Ultrafast Surface-Specific Spectroscopy of Water at a Photoexcited TiO2 Model Water-Splitting Photocatalyst

A critical step in photocatalytic water dissociation is the hole-mediated oxidation reaction. Molecular-level insights into the mechanism of this complex reaction under realistic conditions with high temporal resolution are highly desirable. Here, we use femtosecond time-resolved, surface-specific vibrational sum frequency generation spectroscopy to study the photo-induced reaction directly at the interface of the photocatalyst TiO2 in contact with liquid water at room temperature. Thanks to the inherent surface specificity of the spectroscopic method, we can follow the reaction of solely the interfacial water molecules directly at the interface at timescales on which the reaction takes place. Following the generation of holes at the surface immediately after photoexcitation of the catalyst with UV light, water dissociation occurs on a sub-20 ps timescale. The reaction mechanism is similar at pH 3 and 11. In both cases, we observe the conversion of H2O into Ti−OH groups and the deprotonation of pre-existing Ti−OH groups. This study provides unique experimental insights into the early steps of the photo-induced dissociation processes at the photocatalyst-water interface, relevant to the design of improved photocatalysts

Die Supervision auf dem Weg zur Profession? Professionalisierung im Spannungsfeld von Expansionsbestrebung und Selbstbescheidung

Kühl S. Die Supervision auf dem Weg zur Profession? Professionalisierung im Spannungsfeld von Expansionsbestrebung und Selbstbescheidung. Organisationsberatung - Supervision - Coaching. 2006;13(1):5-18

The utilization of 2-ketogluconate by Hydrogenomonas eutropha H 16.

During growth of Hydrogenomonas eutropha H 16 on 2-ketogluconate, 2-ketogluconate kinase and 2-keto-6-phosphogluconate reductase were formed. These enzymes were absent from cells grown on fructose, gluconate, acetate, succinate or autotrophically. There was no evidence for extracellular oxidation of glucose, fructose or gluconate with the formation of ketogluconic acids. The utilization of 2-ketogluconate is neither subject to catabolite inhibition by hydrogen nor is 2-keto-6-phosphogluconate reductase inhibited by ATP, ADP or phosphoenolpyruvate. The enzyme is characterized by a high affinity for its substrates

Localization and stability of hydrogenases from aerobic hydrogen bacteria.

Alcaligenes eutrophus strains H 16, B 19, G 27 and N9A contained two different hydrogenases. One enzyme catalyzed the reduction of NAD by hydrogen and was strictly localized in the soluble cell fraction, while the second enzyme was found to be particulate and unable to react with NAD. All other tested strains, Alcaligenes paradoxus SA 29, Pseudomonas facilis, P. palleronii RH 2, Pseudomonas sp. strain GA 3, Paracoccus denitrificans, Aquaspirillum autotrophicum SA 32, and Corynebacterium autotrophicum 14g and 7C contained only a single enzyme exclusively bound to membranes. This was established using fractional centrifugation, indicator enzyme systems, gentle methods of cell disintegration and discontinuous sucrose density gradient centrifugation. In cell-free extracts obtained by rough disruption (sonication) of cells, hydrogenase was associated to particles of different size and sedimentation velocity. A partial solubilization of hydrogenase caused by sonication was observed with P. facilis. Without exception, the particulate hydrogenases were found (1) to be unable to reduce pyridine nucleotides, and (2) to reduce methylene blue at an extremely high activity. The eminent reaction rate of 34 μmoles H2 oxidized per min and mg protein has been determined in particle suspensions of Pseudomonas sp. strain GA 3. All hydrogenases were stable during storage under hydrogen atmosphere, except the soluble enzyme from A. eutrophus H 16 which was shown to be more stable under aerobic conditions

Physiological characterization of the hydrogen bacterium <em>Aquaspirillum autotrophicum</em>.

Aquaspirillum autrotrophicum, an aerobic hydrogen bacterium recently isolated from an eutrophic freshwater lake, was characterized physiologically. It grew autotrophically in a fermenter with a doubling time of 4 h. Heterotrophic growth was faster. pH-Optimum ranged from 5.0-7.5, temperature optimum was about 28&deg; C. During autotrophic growth about 10 moles hydrogen were consumed per 1 mole carbon dioxide fixed. Hydrogenase activity is inducible. CO2 did not enhance the oxy-hydrogen reaction by intact cells. The hydrogenase activity was localized in the particulate fraction. The hydrogenase reduced methylene blue and phenazine methosulfate; pyridine nucleotides were not reduced. In cell-free extracts, hydrogenase was sensitive to oxygen. Ribulosebisphosphate carboxylase was present in autotrophically-grown cells and absent from heterotrophically grown cells. Hydrogenase induction in heterotrophically-grown cells followed parabolic kinetics. Oxygen and D-gluconate repressed hydrogenase synthesis, whereas citrate, DL-lactate and pyruvate stimulated its formation. The repressive effect was delayed. The results suggest that the control of hydrogenase synthesis occurred at the transcriptional level, and that mRNA coding for the hydrogenase had a relatively long life span. D-Gluconate was degraded via the Entner-Doudoroff pathway, the enzymes of which were constitutively formed. Enzymes of the pentosephosphate and Embden-Meyerhof pathways (except phosphofructokinase) were present, too. Hydrogen did not inhibit heterotrophic growth. The possible competitive advantage of the physiological properties described with regard to the natural habitat was discussed

Studies on a gram-positive hydrogen bacterium, <em>Nocardia opaca</em> 1 b : III. Purification, stability and some properties of the soluble hydrogen dehydrogenase.

Nocardia opaca strain 1 b has a NAD-dependent hydrogenase (hydrogen dehydrogenase). The enzyme has been purified from autotrophically grown cells and tested for optimal assay conditions and stability. The purification procedure involved protamine sulfate treatment, ammonium sulfate precipitation, and separation by DEAE-cellulose and Sephadex G-200 chromatography and resulted in a 63-fold increase of specific activity at a 11.7% enzyme recovery. The final specific activity was 103 &mu;moles H2/min&middot;mg protein. The purified enzyme was dependent on nickel and magnesium ions at 0.5 and 5.0 mM concentrations, respectively, as well as flavin mononucleotide at a 5-10 &mu;M concentration. Straight enzyme kinetics were achieved by preincubating the enzyme in the presence of NADH2. A high stability of the enzyme was observed in 0.1 M potassium phosphate buffer, pH 6.5, in the presence of 0.5 mM nickel and 5 mM magnesium ions under hydrogen atmosphere. Even under air the enzyme was remarkably stable, although less than under hydrogen. From double reciprocal plots of substrate saturation curves the Michaelis-Menten constants were calculated: For saturating NAD-concentration the Kmwas 0.063 mM H2 and for saturating hydrogen concentration the Kmwas 0.123 mM NAD

&alpha;-Isopropylmalate synthase from Alcaligenes eutrophus H 16 - I. Purification and general properties.

&alpha;-Isopropylmalate (IPM) synthase, the first enzyme in the biosynthesis of l-leucine, was purified to a specific activity of 12 &mu;mole/min x mg protein from the valine-isoleucine double auxotrophic mutant A-81 of the hydrogen bacterium Alcaligenes eutrophus H 16. The activity in crude extracts of derepressed cells was 0.106 &mu;moles of isopropylmalate formed per min and per mg protein. Gel electrophoresis and regel electrophoresis of the isolated main band resulted in several distinct bands, which were not altered by the additions of substrate &alpha;-ketoisovalerate, feedback inhibitor leucine or other effectors. The isoelectric points of the enzyme protein was between 3.9 and 4.0. The molecular weight was 114500 daltons and 100000 respectively in the absence and presence of the feedback inhibitor leucine. The enzyme activity depended strongly on the pH, the optimum is at pH 8.2. The enzyme was could labile and exhibits temperature anomalies

Regulation of the glucose-6-phosphate dehydrogenase of different bacterial species by ATP.

From cell free extracts of Hydrogenomonas facilis, Pseudomonas fluorescens, Escherichia coli, Acetobacter xylinum and A. suboxydans the glucose-6-phosphate dehydrogenases have been purified up to twenty or eighty fold. The enzymes from Hydrogenomonas and Pseudomonas are characterized by specificity for NAD and NADP, by sigmoid substrate (G-6-P)1 saturation curves, and by inhibition by adenosine triphosphate. The E. coli enzyme is specific for NADP and is not inhibited by ATP; the substrate saturation curves are hyperbolic even in the presence of ATP. The enzymes from Acetobacter xylinum and A. suboxydans exhibit specificity for NAD and NADP and hyperbolic substrate saturation curves and are not inhibited by ATP. The results coincide with the idea, the glucose-6-phosphate dehydrogenases functioning only in a degradative pathway are subject to adenylate control with ATP as a negative allosteric effector