Co-ordinating properties of cyclopeptides. Thermodynamic and spectroscopicstudy on the formation of copper(II) complexes with cyclo(Gly-His)(4) and cyclo(Gly-His-Gly)(2) and their superoxide dismutase-like activity

Two cyclopeptides, cyclo(Gly-His), and cyclo(Gly-His-Gly)(2) were synthesiz ed with the specific aim to form copper(II) complexes which are able to mim ic the active site of superoxide dismutase. Proton and copper(II) complexes were thermodynamically characterized. The copper(II) complexes were also s tudied by means of optical and ESR spectroscopy to gain information on thei r structural features and by voltammetry to know about their redox ability Moreover, the antioxidant activity of these complex species was tested agai nst enzymatically generated superoxide radical. Depending on the pH value o f the solution, definite complexes could be characterized, in particular [C u{cyclo(Gly-His)(4)}](2+) and [Cu{cyclo(Gly-His)(4)}H-2] and [Cu{cyclo(Gly- His-Gly)(2)}H-2] are the main species, which were taken into consideration to assay their antioxidant catalytic activity. The ESR studies suggested th at a four-nitrogen co-ordination by means of imidazole nitrogen atoms or de protonated peptide nitrogen atoms forms the environment around copper. At t he same co-ordination level, the redox properties of these compounds parall el their scavenging abilities against O-2(-) which are lower than those of other copper(II) complexes previously tested. The [Cu{cyclo(Gly-His)(4)}](2 +) complex showed higher redox potential and better catalytic ability than [Cu{cyclo(Gly-His)(4)}H-2] and [Cu{cyclo(Gly-His-Gly)(2)}H-2], which have r oughly similar redox potentials and scavenging abilities.