Light-driven system

Electron-anion antiport has been realized, for instance with redox active carriers such as ferrocene derivatives [6.51a] or alkylviologens [6.44-6.46,6.51b] via ferricinium or reduced viologen species, respectively. The latter have been used extensively in light-driven systems and in studies on solar energy conversion [6.44-6.46],... 

However, this also means that chemical conditions within the mats are strongly affected by microbial metabolic activity. As light-driven systems, microbial mats experience dramatic daily fluctuations in metabolic processes, and therefore in the resulting chemical microenvironment [211,212]. It is against this backdrop of diverse metabolic capabilities and dramatically oscillating physical and chemical conditions that the cycling of H2 within microbial mats must be considered. 

Other mononuclear nickel complexes containing thiolate ligands, such as pyri-dine-2-thiolate [127], 2-mercaptoethanol [128], 2-mercaptobenzimidazole [129], or 2-mercaptobenthiazole [129] have proved active in light-driven systems based on fluorescein, erythrosin B, eosin Y, and rose Bengal as photosensitizers. These very simple systems also proved extremely stable upon turnover with 5500 TON after 40 h for the pyridine-2-thiolate complex [127]. Again, bleaching of the photosensitizer has been recognized to be responsible for the decrease of the activity after a few tens of hours. This issue could be solved with the replacement of the molecular photosensitizer by CdSe semiconductor nanocrystals, capped with... 

Mn02 colloids have been studied in light-driven systems, using [Ru(bpy)3] " " as photosensitizer and S20g or [Co(NH3)5Cl] as sacrificial agents,as well as in the presence of dark oxidants like Ce(IV). Unfortunately colloidal manganese oxides display poor performance in all these conditions, probably due to the aggregation of initially small particles (6 nm), to yield a physical separation of the catalyst from the reaction mixture. 

Pea chloroplasts were isolated and incubated as described in Fig. 1. Incorporation by the complete light-driven system is called 100, and represents a rate of incorporation of between 0.5 and 1.0 nmole [ CJleucine per mg chlorophyll per hour. The ATP and ATP-generating system consists of 2 mM ATP, 5 mM creatine phosphate, and 1(X) /i,g/ml creatine phosphokinase. Reproduced from Biochim. Biophys. Acta 319 (1973), p. 228 (Table 1). 

The strong interaction of the 13 /I" redox system with the metal dichalcogenide materials was recently exploited (31) to bring about the visible light-driven process representeiTby equation (10). In 50% by weight H2SO4 the reaction as written... 

Light-driven membrane transport. Cations may be transported through liquid membranes using crown ethers. For example, a typical system is of the type water-phase(I)/organic-phase/water-phase(II). The metal ion is added to water-phase(I) and the crown ether to the organic phase (to yield the liquid membrane). The crown acts as carrier for metal ions from water-phase(I) across the liquid membrane phase into water-phase(II). There have now been a very large number of studies of this type reported and a fuller discussion of this topic is given in Chapter 9. 

Long-term aim of our project is the construction of a biomolecular device for hydrogen production in combination with light-driven water-splitting as it occurs in the natural process of photosynthesis in plants. Such a semiartificial device should combine the best suited components found in various native systems which - up to now - cannot be found in an individual native system due to incompatibilities and/or different origin (pro- and eukaryotic, meso- and thermophilic, aerobic - anaerobic environment). Advantage of such a system is... 

The straightforward concept for the direct light-driven regeneration of flavin-dependent enzymes has been successfully applied for two representative classes of such enzymes a reductase and a monooxygenase. Therefore, it can be expected that this concept can also be applied to other flavin-dependent enzymes, potentially leading to additional practical catalyst systems for applications in synthetic organic chemistry. 

An alternative approach, using semiconductors as light-driven electron donors, has been demonstrated in model systems (Gratzel 1982 Nikhandrov et al. 1988). These are more stable than the photosystems, but show lower photochemical conversion efficiencies owing to short-circuiting of reducing equivalents. The presently used... 

The general approach of graded radiation exposure can also be used to examine light driven processes such as photopolymerization [19]. For example, Lin-Gibson and coworkers used this library technique to examine structure-property relationships in photopolymerized dimethacrylate networks [38] and to screen the mechanical and biocompatibility performance of photopolymerized dental resins [39]. In another set of recent studies, Johnson and coworkers combined graded light exposure with temperature and composition gradients to map and model the photopolymerization kinetics of acrylates, thiolenes and a series of co-monomer systems [40 2]. 

As an attempt to connect the first discussion, which was concerned with diffusion-reaction coupling, with Dr. Williams presentation of enzymes as dynamic systems, I wanted to direct attention to a number of specific systems. These are the energy-transducing proteins that couple scalar chemical reactions to vectorial flow processes. For example, I am thinking of active transport (Na-K ATPase), muscular contraction (actomyosin ATPase), and the light-driven proton pump of the well-known purple... 

Light-driven electron transfer in plant chloroplasts during photosynthesis is accomplished by multienzyme systems in the thylakoid membrane. Our current picture of photosynthetic mechanisms is a composite, drawn from studies of plant chloroplasts and a variety of bacteria and algae. Determination of the molecular structures of bacterial photosynthetic complexes (by x-ray crystallography) has given us a much improved understanding of the molecular events in photosynthesis in general. 

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