Copper(II) complexes with cyclo(L-aspartyl-L-aspartyl) and cyclo(L-glutamyl-L-glutamyl) derivatives and their antioxidant properties
Two new functionalized cyclodipeptides have been synthesized with the aim of obtaining a good model of superoxide dismutase. Better to mimic the active site of this metalloenzyme, these two compounds have been designed to allow a great co-ordination flexibility. Copper(II) complexes with cyclo(-L-aspartyl-L-aspartyl-) or cyclo(-L-glutamyl-L-glutamyl-)bis(histamine)(L) have been thermodynamically and spectroscopically characterized and their antioxidant activity tested against enzymatically generated O2–. Taking into account the speciation of the system, it has been determined that the more active species against O2– is the [CuL2]2+ complex. Electron spin resonance measurements suggest for this species the presence of four imidazole nitrogen atoms in a slightly tetrahedrally distorted co-ordination plane. The [CuLH–2] complex species also possesses four-nitrogen co-ordination involving two deprotonated peptide nitrogen atoms. The complex [CuL2]2+ showed the highest antioxidant activity and reasons for this behaviour are proposed on the basis of spectroscopic and voltammetric data.