A detailed kinetic study was performed for the reaction of the aroxyl radical (ArO • ) with eight vegetable oils 1-8, which contain different concentrations of α-, β-, γ-, and δ-tocopherols and -tocotrienols (-Tocs and -Toc-3s). The second-order rate constants (k s ) and aroxyl radical absorption capacity (ARAC) values for the reaction of ArO • with vegetable oils 1-8 (rice bran 1, perilla 2, rapeseed 3, safflower 4, grape seed 5, sesame 6, extra virgin olive 7, and olive oils 8) were measured in ethanol/chloroform/D 2 O (50:50:1, v/v/ v) solution at 25 C using stopped-flow spectrophotometry. The k s value (16.1 × 10 −3 L g −1 s −1 ) of rice bran oil 1 with the highest activity was 8.0 times larger than that (2.02 × 10 −3 ) of olive oil 8 with the lowest activity. The concentrations (in mg 100 g −1 ) of α-, β-, γ-, and δ-Tocs and -Toc-3s contained in the vegetable oils 1-8 were determined using high performance liquid chromatography-mass spectrometry/mass spectrometry (HPLC-MS/MS). From these results, it was clarified that the ArO • -scavenging rates (k s ) (i.e., the relative ARAC value) obtained for the vegetable oils 1-8 may be well explained as the sum of the product P k AOH-i s AOH-i ½ =10 5 È É of the rate constant (k AOH-i s ) and the concentration ([AOH-i]/10 5 ) of AOH-i (Tocs and Toc-3s) included in vegetable oils. The results suggest that the ARAC assay method might be used in the evaluation of antioxidant activity of general food extracts.

Singlet-oxygen (1O2) quenching, free-radical scavenging, and excited-state intramolecular proton-transfer (ESIPT) activities of hydroxyflavones, anthocyanidins, and 1-hydroxyanthraquinones were studied by means of laser, stopped-flow, and steady-state spectroscopies. In hydroxyflavones and anthocyanidins, the 1O2 quenching activity positively correlates to the free-radical scavenging activity. The reason for this correlation can be understood by considering that an early step of each reaction involves electron transfer from the unfused phenyl ring (B-ring), which is singly bonded to the bicyclic chromen or chromenylium moiety (A- and C-rings). Substitution of an electron-donating OH group at B-ring enhances the electron transfer leading to activation of the 1O2 quenching and free-radical scavenging. In 3-hydroxyflavones, the OH substitution at B-ring reduces the activity of ESIPT within C-ring, which can be explained in terms of the nodal-plane model. As a result, the 1O2 quenching and free-radical scavenging activities negatively correlate to the ESIPT activity. A catechol structure at B-ring is another factor that enhances the free-radical scavenging in hydroxyflavones. In contrast to these hydroxyflavones, 1-hydroxyanthraquinones having an electron-donating OH substituent adjacent to the O–H---OC moiety susceptible to ESIPT do not show a simple correlation between their 1O2 quenching and ESIPT activities, because the OH substitution modulates these reactions.

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Cascade Si LVV Auger electron spectra at the photoexcitation of the Si 1s electron in a SiCl4 molecule have been measured using an electron spectrometer combined with monochromatized undulator radiation. In the instance of the resonant excitation of the Si 1s electron into the vacant molecular orbital a peak with high yield is observed at about 106 eV, an energy considerably higher than the energies of the normal LVV Auger electron. This peak is presumed to originate from the participator decay from the state with two 2p holes and one excited electron into the state with one 2p hole and one valence hole. Following the normal KL23L23 Auger transition, the cascade spectrum shows several peak structures, e.g. 63 eV, 76 eV and 91 eV. The peak at 91 eV is probably assigned to the second step Auger decay into states having a 2p hole together with two valence holes. These findings are similar to experimental results of SiF4. The former two peaks (63 eV and 76 eV) are ascribed to Auger transitions of Si atomic ions produced through molecular ion dissociation after the first step cascade decays, although the peak heights of atomic ions are lower than those of SiF4.

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