EFFECT OF ION-PAIR FORMATION BETWEEN 1,4-BENZOSEMIQUINONE AND ALKALINE-EARTH METAL-CATIONS ON THE SEMIQUINONE DISPROPORTIONATION EQUILIBRIUM AT PHYSIOLOGICAL PH

The decrease in the 1,4-benzosemiquinone (BQ-.) disproportionation constant upon addition of either MgCl2 or BaCl2 was used to determine the corresponding semiquinone-M+2 ion pair formation constant in 10 mM cacodylic-cacodylate buffer, at pH 7.4, utilizing electron paramagnetic resonance (EPR) spectroscopy. These constants are 5.6 +/- 2.8 and 2.5 +/- 1.3 for magnesium and barium, respectively, at an ionic strength of 1.0. A decrease in the BQ-. g-value was also detected upon addition of either MgCl2 or BaCl2 due to ion pair formation, being more pronounced for MgCl2 than for BaCl2. No changes in proton coupling constants were observed by addition of these salts to the BQ-. containing solutions. No ion pair formation was detected if KCl or NaCl were added, up to a concentration of 3.0M.