Abstract
Recently, increasing attention has been paid to quantum mechanical behavior in biology. In this study, we investigated the involvement of quantum mechanical tunneling in the hydrogen-transfer reaction from Trolox, a water-soluble analog of vitamin E (α-tocopherol), to 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) in a phosphate buffer solution (0.05 M, pH 7.0). DPPH• was used as a reactivity model of reactive oxygen species and solubilized in water using β-cyclodextrin (β-CD). The second-order rate constants, kH and kD, in 0.05 M phosphate buffer solutions prepared with H2O (pH 7.0) and D2O (pD 7.0), respectively, were determined for the reaction between Trolox and DPPH•, using a stopped-flow technique at various temperatures (283–303 K). Large kinetic isotope effects (KIE, kH/kD) were observed for the hydrogen-transfer reaction from Trolox to the β-CD-solubilized DPPH• in the whole temperature range. The isotopic ratio of the Arrhenius prefactor (AH/AD = 0.003), as well as the isotopic difference in the activation energies (19 kJ mol−1), indicated that quantum mechanical tunneling plays a role in the reaction.
Highlights
Hydrogen-transfer reactions are cornerstones of the radical-scavenging reactions of antioxidants, such as vitamins C and E (α-tocopherol), flavonoids, and so on, where hydrogen atoms are transferred from antioxidants to oxygen radicals as an initial step
On the other hand, increasing attention has been paid to quantum mechanical behavior in biology in recent years [1], such as hydrogen tunneling [2,3,4,5]
22.5, observed in the hydrogen-transfer reaction from α-tocopherol to aroxyl radical in ethanol, demonstrating that quantum mechanical tunneling plays a role in this reaction [6]
Summary
Hydrogen-transfer reactions are cornerstones of the radical-scavenging reactions of antioxidants, such as vitamins C (ascorbic acid) and E (α-tocopherol), flavonoids, and so on, where hydrogen atoms (or protons and electrons) are transferred from antioxidants to oxygen radicals as an initial step. A phosphate buffer (Figure 1), indicating that quantum mechanical tunneling plays a role smaller than the semi-classical isotope effect for O–H bonds (7.9) [16]. After the mixing of DPPH in water (Milli-Q) with a phosphate buffer solution (0.1 M, pH 7.0) containing Trolox at a volumetric ratio of 1:1 using a stopped-flow technique on a UNISOKU RSP-1000-02NM spectrophotometer (UNISOKU Co., Ltd., Osaka, Japan), which was thermostated with a Thermo Scientific NESLAB RTE-7 Circulating Bath
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