Great structural variety of complexes in copper(II)-oligoglycine systems:: Microspeciation and coordination modes as studied by the two-dimensional simulation of electron paramagnetic resonance spectra

A series of isotropic EPR spectra recorded at various concentrations and pH (in the range 2-12) on equilibrium systems containing copper(II) and diglycine, triglycine, or tetraglycine were analyzed. A purely mathematical method, matrix rank analysis gave the number of independent EPR-active species. Two-dimensional evaluation then resulted in the formation constants and magnetic parameters of 14 metal complexes (including microspecies) in each system. The independent paramagnetic species formed with each ligand are as follows: Cu2+ (aqua complex), [CuLH](2+), [CuL](+), [CuLH-1], [CuLH-2](-), [CuL2H2](2+), [CuL2H](+), [CuL2], [CuL2H-1](-), and [CuL2H-2](2-). Moreover, for cliglycine, the diamagnetic complex [Cu2L2H-3](-), and for triglycine and tetraglycine, the EPR-active species [CuLH-3](2-) were identified. Further, equilibria of two microspecies were demonstrated for [CuL2], [CuL2H-1](-), and [CuL2H-2](2-). The magnetic parameters allowed a detailed description of the coordination modes. The most important findings: (1) For the mono complexes, the in-plane a-bonds between copper(II) and the equatorial N donors are particularly strong when the same ligand forms several adjacent chelate rings with the participation of amino N, deprotonated peptide N(s), and the carboxylate group. (2) Structures with coupled chelate rings are likewise favored in the bis complexes. Different protonation states of the two ligands are observed in the major isomer of [CuL2] ((LH-1 + LH) coordination), and in the isomers of [CuL2H-2](2-) ((LH-2 + L) coordination) for triglycine and tetraglycine.