Reaction of oxygen with 6-hydroxydopamine catalyzed by Cu, Fe, Mn, and V complexes: Identification of a thermodynamic window for effective metal catalysis

In the autoxidation of 6-hydroxydopamine, we investigated the reactivity of metals and metal complexes with a range of abilities to catalyse the react ion with oxygen. Comparing the catalytic effectiveness of aquo metals at pH 7.4, copper accelerated autoxidation 61-fold, iron 24-fold, manganese 7.3- fold, and vanadium 5.7-fold. Copper was thus the most effective catalyst de spite being the weakest oxidant, indicating reduction of oxygen as rate lim iting. EDTA, which decreases the reduction potential of Fe(III)/Fe(II), inc reased catalysis by iron 74% to almost that of aquo copper. Conversely, EDT A inhibited catalysis by copper, manganese, and vanadium. Desferrioxamine s trongly inhibited catalysis by all of the metals. Histidine prevented catal ysis by copper, accelerated catalysis by iron (43%), and had little effect on catalysis by manganese or vanadium, ADP and phytate inhibited catalysis by iron and manganese (50% or more), accelerated catalysis by vanadium (10- 27%), and had no effect on catalysis by copper. The effects of the ligands largely reflected their influence on the reduction potential of the metal. Accordingly, addition of NaBr, which increases the reduction potential of C u(II)/Cu(I), inhibited by 50%. In contrast, Na2SO4 augmented catalysis by c opper 3-fold. Consistent with effects of OH- on reduction potentials and on metal coordination to 6-hydroxydopamine, an increase in pH to 8.0 decrease d catalysis by copper and iron, but increased that of manganese 10-fold. In conclusion, the catalytic effectiveness of the metal-ligand complexes are largely attributable to their reduction potential, with steric accessibilit y playing secondary roles. The results delineate a window of catalytically effective potentials suitable for facile reduction and reoxidation by oxyge n. By extension the results identify factors determining the pro- and antio xidant roles of ligands in metal mediated reduction of oxygen. (C) 2001 Aca demic Press.