The mechanism of water photooxidation reaction at atomically flat n-TiO(2) (rutile) surfaces was investigated in aqueous solutions of various pH values, using photoluminescence (PL) measurements. The PL bands, which peaked at around 810 and 840 nm for the (110) and (100) surfaces, respectively, were assigned to radiative transitions between conduction-band electrons and surface-trapped holes (STH), [Ti-O=Ti(2)](s)+, formed at triply coordinated (normal) O atoms at the surface lattice. The PL intensity (I(PL)) decreased stepwise with increasing solution pH, namely, it sharply decreased at around pH 4, near the point of zero charge of TiO(2) (about 5), and then rapidly decreased to zero near pH 13. The first sharp decrease around pH 4 is attributed to the increased rate of nucleophilic attack of a water molecule to a hole at a site of surface bridging oxygen (Ti-O-Ti), the density of which increases with increasing pH. The nucleophilic attack is regarded as the main initiating step of the water oxidation reaction in low and intermediate pH. The high PL intensity at low pH is ascribed to slow nucleophilic attack owing to a very low density of Ti-O-Ti by its protonation at the low pH. The second sharp decrease near pH 13 is attributed to formation of surface anionic species like Ti-O- which can be readily oxidized by photogenerated holes. Interrelations between reaction intermediates proposed in this work and those reported by time-resolved laser spectroscopy are discussed.

Multiple internal reflection infrared spectroscopy was applied to in situ investigations of surface intermediates of photocatalytic reactions on nanocrystalline TiO(2) films in contact with aqueous solutions. UV irradiation in the presence of dissolved O(2) caused the appearance of new bands peaked at 943, 838, and 1250-1120 cm(-)(1) together with intensity changes in other bands. Investigations of influences of the solution pH, the presence or absence of hole and electron scavengers, and isotopic H(2)O --> D(2)O exchange on the spectral changes have revealed that the primary step of photocatalytic O(2) reduction is the formation of the surface peroxo species, Ti(O(2)), giving the 943 cm(-)(1) band, probably with the surface superoxo species, TiOO., as a precursor, in neutral and acidic solutions. The surface peroxo species is then transformed to the surface hydroperoxo, TiOOH, giving the 838 and 1250-1120 cm(-)(1) bands, by protonation in the dark. This is, to our knowledge, the first direct in situ spectroscopic detection of primary intermediates for the photocatalytic O(2) reduction in aqueous solutions. On the basis of the assignment, a possible reaction scheme for various processes of the photocatalytic O(2) reduction is proposed, which is in harmony with other spectral changes induced by the UV irradiation.

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Proton pump inhibitors (PPIs) are widely used in the treatment of acid-related diseases. However, several unmet medical needs, such as suppression of night-time acid secretion and rapid symptom relief, remain. In this study, we investigated the pharmacological effects of 1-[5-(2-fluorophenyl)-1-(pyridin-3-ylsulfonyl)-1H-pyrrol-3-yl]-N-methylmethanamine monofumarate (TAK-438), a novel potassium-competitive acid blocker (P-CAB), on gastric acid secretion in comparison with lansoprazole, a typical PPI, and SCH28080 [3-(cyanomethyl)-2-methyl,8-(phenylmethoxy)imidazo(1,2-a)pyridine], a prototype of P-CAB. TAK-438, SCH28080, and lansoprazole inhibited H ϩ ,K ϩ -ATPase activity in porcine gastric microsomes with IC 50 values of 0.019, 0.14, and 7.6 M, respectively, at pH 6.5. The inhibitory activity of TAK-438 was unaffected by ambient pH, whereas the inhibitory activities of SCH28080 and lansoprazole were weaker at pH 7.5. The inhibition by TAK-438 and SCH28080 was reversible and achieved in a K ϩ -competitive manner, quite different from that by lansoprazole. TAK-438, at a dose of 4 mg/kg (as the free base) orally, completely inhibited basal and 2-deoxy-D-glucose-stimulated gastric acid secretion in rats, and its effect on both was stronger than that of lansoprazole. TAK-438 increased the pH of gastric perfusate to a higher value than did lansoprazole or SCH28080, and the effect of TAK-438 was sustained longer than that of lansoprazole or SCH28080. These results indicate that TAK-438 exerts a more potent and longer-lasting inhibitory action on gastric acid secretion than either lansoprazole or SCH28080. TAK-438 is a novel antisecretory drug that may provide a new option for the patients with acid-related disease that is refractory to, or inadequately controlled by, treatment with PPIs.