Journal of Physical Chemistry B, Vol.116, No.8, 2615-2621, 2012
Kinetic Study of the alpha-Tocopherol-Regeneration Reaction of Ubiquinol-10 in Methanol and Acetonitrile Solutions: Notable Effect of the Alkali and Alkaline Earth Metal Salts on the Reaction Rates
A kinetic study of regeneration reaction of alpha-tocopherol (alpha-TocH) by ubiquinol-10 has been performed in the presence of four kinds of alkali and alkaline earth metal salts (LiClO4, NaClO4, NaI, and Mg(ClO4)(2)) in methanol and acetonitrile solutions, using double-mixing stopped-flow spectrophotometry. The second-order rate constants (k(r)'s) for the reaction of alpha-tocopheroxyl (alpha-Toc center dot) radical with ubiquinol-10 increased and decreased notably with increasing concentrations of metal salts in methanol and acetonitrile, respectively. The k(r) values increased in the order of no metal salt < NaClO4 similar to NaI < LiClO4 < Mg(ClO4)(2) at the same concentration of metal salts in methanol. On the other hand, in acetonitrile, the k(r) values decreased in the order of no metal salt > NaCIO4 similar to NaI > LiClO4 > Mg(ClO4)(2) at the same concentration of metal salts. The metal salts having a smaller ionic radius of cation and a larger charge of cation gave a larger k(r) value in methanol, and a smaller k(r) value in acetonitrile. The effect of anion was almost negligible in both the solvents. Notable effects of metal cations on the UV vis absorption spectrum of alpha-Toc center dot radical were observed in aprotic acetonitrile solution, suggesting complex formation between alpha-Toc center dot and metal cations. On the other hand, effects of metal cations were negligible in protic methanol, suggesting that the complex formation between alpha-Toc center dot and metal cations is hindered by the hydrogen bond between alpha-Toc center dot and methanol molecules. The difference between the reaction mechanisms in methanol and acetonitrile solutions was discussed on the basis of the results obtained. High concentrations of alkali and alkaline earth metal salts coexist with alpha-TocH and ubiquinol-10 in plasma, blood, and many tissues, suggesting the contribution of the metal salts to the above regeneration reaction in biological systems.