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...

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Bibliographic Details
Published in:Chemphyschem 2007-08, Vol.8 (12), p.1863-1869
Main Authors: Lebedev, Alexander V., Ivanova, Marina V., Timoshin, Alexander A., Ruuge, Enno K.
Format: Article
Language:English
Subjects:
acid-base equilibrium
alkaline earth metals
autoxidation
Catechols - chemistry
Cations, Divalent
Dopamine - chemistry
dopamines
Free Radicals - chemistry
Hydrogen-Ion Concentration
Hydrolysis
Isoproterenol - chemistry
Metals, Alkaline Earth - chemistry
Norepinephrine - chemistry
Oxidation-Reduction
Oxygen - chemistry
Zinc - chemistry
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Summary: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+–catecholate complexes is accompanied by an increase in the dissociation constants (Kai ) of their phenolic hydroxyls. At pH>pKai 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 rate 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+–catecholate 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. Leading role for calcium: The effects of Ca2+ on the free‐radical chain reactions of catecholate complexes are investigated. In Ca2+‐free solutions, pyrocatechol autoxidation (see figure) generates a weak steady‐state EPR signal (g=2.0045). On addition of calcium the Ca2+–semiquinonate spectrum (g=2.0043) is observed.
ISSN:1439-4235
1439-7641
DOI:10.1002/cphc.200700296