Water as electron donor

Depending on the system under study, oxygen production or consumption is measured. In both cases the use of artificial electron acceptors and electrondonors is necessary (2). The classical electron acceptor for the whole electron transport chain with water as electron donor is methyl-viologen. The same acceptor is used when one measures the photosystem I (PSI) electron transport but now with different artificial electron donors after inhibition of photosystem 2 (PS2) with DCMU (2). In both cases an oxygen consumption is registered due to the auto-oxidation of the reduced methylviologen (3). 

PS 2 activity is measured as an oxygen production with water as electron donor and with artificial electron acceptors who withdrawn the electrons somewhere between PS 2 and PS 1(4). 

Photosystem 2 activity was measured using water as electron donor and with DCPIP, TMQ, DAD, PD, TMPD, BQ and DMBQ as electron acceptors. The reaction medium contains 2mM NaCl, 5mM MgCl2, 2mM K2HPO4 lOmM Tricine-NaOH pH 7.8, 48mM K3Fe(CN>5. 

Photoautotrophs use light from the sun as source of energy and water as electron donor with formation of dioxygen. In terrestrial environments, plants and microorganisms are the predominant variety of photoautotrophs, while in aquatic systems, algae and (cyano)bacteria depend on this pathway. The process that occurs in a leaf is shown in Fig. 39.4 and highlighted in the follow-up of this section. 

The purple sulfur bacteria and the green sulfur bacteria use hydrogen sulfide as electron donor in photosynthesis, thereby producing elemental sulfur. (In fact, this mode of photosynthesis is older than the mode of cyanobacteria, algae, and plants, which uses water as electron donor and liberates oxygen.)... 

Seawater is circulated below the sea bed where it is heated by volcanic activity before being re-injected into the sea at high pressure and temperature. The pressure prevents water from boiling until the temperature reaches 725 K. The superheated water dissolves minerals from around the vent that then precipitate as the water temperature cools. This gives the vents their black smoker appearance. Many small molecules such as H2, H2S and Mn2+ do not precipitate but remain in their reduced formed and are available as electron donors. 

An astonishing recent discovery is that there are bacteria living deep in the Earth s crust. Colonies of anaerobic bacteria have been isolated from boreholes 1,500 m deep in basaltic rock formations. The bacteria use H2 as electron donor, which may originate from fermentation of organic matter, or from a purely inorganic reaction of iron of the Earth s core with water (Stevens and McKinley 1995 Anderson et al. 1998). 

Figure 1. Representative polarographic traces that depict inhibition by energy transfer inhibitors and allelochemicals of ADP-stimulated electron transport in isolated spinach thylakoids and circumvention of the inhibition by an uncoupler (FCCP, 2 pM). Trace A chlorotributyltin (TBT, 1 pM) trace B phlorizin (400 pM) trace C DCCD (20 pM) trace D quercetin (200 pM) trace E naringenin (1 mM). Water served as electron donor and methyl viologen as electron acceptor. Rates of oxygen utilization, that resulted from the autooxidation of methyl viologen, expressed as pmol 0 consumed/mg Chi h, are indicated parenthet ically.

This class includes enzymes that use diphenols or related compounds as electron donors and oxygen as the acceptor, thereby forming the oxidized donor and water. Members include catechol oxidase (E.C. 1.10.3.1), laccase (E.C. 1.10.3.2), and o-aminophenol oxidase (E.C. 1.10.3.4). Laccase is also known as / -diphenoloxidase. whereas catechol oxidase is also known as diphenoloxidase, phenoloxidase, polyphenoloxidase, o-diphenolase, phenolase and tyrosinase. Many of these names are also used in reference to a different enzyme, monophenol monooxygenase (E.C. 1.14.18.1). This enzyme will be discussed further in Section 1.8.2.2. 

Figure 6. Hydrogen production at 25°C in deaerated solutions as a function of catalyst (rhodium) concentration during the first two hours of irradiation using 350 nm cut-off and water filters. Plotted are the amount of hydrogen produced in 25 ml DHP vesicle solution.and measured in the gas phase (16 ml) by GC s 2 x 10- J M DHP, 2 x 10 M CdS symmetrically distributed on both sides of the vesicles, and 10 J M PhSH as electron donor pH approximately 7 at sonication and during photolysis. Concentrations of the catalyst, reduced by uv irradiation prior to visible light photolysis ( - Dl 1 ° ...

We have examined the photosensitized reduction of a series of + 4,4 -bipyridinium salts, CnV2+, (2), (where n=l-16) with Ru(bpy)l as sensitizer and (NHi jEDTA as electron donor in a water-in-toluene microemulsion media. Under steady state illumination the quantum yield of CjjVt" formation strongly depends on the alkyl chain length of the electron acceptor (Figure 4). It improves as the hydro-... 

The water-soluble Wilkinson-type catalyst chlorotris(diphenylphosphinoben-zene-m-sulfonate)rhodium(I), RhQfdpm) (19), acts as catalyst for H2-evolution [158], hydrogenation and hydroformylation [159]. In a photosystem composed of Ru(bpy)i+ as photosensitizer, ascorbic acid, HA, as electron donor and RhCl(dpm)3, hydrogen evolution proceeds with a quantum efficiency corresponding to (p = 0.033. In the presence of ethylene or acetylene, hydrogen evolution is blocked and hydrogenation of the unsaturated organic substrates predominates. Table 6 summarizes the quantum yields for H2-evolution and... 

Figure 1 Simplified scheme illustrating in space-energy coordinates, the photogeneration, the bulk and surface recombination, the reaction with dioxygen, hydroxide ions, water and electron-donor pollutants, of charge carriers in an n-type semiconductor such as Ti02.

Surface OH groups can trap holes, and adsorbed water molecules can act as electron donors, creating hydroxyl radicals (Fig. 1). Also, water can be involved in the transformation of organic radical cations, e.g.,... 

In particular, evidence of the role of adsorbed water as an electron donor has been presented in which the position of the Fermi level was varied (see Fig. 2) (Fritzsche and Tanielian, 1981). It has also been demonstrated that NH3, CO, and (CF13)20 (dimethyl ether) act as electron donors, while 02 acts as an electron acceptor. It is interesting to note that in order to restore... 

Photochemical Hydrogen Production using Cadmium Sulfide Suspensions in Aerated Water Cysteine or EDTA used as electron donor and platinized samples used as in Entry 5. The H2 yield only marginally reduced in the presence of O2. 492... 

Fig. 9.14. Illustration of the mechanism of surface band bending by adsorbed water molecules which act as electron donors.

Special cases of charge-transfer spectra are the so-called charge-transfer-to-solvent (CTTS) spectra [17, 68]. In this type of CT transitions, solute anions may act as electron-donors and the surrounding solvent shell plays the role of the electron-acceptor. A classical example of this kind of CTTS excitation is the UV/Vis absorption of the iodide ion in solution, which shows an extreme solvent sensitivity [68, 316]. Solvent-dependent CTTS absorptions have also been obtained for solutions of alkali metal anions in ether or amine solvents [317]. Quantum-mechanical molecular simulations of the CTTS spectra of halide ions in water are given in reference [468]. 

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