Photochemical formation and decay of tocopheroxyl radical in vitamin E emulsion: A laser-photolysis study

In order to investigate effect of the inhomogeneous dispersion system on photogeneration and decay dynamics of the tocopheroxyl radical (Toc), transient absorption and steady-state and time-resolved EPR measurements on photolysis of vitamin E (VE) were carried out. Tocopherol (TocH) is photo-decomposed through electron-detachment in the excited state and the subsequent proton releasing, which produce Toc and the solvated electron (esolv−). The decay of Toc is explained by (i) a rapid decay within 2 μs by recombination between Toc and esolv−, (ii) a second-order decay observed in the range of 5–150 μs by reaction between Toc and the superoxide anion radical (O2−), and (iii) a slow decay in >10 ms by bimolecular self-quenching of Toc. The Toc decay in the VE emulsion was very different from that in the homogeneous solution. In the emulsion, the Toc yield was small, the contribution of the reaction with O2− was rather small, and the decay due to the bimolecular reaction of Toc was much accelerated. In the emulsion, Toc could not be detected by steady-state EPR, although it could in ethanol. These differences should be caused by the oil-droplet formation in the emulsion. From these results, we can expect that the photostability of the VE emulsion is superior to that of the VE homogeneous solutions.