Superoxide radical anion scavenging and dismutation by some Cu2+ and Mn2+ complexes: A pulse radiolysis study

Copper (Cu) and manganese (Mn) ions are catalytic centers, in complexed form, in scavenging and dismutation process of superoxide radicals anion (O2.−) by superoxide dismutase enzyme. In the present work, fast reaction kinetics and mechanism of scavenging and dismutation of O2.− by Cu2+, Mn2+ and their complexes formed with some natural ligands have been studied using pulse radiolysis technique. Catechol, gentisic acid, tetrahydroxyquinone, tyrosine, tryptophan, embelin and bilirubin have been used as low molecular weight natural ligands for Cu2+ and Mn2+ to understand superoxide radical scavenging and dismutation reactions. These complexes have been found to be efficient scavengers of O2.− (k~107-109 M−1 s−1). The effects of nature of metal ion and ligand, and stoichiometry of complex on scavenging reaction rate constants are reported. Higher scavenging rate constants have been observed with complexes of: (1) Cu2+ as compared to Mn2+, and (2) at [ligand]/[metal] ratio of one as compared to two. A clear evidence of O2.− dismutation by free metal ions and some of the complexes has been observed. The study suggests that complexes of Cu2+ and Mn2+ with small natural ligands can also act as SOD mimics.