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Effect of Group II Metal Cations on Catecholate Oxidation
Author(s) -
Lebedev Alexander V.,
Ivanova Marina V.,
Timoshin Alexander A.,
Ruuge Enno K.
Publication year - 2007
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200700296
Subject(s) - chemistry , autoxidation , deprotonation , catechol , potentiometric titration , electron paramagnetic resonance , metal , inorganic chemistry , comproportionation , photochemistry , dissociation (chemistry) , reaction rate constant , redox , oxygen , metal ions in aqueous solution , kinetics , electrochemistry , ion , organic chemistry , physics , electrode , nuclear magnetic resonance , quantum mechanics
The unexpected effects of Ca 2+ 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 Ca 2+ –catecholate complexes is accompanied by an increase in the dissociation constants ( K ai ) of their phenolic hydroxyls. At pH>p K ai and in the presence of alkaline‐earth metal cations, the rate of catecholate oxidation increases (Ca 2+ , Mg 2+ > Sr 2+ , Ba 2+ ), 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 Ca 2+ 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 M 2+ –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.