Effect of Group II Metal Cations on Catecholate Oxidation

Abstract

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>pK(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.

Potent 2,2-Diphenyl-1-picrylhydrazyl Radical-Scavenging Activity of Novel Antioxidants, Double-Stranded Tyrosine Residues Conjugating Pyrocatechol

“…The formation of DPPH 2 was also identified from the peaks of two -NH-at 8.56 and 9.06 ppm, respectively, in CD 3 OD by 1 H-NMR. On the other hand, although pyrCat reduces oxygene O 2 to generate superoxide anion radical (˙O 2 Ϫ ) in one electron transfer pathway, 24) it was found that compounds 5a and b do not generate ˙O 2 Ϫ in 0.1 M KH 2 PO 4 buffer (pH 7.5) (and 90% ethanol) by the chemical luminescence method (data not shown here) (Eq. 4).…”

Influence of cement type on resistance to organic acids

“…Catechol forms strong complexes with aluminium and iron (Table 8). Whilst complex formation with calcium ions has not been conclusively demonstrated, their presence leads to modification of the pKa of catechol (in stepwise terms: pKa1 = 9.04; pKa2 = 1.64), indicating that it is likely (Lebedev, Ivanova, Timoshin and Ruuge, 2007). The catecholate ion forms relatively strong complexes with Si.…”

The Electrochemical Reaction Mechanism and Applications of Quinones

“…5 Shift of E r1 and E r2 by metal cation complexation affects semiquinone stability and several accounts show that metal ion can increase the autoxidation rate of catechols. 47,48 It is still of considerable interest to understand the influence of various factors on semiquinone radical stability and electronic structure calculations have been performed. 49,50 Band assignments of vibrational peaks along with ab initio calculation helped elucidate electronic structure of semiquinones as well.…”