Water Cleavage Research Articles

Radiation-induced lysosomal iron reactivity: Implications for radioprotective therapy

A novel mechanism of radiosensitization involves radiation-enhanced autophagy of damaged mitochondria and various metalloproteins, by which iron accumulates within lysosomes. Hydrogen peroxide, formed by the radiolytic cleavage of water, generates in the presence of lysosomal redox-active iron extremely reactive hydroxyl radicals by Fenton-type chemistry. Subsequent peroxidative damage of lysosomal membranes initiates release of harmful content from ruptured lysosomes that triggers a cascade of events eventuating in DNA damage and apoptotic or necrotic cell death. This article reviews the role of lysosomal destabilization in radiation-induced cell damage and death. The potential effects of iron chelation therapy targeted to the lysosomes for protection of normal tissues against unwanted effects by radiation is also discussed.

Visible Light-Induced Water Oxidation on Mesoscopic α-Fe2O3 Films Made by Ultrasonic Spray Pyrolysis

Alpha-Fe(2)O(3) films having a mesoscopic leaflet type structure were produced for the first time by ultrasonic spray pyrolysis (USP) to explore their potential as oxygen-evolving photoanodes. The target of these studies is to use translucent hematite films deposited on conducting fluorine doped tin oxide (FTO) glass as top electrodes in a tandem cell that accomplishes the cleavage of water into hydrogen and oxygen by sunlight. The properties of layers made by USP were compared to those deposited by conventional spray pyrolysis (SP). Although both types of films show similar XRD and UV-visible and Raman spectra, they differ greatly in their morphology. The mesoscopic alpha-Fe(2)O(3) layers produced by USP consist mainly of 100 nm-sized platelets with a thickness of 5-10 nm. These nanosheets are oriented mainly perpendicularly to the FTO support, their flat surface exposing (001) facets. The mesoscopic leaflet structure has the advantage that it allows for efficient harvesting of visible light, while offering at the same time the very short distance required for the photogenerated holes to reach the electrolyte interface before recombining with conduction band electrons. This allows for water oxidation by the valence band holes even though their diffusion length is only a few nanometers. Distances are longer in the particles produced by SP favoring recombination of photoinduced charge carriers. Open-circuit photovoltage measurements indicate a lower surface state density for the nanoplatelets as compared to the round particles. These factors explain the much higher photoactivity of the USP compared to the SP deposited alpha-Fe(2)O(3) layers. Addition of hydrogen peroxide to the alkaline electrolyte further improves the photocurrent-voltage characteristics of films generated by USP indicating the hole transfer from the valence band of the semiconductor oxide to the adsorbed water to be the rate-limiting kinetic step in the oxygen generation reaction.

Fractionation of the three stable oxygen isotopes by oxygen-producing and oxygen-consuming reactions in photosynthetic organisms.

The triple isotope composition (delta17O and delta18O) of dissolved O2 in the ocean and in ice cores was recently used to assess the primary productivity over broad spatial and temporal scales. However, assessment of the productivity with the aid of this method must rely on accurate measurements of the 17O/16O versus 18O/16O relationship in each of the main oxygen-producing and -consuming reactions. Data obtained here showed that cleavage of water in photosystem II did not fractionate oxygen isotopes; the delta18O and delta17O of the O2 evolved were essentially identical to those of the substrate water. The fractionation slopes for the oxygenase reaction of Rubisco and respiration were identical (0.518 +/- 0.001) and that of glycolate oxidation was 0.503 +/- 0.002. There was a considerable difference in the slopes of O2 photoreduction (the Mehler reaction) in the cyanobacterium Synechocystis sp. strain PCC 6803 (0.497 +/- 0.004) and that of pea (Pisum sativum) thylakoids (0.526 +/- 0.001). These values provided clear and independent evidence that the mechanism of O2 photoreduction differs between higher plants and cyanobacteria. We used our method to assess the magnitude of O2 photoreduction in cyanobacterial cells maintained under conditions where photorespiration was negligible. It was found that electron flow to O2 can be as high as 40% that leaving photosystem II, whereas respiratory activity in the light is only 6%. The implications of our findings to the evaluation of specific O2-producing or -consuming reactions, in vivo, are discussed.

Beyond Oil and Gas: The Methanol Economy

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reaction pattern of Photosystem II: oxidative water cleavage and protein flexibility

This short communication addresses three topics of photosynthetic water cleavage in Photosystem II (PS II): (a) effect of protonation in the acidic range on the extent of the 'fast' ns kinetics of P680+. reduction by YZ, (b) mechanism of O-O bond formation and (c) role of protein flexibility in the functional integrity of PS II. Based on measurements of light-induced absorption changes and quasielastic neutron scattering in combination with mechanistic considerations, evidence is presented for the protein acting as a functionally active constituent of the water cleavage machinery, in particular, for directed local proton transfer. A specific flexibility emerging above a threshold of about 230 K is an indispensable prerequisite for oxygen evolution and plastoquinol formation.

Cleavage and Recovery of Molecular Water in Silica

The network response associated with the incorporation and reactivity of water molecules in bulk phases of amorphous and crystalline silica are investigated using density functional theory. The extent of network relaxation is found to change the relative stabilities of the reactant and product states. A highly reactive site, with a low activation barrier, is associated with a highly strained site in which network relaxation significantly stabilizes the silanol state by effectively annealing the local structure. Diffusion and exchange reaction paths are found to likely be associated with minimum energy paths in which the stability of the product and reactant states are equal. These latter paths are associated with minimal network response, although the ability of the silanol groups to take on several conformations has an overall effect of changing the stability along a given reaction path.

Thermostability and Ca2+Binding Properties of Wild Type and Heterologously Expressed PsbO Protein from Cyanobacterial Photosystem II

Oxygenic photosynthesis takes place in the thylakoid membrane of cyanobacteria, algae, and higher plants. Initially light is absorbed by an oligomeric pigment-protein complex designated as photosystem II (PSII), which catalyzes light-induced water cleavage under release of molecular oxygen for the biosphere on our planet. The membrane-extrinsic manganese stabilizing protein (PsbO) is associated on the lumenal side of the thylakoids close to the redox-active (Mn)(4)Ca cluster at the catalytically active site of PSII. Recombinant PsbO from the thermophilic cyanobacterium Thermosynechococcus elongatus was expressed in Escherichia coli and spectroscopically characterized. The secondary structure of recombinant PsbO (recPsbO) was analyzed in the absence and presence of Ca(2+) using Fourier transform infrared spectroscopy (FTIR) and circular dichroism spectropolarimetry (CD). No significant structural changes could be observed when the PSII subunit was titrated with Ca(2+) in vitro. These findings are compared with data for spinach PsbO. Our results are discussed in the light of the recent 3D-structural analysis of the oxygen-evolving PSII and structural/thermodynamic differences between the two homologous proteins from thermophilic cyanobacteria and plants.

Mesoscopic Solar Cells for Electricity and Hydrogen Production from Sunlight

Abstract The quality of human life depends to a large degree on the availability of energy sources. The present annual worldwide energy consumption has already attained a level of over 400 exajoules and is expected to further augment steeply from the increase in world population and the rising demand of energy in the developing countries. This implies enhanced depletion of fossil fuel reserves leading to further aggravation of the environmental pollution. Quality of life on earth is threatened unless renewable energy resources can be developed in the near future. Chemistry is expected to make important contributions to identify environmentally friendly solutions of the energy problem. One attractive strategy discussed in this article is the development of systems that mimic natural photosynthesis in the conversion solar energy. The task to be accomplished by these systems is to harvest sunlight and convert it to electricity or drive an uphill chemical reaction. Learning from the concepts used by green plants, we have developed a molecular photovoltaic device whose overall efficiency for solar energy conversion to electricity has already attained 11%. The system is based on the sensitization of nanocrystalline oxide films by charge-transfer sensitizers. The underlying fundamental processes of light trapping by the sensitizer, heterogeneous electron transfer from the electronically excited chromophore into the conduction band of the semiconductor oxide and the percolative migration of the injected electrons through the mesoporous film to the collector electrode, will be described below in detail. The low cost and ease of production of the new cell should benefit large scale applications. These systems will undoubtedly promote the acceptance of renewable energy technologies, not least by setting new standards of convenience and economy. The nanocrystalline semiconductor junctions developed first for dye sensitized solar cells have meanwhile found a realm of other applications ranging from electrochromic and electroluminescent displays to high power lithium insertion batteries. They are also used in tandem cells for the cleavage of water into hydrogen and oxygen by sunlight.

Solar water splitting to generate hydrogen fuel—a photothermal electrochemical analysis

Theory 1 1 From a similar paper reprinted with permission from S. Licht, J. Phys. Chem., B 107 (2003) 4253. Copyright 2003 American Chemical Society. and experiment 2 2 From a similar paper reprinted with permission from S. Licht, Halperin, L. Kalina, M. Halperin, N. Chem. Commun. 2003, 3006—Reproduced by permission of The Royal Society of Chemistry. are derived for highly efficient photothermal electrochemical solar water splitting to provide clean, renewable sources of hydrogen fuel. The theory derives semiconductor band-gap-restricted, thermal enhanced, solar water splitting efficiencies, combining photodriven charge transfer, with excess sub-band-gap insolation to lower the water potential. Fundamental water thermodynamics and solar photosensitizer constraints determine solar to hydrogen fuel conversion efficiencies in the 50% range over a wide range of insolation, temperature, pressure and photosensitizer band-gap conditions. Experimentally, a novel physical chemical process within molten NaOH is demonstrated, in which an external single, small band-gap photosentizer, such as Si, can drive the energetics of water cleavage. This is accomplished by tuning (decreasing) the water splitting electrochemical potential, E H 2O , rather than tuning the photosensitizer band gap; this diminished potential is due to (i) thermodynamic temperature induced decrease of E H 2O with increasing temperature, and (ii) partial recombination of the cleavage products.

Physicochemical properties and photocatalytic hydrogen evolution of TiO2 films prepared by sol–gel processes

Anatase TiO2 films were obtained on glass substrates using a sol–gel method using titanium isopropoxide as a precursor. The thickness of the film was about 140 nm for one coating, and the thickness is controlled by the number of coating cycles. The spectra of UV-VIS absorption indicated that the absorption edge of the TiO2 films is ca. 385 nm, corresponding to the band gap energy of 3.20 eV. We obtained TiO2 films having a high activity for the hydrogen evolution from photocatalytic water cleavage. By loading with 0.3 wt% Pt rate of hydrogen production increases. No influence of film thickness and calcination temperature on the photocatalytic property is observed.

The Use of Ultrasound for the Degradation of Pollutants in Water: Aquasonolysis – A Review

AbstractThe degradation of organic compounds in water is the subject of a number of fundamental and applied investigations. Aquasonolysis of organic compounds is an advanced technology for the purification of contaminated water. The process is based on the phenomenon of acoustic cavitation. Dominant reactions of volatile compounds are: cavitative high‐temperature gas‐phase reactions and thermal‐oxidative decay in bubbles. Nonvolatile compounds are mainly decomposed by OH radicals in the aqueous solution (sonolytical cleavage of water). This article reviews mechanisms and kinetics of pollutant decomposition in water and discusses the influence of different experimental parameters. An overview of the application spectrum of aquasonolysis is provided, analyzing results gathered in experiments completed in our group as well as results reported in the literature.

Platinum Group Metals and their Oxides in Semiconductor Photosensitisation

The basic principles of semiconductor photochemistry, particularly using titania as a semiconductor photocatalyst, are discussed. When a platinum group metal or its oxide is deposited onto the surface of a sensitised semiconductor the overall efficiency of the reactions it takes part in are often improved, especially when the deposits are used as hydrogen and oxygen catalysts, respectively. Methods of depositing metal or metal oxide are examined, and a particular focus is given to a photodeposition process that uses a sacrificial electron donor. Platinum group metal and platinum group metal oxide coated semiconductor photocatalysts are prominent in heterogeneous systems that are capable of the photoreduction, oxidation and cleavage of water. There is a recent renaissance in work on water-splitting semiconductor-sensitised photosystems, but there are continued concerns over their irreproducibility, longevity and photosynthetic nature.

Electrochemical potential tuned solar water splitting.

As a fundamental step towards a clean, renewable source of H2, a novel physical chemical process within molten NaOH in which an external single, small bad gap photosentizer, such as Si, can drive the energetics of water cleavage is demonstrated, and is accomplished by tuning (decreasing) the water splitting electrochemical potential, EH2O, rather than tuning the photosensitizer band gap; this diminished potential is due to (i) a thermodynamic temperature induced decrease of EH2O with increasing temperature, and (ii) a partial recombination of the cleavage products.

Synthesis and photocatalytic properties of TiO2 and Pt pillared HCa2Nb3O10 doped with various rare earth ions

TiO2 and Pt pillars were constructed in the interlayer of H1−xCa2−xLnxNb3O10 (Ln=La, Nd, Eu, x=0–0.75) by the stepwise intercalation reactions. The amounts of TiO2 and Pt incorporated were 0.5–2.9 and 0.04–0.19 wt.%, respectively. The height of TiO2 and Pt pillar was less than 0.5 nm. TiO2 and/or Pt pillared compounds were capable of water cleavage into hydrogen and oxygen by the band gap irradiation without a sacrificial hole scavenger although no water cleavage proceeded by using H1−xCa2−xLnxNb3O10 separately. The photocatalytic activity for water cleavage was in the order H1−xCa2−xLnxNb3O10/Pt>H1−xCa2−xLnxNb3O10/(Pt, TiO2)>H1−xCa2−xLnxNb3O10/TiO2 and that of Pt pillared nanocomposite was in the order H1−xCa2−xLaxNb3O10/Pt>H1−xCa2−xEuxNb3O10/Pt>H1−xCa2−xNdxNb3O10/Pt.

Identification of free radicals produced during phacoemulsification

Purpose To detect, identify, and quantitate free radicals produced during conditions similar to phacoemulsification cataract surgery. Setting Research laboratory at the Biotechnology Center, Utah State University, Logan, Utah, USA. Methods All experiments were performed using a Series Ten Thousand phacoemulsifier (Alcon Laboratories) modified to make a 10 mL continuous circulation loop (to increase sensitivity). The irrigating solution was passed through a 3 mL chamber in line with the circulation loop, and electron spin resonance spin trapping methods were used to detect, identify, and quantitate free radical production during phacoemulsification. As an additional indication of hydroxyl radical production, the hydroxylation of salicylate and thiocyanate was detected by high-performance liquid chromatography and spectrophotometry, respectively. Results The hydroxyl radical was formed when phacoemulsification was performed in the presence of solutions containing spin trap in double deionized water or balanced salt solution (BSS®). Hydroxyl radical production was linear with respect to phacoemulsification time. Production of the hydroxyl radical was not observed when phacoemulsification was performed with anaerobic solutions, indicating a requirement for oxygen in radical production. The concentration of trapped hydroxyl radical was reduced in the presence of balanced salt solution with bicarbonate, dextrose, and glutathione (BSS Plus®). Upon phacoemulsification, both salicylate and thiocyanate underwent hydroxylation when included in the irrigating solution, confirming the generation of the hydroxyl radical. Additional tests discounted the formation of superoxide or hydrogen peroxide during phacoemulsification. Conclusions Hydroxyl radical was produced by phacoemulsification in the presence of aerobic solutions. Hydroxyl radical production was dependent on the presence of molecular oxygen and was not generated as a result of the homolytic cleavage of water. The amount of hydroxyl radical detected was directly proportional to phacoemulsification time and was reduced in the presence of BSS Plus. Other reactive oxygen species such as superoxide, hydrogen peroxide, and ozone were not detected during phacoemulsification under these conditions.

Modification of the interlayer in K[Ca2Nan−3NbnO3n+1] and their photocatalytic performance for water cleavage

Layered compounds K[Ca2Nan−3NbnO3n+1] (3 ≤ n ≤ 6) were prepared by solid reaction at high temperatures, and the processes for the modification of the interlayers by protonation, intercalation and pillaring were investigated. n-hexylamine could easily be intercalated into the interlayers of H[Ca2Nan−3NbnO3n+1] and enhanced significantly the interlayer distance, which facilitated the incorporation and reaction of tetraethyl orthosilicate with n-hexylamine intercalated compounds. The analysis of XRD, TG-DTA and TEM indicated that silica-pillared layered compounds were formed and had high thermal stability. By comparing the rates of hydrogen evolution from water cleavage on various compounds, the enhancement in the activity of proton-exchanged and silica-pillared compounds was examined in the case of loading Pt. The promotion effect might be due to the adsorption and combination of Pt on the surface of compounds proton-exchanged and pillared.

Synthesis and photochemical properties of Cu2+ doped layered hydrogen titanate

Cu2+ doped layered hydrogen titanate was prepared by the calcination of K2CO3, TiO2 and CuO mixtures with the K2CO3:TiO2:CuO molar ratio of 1:2.5(1−x):2.5x at 1200°C for 5 h followed by an ion-exchange reaction in 1 M HCl solution. The crystalline phase changed from monoclinic hydrogen tetratitanate to an orthorhombic lepidocrocite-type hydrogen titanate by increasing the amount of Cu2+ doped. Both compounds could be excited by visible light irradiation (λ>400 nm) and were capable of hydrogen gas evolution from an aqueous methanol solution, where the photocatalytic activity of Cu2+ doped hydrogen tetratitanate was slightly greater than that of Cu2+ doped lepidocrocite-type hydrogen titanate. The photocatalytic activity of Cu2+ doped hydrogen tetratitanate was enhanced by constructing Pt and TiO2 pillars in the interlayer, and the incorporation of Pt in Cu2+ doped hydrogen tetratitanate enabled the cleavage of water into hydrogen and oxygen by irradiating visible light (λ>400 nm) without a sacrificial hole acceptor.

Hydrogen production by steam–iron process

The steam–iron process is one of the oldest methods of producing hydrogen. It is a cyclic process for water cleavage, whereby coal is consumed. Coal is gassified to a lean reducing gas, containing carbon monoxide and hydrogen. This gas reacts with iron oxides (haematite Fe 2O 3, magnetite Fe 3O 4, wuestite FeO) to produce a reduced form of iron oxide (wuestite FeO, iron Fe). The reduced iron oxide is re-oxidised with steam to form magnetite and hydrogen. After studies concerning theoretical limitations, the subsequent practical realisation by construction of a suitable laboratory prototype reactor was performed. Further, the investigation and optimisation of process variables, accompanied by respective chemical analyses, and finally the simulation of the whole process and the design of a demonstration plant for electricity generation system in the range of 10 MW were carried out. The resulting overall efficiency (heat and electricity) of the respective power plant was calculated as 35% and the electrical efficiency at about 25%. The operation of the small scale “Sponge Iron Reactor” (SIR) showed that the hydrogen produced is sufficiently pure for use in any kind of fuel cell (CO <10 ppm).

Synthesis and photocatalytic properties of titania pillared H4Nb6O17 using titanyl acylate precursor

Titania pillared H 4 Nb 6 O 17 has been synthesised by the intercalation of [Ti(OH) x (CH 3 CO 2 ) y ] z+ followed by photodecomposition with UV light irradiation. The incorporation of TiO 2 in the interlayer of H 4 Nb 6 O 17 has been confirmed by powder X-ray diffraction, DTA, UV-VIS reflectance and BET measurements. The incorporation of TiO 2 in the interlayer of H 4 Nb 6 O 17 results in enhanced water cleavage by band gap irradiation. The photocatalytic activity of TiO 2 pillared H 4 Nb 6 O 17 prepared using [Ti(OH) x (CH 3 CO 2 ) y ] z+ was higher than that of a sample prepared using a TiO 2 sol solution.

Intercalation of titanium oxide in layered H2Ti4O9 and H4Nb6O17 and photocatalytic water cleavage with H2Ti4O9/(TiO2, Pt) and H4Nb6O17/(TiO2, Pt) nanocomposites

Titanium oxide and platinum have been intercalated into H 2 Ti 4 O 9 and H 4 Nb 6 O 17 by the successive reactions of H 2 Ti 4 O 9 and H 4 Nb 6 O 17 with [Pt(NH 3 ) 4 ]Cl 2 , n-C 3 H 7 NH 2 and acidic TiO 2 colloid solutions followed by UV light irradiation. The thicknesses of titanium oxide and platinum were less than 0.7 nm. The band gap energy of the titanium oxide incorporated was less than 3.4 eV. The rate constants for the charge injection from excited TiO 2 into the conduction band of H 2 Ti 4 O 9 and H 4 Nb 6 O 17 were determined to be 0.11×10 9 and 0.12×10 9 s -1 , respectively, by measuring the TiO 2 emission lifetimes in TiO 2 incorporatednanocomposites. H 2 Ti 4 O 9 /(TiO 2 , Pt) and H 4 Nb 6 O 17 /(TiO 2 ,Pt) nanocomposites (intercalated with both TiO 2 and Pt) werecapable of water cleavage into hydrogen and oxygen without a hole scavenger, at 0.088–0.104 mmol h -1 , following irradiation from a 450 W Hg lamp, although no noticeable gas was evolved in the presence of H 2 Ti 4 O 9 /TiO 2 , H 4 Nb 6 O 17 /TiO 2 , H 2 Ti 4 O 9 /Pt, H 4 Nb 6 O 17 /Pt and unsupported TiO 2 /Pt. The hydrogen and oxygen evolution was linear with time and the niobate system was fairly efficient.