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

Abstract

The measurements of the UV-vis absorption spectra of α-, β-, γ-, and δ-tocopheroxyl (α-, β-, γ-, and δ-Toc(•)) radicals were performed by reacting aroxyl (ArO(•)) radical with α-, β-, γ-, and δ-tocopherol (α-, β-, γ-, and δ-TocH), respectively, in acetonitrile solution including three kinds of alkali and alkaline earth metal salts (LiClO(4), NaClO(4), and Mg(ClO(4))(2)) (MX or MX(2)), using stopped-flow spectrophotometry. The maximum wavelengths (λ(max)) of the absorption spectra of the α-, β-, γ-, and δ-Toc(•) 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(•)···M(+) (or M(2+))) complex formations. Similarly, the values of the apparent molar extinction coefficient (ε(max)) increased drastically with increasing concentrations of metal salts in acetonitrile and approached some constant values. The result suggests that the formations of Toc(•) dimers were suppressed by the metal ion complex formations of Toc(•) radicals. The stability constants (K) were determined for Li(+), Na(+), and Mg(2+) complexes of α-, β-, γ-, and δ-Toc(•). The K values increased in the order of NaClO(4) < LiClO(4) < Mg(ClO(4))(2), being independent of the kinds of Toc(•) radicals. Furthermore, the K values increased in the order of δ- < γ- < β- < α-Toc(•) 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 λ(max) value, a larger ε(max) value, and a larger K value. The result of the DFT molecular orbital calculations indicated that the α-, β-, γ-, and δ-Toc(•) radicals were stabilized by the (1:1) complex formation with metal cations (Li(+), Na(+), and Mg(2+)). Stabilization energy (E(S)) due to the complex formation increased in the order of Na(+) < Li(+) < Mg(2+) complexes, being independent of the kinds of Toc(•) radicals. The calculated result also indicated that the metal cations coordinate to the O atom at the sixth position of α-, β-, γ-, and δ-Toc(•) radicals.

Natural organic matter-cations complexation and its impact on water treatment: A critical review

“…In anthropogenically influenced waters however, nickel is mainly complexed to EDTA and can account for about 87% of all EDTA sites bonded (Baken et al, 2011). The complexation of Na þ and Mg 2þ with tocopheroxyl radicals results in an increase in UV-absorbance (Mukai et al, 2012). Trivalent cations (Al, Fe, Cr) complexes with anthocyanins and its derivatives resulted in a stronger decrease in absorbance than Mg 2þ (Sigurdson et al, 2016).…”

Aroxyl-Radical-Scavenging Rate Increases Remarkably under the Coexistence of α-Tocopherol and Ubiquinol-10 (or Vitamin C): Finding of Synergistic Effect on the Reaction Rate

“…50,51 Similarly, the anion moiety (AsH − ) of Na + AsH − will also interact with α-TocH •+ cation and form a complex with a α-TocH •+ cation [α-TocH •+ ---AsH − ]. As a result, the energy level of the electron-transfer intermediate [ArO: − ---α-TocH •+ ] will be stabilized (that is, the activation energy (E act ) will decrease), and thus, the k s α-TocH (two) value increases.…”

Effect of tocopherols, tocotrienols, β-carotene, and chlorophyll on the photo-oxidative stability of red palm oil

“…2B). The tocopheroxyl radical has a higher molar extinction coefficient than tocopherol on the UV-vis spectrum (20). In the TAG+Toc+Car model system, change of tocopherol to the tocopheroxyl radical occurred at a slower rate and became measurable due to the anti-oxidant activity of β-carotene.…”