ESR Spectrum of HO2 in Solutions of Hydrogen Peroxide in Water at 77°K

Role of Interstitial Voids in Oxides on Formation and Stabilization of Reactive Radicals: Interstitial HO2 Radicals in F2-Laser-Irradiated Amorphous SiO2

A procedure to produce stable hydroperoxy radicals (HO*2) in bulk amorphous SiO(2) (a-SiO(2)) has been developed. Oxygen molecules incorporated in the interstitial voids in a-SiO(2) react with mobile hydrogen atoms (H(0)) generated by the photolysis of silanol (SiOH) groups with F(2)-laser light (lambda = 157 nm, hnu = 7.9 eV), resulting in the efficient creation of interstitial HO*2. The high yield of HO*2 suggests that the collisions of the reaction intermediate with the void wall play an important role in dissipating the excess energy of the intermediate instead of the triple collision observed in the gas phase reaction. The resultant HO*2 is thermally stable up to 100 degrees C.

Single Electron Traps at the Surface of Polycrystalline MgO: Assignment of the Main Trapping Sites

Paramagnetic centers at the surface of ionic oxides in the form of trapped electrons can be generated by exposure of the material to alkali metal or hydrogen atoms or of molecular hydrogen under UV irradiation. For many years, it has been assumed that the resulting paramagnetic centers consist of oxygen vacancies filled by one electron. High-resolution electron spin resonance spectra and ab initio quantum chemical calculations show that the paramagnetic centers consist of (H(+))(e(-)) electron pairs formed at morphological irregularities of the surface. At least three different kinds of (H(+))(e(-)) centers, [A], [B], and [C], have been identified with abundances of 80%, 10%, and 8%, respectively. In this work, we compare a wide set of measured and computed g-factors and hyperfine coupling constants of the unpaired electron with the surrounding (25)Mg, (17)O, and (1)H nuclei and we propose a general assignment of the centers. (H(+))(e(-)) pairs formed at Mg(4c) ions at steps and edges account for species [A], centers formed at Mg(4c) ions at reverse corners correspond to species [B], and species [C] originates from (H(+))(e(-)) pairs formed at Mg(3c) ions at corners and kinks.

Increased generation of hydrogen peroxide possibly from mitochondrial respiratory chain after UVB irradiation of murine fibroblasts

The purpose of this study is to detect the generation of active oxygens in UVB-irradiated murine fibroblasts and to propose new mechanisms. Decreased survival of fibroblasts under UVB irradiation was partially recovered by addition of catalase, DMSO or deferoxamine, suggesting the contribution of several types of active oxygen species. Then we examined the formation of active oxygen species and found that fibroblasts under UVB irradiation generated superoxide anion radicals (.O2-), intracellular H2O2, and hydroxyl radicals as estimated by the ESR-spin trapping method. Addition of thenoyltrifluoroacetone, which is an inhibitor of the mitochondrial respiratory chain, decreased 29% of the intracellular H2O2 levels in UVB-irradiated cells, but allopurinol, which is an inhibitor of xanthine oxidase, had no effect on them. On the basis of these results, we propose a a possible mechanism for damage of murine fibroblasts exposed to UVB in terms of generation of active oxygen species. The mitochondrial respiratory chain reaction stimulated by UVB irradiation enhances the generation of .O2-, which is in turn dismutated to H2O2 and O2 by superoxide dismutase. H2O2 is then converted to hydroxyl radicals, catalyzed by trace elements such as iron, as suggested by Fenton-like reaction. Thus, hydroxyl radicals with higher reaction rate-constants than those of other active oxygen species to biomolecules are indicated to be responsible for the cytotoxicity in cells under UV irradiation.