Notable Effects of the Metal Salts on the Formation and Decay Reactions of α-Tocopheroxyl Radical in Acetonitrile Solution. The Complex Formation between α-Tocopheroxyl and Metal Cations

Abstract

The measurement of the UV-vis absorption spectrum of α-tocopheroxyl (α-Toc(•)) radical was performed by reacting aroxyl (ArO(•)) radical with α-tocopherol (α-TocH) in acetonitrile solution including four kinds of alkali and alkaline earth metal salts (MX or MX(2)) (LiClO(4), LiI, NaClO(4), and Mg(ClO(4))(2)), using stopped-flow spectrophotometry. The maximum wavelength (λ(max)) of the absorption spectrum of the α-Toc(•) at 425.0 nm increased with increasing concentration of metal salts (0-0.500 M) in acetonitrile, and it approached constant values, suggesting an [α-Toc(•)-M(+) (or M(2+))] complex formation. The stability constants (K) were determined to be 9.2, 2.8, and 45 M(-1) for LiClO(4), NaClO(4), and Mg(ClO(4))(2), respectively. By reacting ArO(•) with α-TocH in acetonitrile, the absorption of ArO(•) disappeared rapidly, while that of α-Toc(•) appeared and then decreased gradually as a result of the bimolecular self-reaction of α-Toc(•) after passing through the maximum. The second-order rate constants (k(s)) obtained for the reaction of α-TocH with ArO(•) increased linearly with an increasing concentration of metal salts. The results indicate that the hydrogen transfer reaction of α-TocH proceeds via an electron transfer intermediate from α-TocH to ArO(•) radicals followed by proton transfer. Both the coordination of metal cations to the one-electron reduced anions of ArO(•) (ArO:(-)) and the coordination of counteranions to the one-electron oxidized cations of α-TocH (α-TocH(•)(+)) may stabilize the intermediate, resulting in the acceleration of electron transfer. A remarkable effect of metal salts on the rate of bimolecular self-reaction (2k(d)) of the α-Toc(•) radical was also observed. The rate constant (2k(d)) decreased rapidly with increasing concentrations of the metal salts. The 2k(d) value decreased at the same concentration of the metal salts in the following order: no metal salt > NaClO(4) > LiClO(4) > Mg(ClO(4))(2). The complex formation between α-Toc(•) and metal cations may stabilize the energy level of the reactants (α-Toc(•) + α-Toc(•)), resulting in the decrease of the rate constant (2k(d)). The alkali and alkaline earth metal salts having a smaller ionic radius of cation and a larger charge of cation gave larger K and k(s) values and a smaller 2k(d) value.

Ultrafast Vibrational Spectroscopic Studies on the Photoionization of the α-Tocopherol Analogue Trolox C

“…Photoionization of tocopherols as shown here may be regarded as a half-reaction of the biochemical process. Our results show that due to the very high acidity of the tocopheroxyl radical cation, in the transition state (shown in Scheme ) proton transfer would proceed extremely rapidly and the reaction may be conveniently viewed as a concerted electron–proton transfer. , …”

“…Within the context of α-tocopherol as an antioxidant in biochemical systems, the postulated involvement of the radical cation in the transition state has been supported by experiments showing that the rate of reduction of aroxyl radicals by αtocopherol is increased by the presence of metal salts. 42,43 This effect was suggested to occur through stabilization of the transition state in which electron transfer creates the tocopherol radical cation−aroxyl anion transition state complex as shown in Scheme 4. Photoionization of tocopherols as shown here may be regarded as a half-reaction of the biochemical process.…”

“…Scheme 4. Transition State Stabilization by Metal Salts in the Reduction of Aroxyl Radicals (ArO • ) by Tocopherol As Proposed by Kohno et al43 …”

Can aluminum, a non-redox metal, alter the thermodynamics of key biological redox processes? The DPPH-QH 2 radical scavenging reaction as a test case.

“…This supported previous studies where it was established that radical scavenging reactions can be affected by redox inactive metals, e.g., Mg(II), Al(III) and Sc(III). [21][22][23][24][25][26][27][28][29][30] In the present work, we give computational support to this study, determining how the presence of aluminum can alter the oxidation/reduction potential of these species. To achieve our goal, we investigated the different coordination modes that aluminum can form with DPPH• and QH 2 mimicking the experimental conditions and assuming an outer-sphere mechanism, as done in the reference experimental study.…”

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

“…In previous works, , in order to clarify the effect of alkali and alkaline earth metal ions on the scavenging reaction of the free radical by α-TocH, we measured the second-order rate constants ( k s ) for the reaction of α-TocH with 2,6-di- t -butyl-4-(4-methoxyphenyl)phenoxyl (aroxyl, ArO • ) (see Figure ) in protic methanol and aprotic acetonitrile solvents including various concentrations of metal salts (LiI, LiClO 4 , NaI, NaClO 4 , KI, and Mg(ClO 4 ) 2 ) (reaction ), using stopped-flow spectrophotometry. The effects of these metal salts on the bimolecular self-reaction rate (2 k d ) (reaction ) and UV–vis absorption spectrum of α-Toc • were also studied. ArO • + TocH → k normals ArOH + Toc • Some notable effects of the metal salts were observed for (i) the rate constant ( k s ), (ii) the rate constant (2 k d ), and (iii) the maximum wavelength (λ max ) (and the molar extinction coefficient (ε max )) of α-Toc • in acetonitrile . Similarly, the effect of the metal salts was observed for the k s value in methanol .…”