Notable Effects of Metal Salts on UV-Vis Absorption Spectra of α-, β-, γ-, and δ-Tocopheroxyl Radicals in Acetonitrile Solution. The Complex Formation between Tocopheroxyls and Metal Cations

The measurements of the UV-vis absorption spectra of alpha-, beta-, gamma- and delta-tocopheroxyl (alpha-, beta-, gamma-, and delta-Toc(center dot)) radicals were performed by reacting aroxyl (ArO center dot) radical with alpha-, beta-, gamma- and delta-tocopherol (alpha-, beta-, gamma-, and delta-TocH), respectively, in acetonitrile solution including three kinds of alkali and alkaline earth metal salts (LiClO4, NaClO4, and Mg(ClO4)(2)) (MX or MX2), using stopped-flow spectrophotometry. The maximum wavelengths (lambda(max)) of the absorption spectra of the alpha-, beta-, gamma-, and delta-Toc(center dot) located at 425-428 nm without metal salts increased with increasing concentrations of metal salts (0-0.500 M) in acetonitrile and approached some constant values, suggesting (Toc(center dot)center dot center dot center dot M+ (or M2+)) complex formations. Similarly, the values of the apparent molar extinction coefficient (epsilon(max)) increased drastically with increasing concentrations of metal salts in ac:etonitrile and approached some constant values. The result suggests that the formations of Toc(center dot) dimers were suppressed by the metal ion complex formations of Toc' radicals. The stability constants (K) were determined for Li+, Na+, and Mg2+ complexes of alpha-, beta-, gamma-, and delta-Toc(center dot). The K values increased in the order of NaClO4 < LiClO4 < Mg(ClO4)(2), being independent of the kinds of Toc(center dot) radicals. Furthermore, the K values increased in the order of delta- < gamma < beta- < alpha-Toc(center dot) radicals for each metal salt. The alkali and alkaline earth metal salts having a smaller ionic radius of the cation and a larger charge of the cation gave a larger shift of the lambda(max) value, a larger epsilon(max) value, and a larger K value. The result of the DFT molecular orbital calculations indicated that the alpha-, beta-, gamma-, and delta-Toc(center dot) radicals were stabilized by the (1:1) complex formation with metal cations (Li+, Na+, and Mg2+). Stabilization energy (E-S) due to the complex formation increased in the order of Na+ < Li+ < Mg2+ complexes, being independent of the kinds of Toc(center dot) radicals. The calculated result also indicated that the metal cations coordinate to the O atom at the sixth position of alpha-, beta-, gamma-, and delta-Toc(center dot) radicals.