Effect of group II metal cations on catecholate oxidation

The unexpected effects of Ca2+ on the free-radical chain reactions of dopamine, norepinephrine, isoproterenol, and pyrocatechol oxidation are studied using oxygen consumption measurements, EPR-spectroscopy, UV/VIS spectrophotometry, and by potentiometric titration. It is found that the formation of Ca2+-Cat(-) echolate complexes is accompanied by an increase in the dissociation constants (K-aj) of their phenolic hydroxyls. At pH > pK(ai) and in the presence of alkaline-earth metal cations, the rate of catecholate oxidation increases (Ca2+, Mg2+ > Sr2+, Ba2+), whereas on addition of Zn ions the rote decreases. The effects of Group II metal cations on catecholate autoxidation are concomitant with a transient increase of the EPR signal for metal-semiquinonate complexes. Therefore, the effects of Ca2+ and other alkaline-earth metal cations on catecholate autoxidation can be defined as 1) additional deprotonation of catechol OH-groups involved in the formation of M2+-catecholote complexes, the latter exceeding catechols in the susceptibility to dioxygen-induced oxidation and 2) formation of relatively stable free-radical intermediates responsible for chain propagation.