Dimer Formation of Copper(II) Peptide Complexes and Related Ones in Aqueous Solution and Their Dimeric Structures as Studied by ESR

Abstract

It has been revealed by ESR spectroscopy that the 1: 1 complexes of copper(II) with triglycine(glycylglycylglycine), tetraglycine, pentaglycine, hexaglycine, and octaglycine in aqueous solutions at pH 10 form dimers in equilibrium with monomers. The copper(II)complexes of glycinamide, glycylglycinamide, and biuret at pH 12 also form dimers. The equilibrium constant of 2 monomer ⇔ dimer for the tetraglycine complex has been determi ned to be 460 mol·dm^-3 at about 0°C by analyzing the concentration dependence of the intensity ratios of dimer and monomer ESR signals. Spin-exchange interaction energies for. the present dimers have been estimated to be of the order of -1 cm-₁ from the temperature dependence of intensity of ΔM=2 transitions. All the dimer ESR spectra observed were analyzed by computer simulation in order to estimate the structural parameters of r and ξ for parallel-planar dimers, where r is the Cu-Cu distance and ξ, the angle between the Cu-Cu direction and the normal to the molecular plane. The results indicate that the dimeric structures of the above complexes (r=4.0-4.3Å;ξ≤15°) are different from those in crystals, except for the triglycine complex (r=3.3A;≤=40°). It has been concluded that coordination of two or more deprotonated peptide nitrogens is required for the dimerization of the present kind of complexes. This may suggest that a so-called π-π interaction between two molecules with the π-electron system made up of the metal and coordinating deprotonated peptide(amide) groups is a possible driving force for the dimerization.