A new free radical absorption capacity assay method is proposed with use of an aroxyl radical (2,6-di-tert-butyl-4-(4'-methoxyphenyl)phenoxyl radical) and stopped-flow spectroscopy and is named the aroxyl radical absorption capacity (ARAC) assay method. The free radical absorption capacity (ARAC value) of each tocopherol was determined through measurement of the radical-scavenging rate constant in ethanol. The ARAC value could also be evaluated through measurement of the half-life of the aroxyl radical during the scavenging reaction. For the estimation of the free radical absorption capacity, the aroxyl radical was more suitable than the DPPH radical, galvinoxyl, and p-nitrophenyl nitronyl nitroxide. The ARAC value in tocopherols showed the same tendency as the free radical absorption capacities reported previously, and the tendency was independent of an oxygen radical participating in the scavenging reaction and of a medium surrounding the tocopherol and oxygen radical. The ARAC value can be directly connected to the free radical-scavenging rate constant, and the ARAC method has the advantage of treating a stable and isolable radical (aroxyl radical) in a user-friendly organic solvent (ethanol). The ARAC method was also successfully applied to a palm oil extract. Accordingly, the ARAC method would be useful in free radical absorption capacity assay of antioxidative reagents and foods.

Measurements of aroxyl radical (ArO•)‐scavenging rate constants (ks AOH ) of antioxidants (AOHs) (α‐tocopherol (α‐TocH) and three catechins (CatHs) (ie, epicatechin (EC), epigallocatechin (EGC), and epigallocatechin gallate (EGCG)) were performed in ethanol solution, using stopped‐flow spectrophotometry. ks AOH values were measured not only for each AOH, but also for the mixtures of two AOHs (α‐TocH and CatH). A notable synergistic effect that the ksαnormal- TocH value of α‐TocH increases 1.29, 1.84, and 1.65 times under the coexistence of constant concentrations of EC, EGC, and EGCG, respectively, was observed for the solutions including α‐TocH and CatH. Similarly, ks CatH values of CatHs (EC, EGC, and EGCG) increased 1.72, 2.25, and 2.34 times under the coexistence of constant concentrations of α‐TocH, respectively. UV‐Vis absorption of α‐tocopheroxyl radical (α‐Toc•) (λmax = 428 nm), which had been produced by reaction of α‐TocH with ArO•, decreased remarkably under the coexistence of α‐TocH and CatHs due to the fast α‐TocH‐regeneration reaction by CatHs. The result suggests that the prooxidant reaction due to α‐Toc• is suppressed by the coexistence of CatHs. By analyzing the formation and decay curves of α‐Toc•, it has been ascertained that one molecule of EGCG having three OH groups at B‐ring may rapidly regenerate three molecules of α‐Toc• to α‐TocH.

Spectator resonant Auger electron spectra with the Si 1s photoexcitation of Cl3SiSi(CH3)3 have been measured using an electron spectroscopic technique combined with undulator radiation. The transition with the highest intensity in the total ion yield (TIY) spectrum, coming from excitation of a Si 1s electron on the Cl-side into a vacant valence orbital, generates the resonant Auger decay in which the excited electron remains in this valence orbital. Photoexcitation of 1s electrons into some Rydberg orbitals induces Auger shake-down transitions, because higher-lying Rydberg orbitals in the two Si atoms closely positioned hold spatially overlapping considerably. A broad TIY peak slightly above the 1s ionization thresholds appreciably yields resonant Auger decays in which a slow photoelectron is re-captured into a higher-lying Rydberg orbital. The normal Auger peak shape at this photon energy is distorted due to a post-collision interaction effect. These findings provide a clear understanding on properties of the excited orbitals which are ambiguous in the measurement of the TIY only.

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