THERMODYNAMIC AND SPECTROSCOPIC CHARACTERIZATION AND INVITRO O-2 SCAVENGER ACTIVITY OF COPPER(II) GLYCYL-L-HISTIDYL-GLYCYL-L-HISTIDINE COMPLEXES

Thermodynamic and spectroscopic (electron spin resonance, electronic and circular dichroism) studies have been carried out on the complex species which exist in the copper(II)-glyCyl-L-histidyl-glycyl-L-histidine (HL) system in aqueous solution. The thermodynamic results suggest the formation of three important species: [CuL]+, [CuH-1L] and [CuH-2L]-, the first being present in the range pH 3.0-5.5, the second and third in different ratios between pH 5.5 and 7.8. The complex [CuL]+ has a pseudo-octahedral structure, the equatorial plane of which involves the amino group, the first peptide nitrogen, an imidazole nitrogen and a water molecule; [CuH-1L] has the same donor atoms in the equatorial plane and the other imidazole nitrogen binds in an apical site, giving a square-pyramidal geometry. For [CuH-2L]-, in which a second peptide nitrogen is deprotonated, the geometry is distorted and can be considered intermediate between a square pyramid with a tetrahedrally distorted plane and a distorted trigonal bipyramid. In vitro O2- scavenging experiments show that the most active species is [CuH-2L]-. The superoxide dismutase-like activity has been rationalized by considering the geometry of the copper(II) complex species.