A step forward in the development of superoxide dismutase mimetic nanozymes: the effect of the charge of the surface on antioxidant activity

Two binucleating hezaaza macrocycles containing a pyridinol spacer have been prepared and characterised. Protonation studies indicate the deprotonation of the phenol group at relatively low pH values with the concomitant occurrence of a keto-enolic equilibrium. These ligands readily form binuclear Cu(2+) and Zn(2+) complexes as denoted by potentiometric and spectroscopic studies. The binding of the metals yields to the ready deprotonation of the phenol with the stabilisation of the keto form that results in complexes of greater stabilities than the analogous ones containing pyridine as spacer instead of pyridine. Mixed Cu(2+)–Zn(2+)-complexes were also detected in aqueous solutions containing equimolar amounts of Cu(2+), Zn(2+) and ligands. The binuclear Cu(2+) complexes show significant SOD activity as proved by the McCord–Fridovich assays. The binuclear Cu(2+) complexes of the ligands grafted to boehmite nanoparticles (BNPs) show a remarkable increase in SOD activity, which reaches 8-fold in one of the systems. The observed increase can be ascribed to the positive ζ-potential of the BNPs since the same complexes anchored to silica nanoparticles with negative ζ-potential do not show any apparent increase in activity. This behaviour is reminiscent of the positively charged funnel found in CuZnSOD, which has the electroactive copper ion at its end.