazole nitrogens. The second 'L' lig and in the [CuL2] complexes is either bound in the former way, or its imida zole 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 th e imidazole ring, which is followed by the proton loss of the equatorial wa ter molecule in the highly alkaline region. (C) 2000 Elsevier Science Ltd A ll rights reserved.