Complexation between copper(II) ion and large excess histidine in aqueous solution.

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The complexation between copper(II) ion and large excess histidine has been studied by visible absorption, proton and C-13 NMR spectral methods, in comparison with histamine–copper(II) system. The predominant species were considered to be ML2H+ and ML2H22+ at a=0 and ML2 at a=1 and 2, respectively from the spectral data in visible region, where a represents moles of base added per mole of diprotonated ligand. In ML2H+, one histidine is bound histamine-like with a carboxylate coordinated in apical position and the other glycine-like with a protonated imidazolium group, and in ML2H22+, both histidine molecules are bound glycine-like. Two histidine molecules are both tridentate with apical carboxylates in ML2. In histamine–copper(II) system, the coordination of the third histamine molecule in the apical position was suggested at a=1 and 2. The proton and the G-13 NMR spectra were reasonably interpreted in view of the above situation. A difference in the line-broadening of PMR signals was observed between histidine–copper(II) and histamine–copper(II) systems. This is mainly due to the difference in the exchange rates of the coordinated ligands of two systems.
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