Photosynthesis Photosynthetic system

Jimenez R and Fleming G R 1996 Ultrafast spectroscopy of photosynthetic systems Biophysical Techniques In Photosynthesis ed J Amesz and A J Hoff (Dordrecht Kluwer) pp 63-73... 

A decade after the discovery of the Rieske protein in mitochondria (90), a similar FeS protein was identified in spinach chloroplasts (91) on the basis of its unique EPR spectrum and its unusually high reduction potential. In 1981, the Rieske protein was shown to be present in purified cytochrome Sg/complex from spinach (92) and cyanobacteria (93). In addition to the discovery in oxygenic photosynthesis, Rieske centers have been detected in both single-RC photosynthetic systems [2] (e.g., R. sphaeroides (94), Chloroflexus (95)) and [1] (Chlo-robium limicola (96, 97), H. chlorum (98)). They form the subject of a review in this volume. 

Photosynthesis is the reverse of reaction (30.1) the formation of carbohydrates and oxygen from water and carbon dioxide with solar energy. Photosynthesis occurs in the chloroplasts contained in the cells of green plants. The chloroplasts hold two types of photosynthetic systems, which are called PSl and PS 11. These systems... 

From a fundamental viewpoint, carbon dioxide reduction is a model reaction which can help us to understand better the mechanism of natural photosynthesis.11 Development of artificial photosynthetic systems, by mimicking functions of green plants, is one of... 

Photochemical fixation of carbon dioxide is a function of green plants and some bacteria in nature in the form of photosynthesis. All living organisms on the Earth are indebted directly or indirectly to photosynthesis. Thus, many attempts have been made to simulate the photosynthetic system and make artificial systems, although to date very little success has been achieved. 

Another important area is the use of photochemistry—chemistry that results from light absorption—to perform transformations that are not otherwise possible. The practical applications of photosynthesis were based on fundamental work to learn the new pathways that light absorption makes possible, but the work on these synthetic methods has also added to our basic understanding of the reaction mechanisms. The important natural process of photosynthesis also inspires some work in photochemistry, where the challenge is one of producing artificial photosynthetic systems that could use sunlight to drive the formation of energetic materials. 

These bacteria cannot in general oxidize water and must live on more readily oxidizable substrates such as hydrogen sulfide. The reaction centre for photosynthesis is a vesicle of some 600 A diameter, called the chromato-phore . This vesicle contains a protein of molecular weight around 70 kDa, four molecules of bacteriochlorophyll and two molecules of bacteriopheophy-tin (replacing the central Mg2+ atom by two H+ atoms), an atom Fe2+ in the form of ferrocytochrome, plus two quinones as electron acceptors, one of which may also be associated with an Fe2+. Two of the bacteriochlorophylls form a dimer which acts as the energy trap (this is similar to excimer formation). A molecule of bacteriopheophytin acts as the primary electron acceptor, then the electron is handed over in turn to the two quinones while the positive hole migrates to the ferrocytochrome, as shown in Figure 5.7. The detailed description of this simple photosynthetic system by means of X-ray diffraction has been a landmark in this field in recent years. 

Based on photosynthesis and the energy cycie on the earth, an artificial photosynthetic system was proposed (Fig. 19.2) tc create fuels from solar energy and water by utilizing photocatalytic reactions.95 This scheme takes minimum requirements to achieve photochemical energy conversion. 

The key feature of photosynthesis is the ability to carry charge spatially away from an excited state reaction centre before the usually highly efficient and biochemically useless recombination can take place. In photosynthetic systems, charge separation occurs about 108 times faster than recombination, a ratio that is impossible to reach in normal chemical reactions. This phenomenon is achieved by the spatial anchoring of the components at particular orientations to one another within a non polar region of the membrane anchored protein, thus preventing free diffusion and allowing a vectorial uphill chemical reaction. 

Abstract. Electronic energy transfer is reviewed with a particular emphasis on its role in photosynthesis. The article describes the advances in theory that have been motivated by studies of photosynthetic light harvesting antenna proteins. Noting that most theoretical work presently focuses on just a few photosynthetic systems, the extraordinary scope and diversity of systems actually found in nature is described. 

The objectives of this account are to review the problems involved in tailoring man-made photosynthetic systems and to highlight the scientific accomplishments in artificial photosynthesis. The chemical methodology of linking catalysts, biocatalysts and photosystems into integrated photosynthetic assemblies will be discussed. 

G. Charles Dismukes is professor of chemistry at Princeton University and an affiliated member of the Princeton Environmental Institute and the Princeton Materials Institute. His research interests focus on biological and chemical methods for solar-based fuel production, photosynthesis, metals in biological systems, and tools for investigating these systems. His published works describe the biology and chemistry of oxygen production in natural photosynthetic systems, the synthesis and characterization of bioinspired catalysts for renewable energy production, the use of microorganisms... 

The role of quinones in photobiological reactions involving chlorophyll has also been investigated (405,406). Despite the great effort and the multidisciplinary approach, progress in this field is slow because of the enormous complexity involved in photosynthetic systems. Hopefully the recent advances in experimental ESR technique, including the coupling of rapid scan ESR to flash photolysis, will help to elucidate the nature of the physical and chemical processes in photosynthesis. 

We can use the photosynthetic enhancement effect to study the pigments associated with each of the two photochemical systems involved in photosynthesis. The system containing the pigments absorbing beyond 690 nm was... 

From a functional point of view, this important property can be readily built into low molecular weight chromophore assemblies acting as artificial reaction centers (coordination compounds, the population of CT states is directly related to the concept of light-induced charge separation in photosynthesis. Whenever such CT states are photoreactive and lead to the formation of the same kind of permanent redox products as observed in photosynthesis, the most essential features of the primary light reactions have been successfully duplicated. In a more strict sense, this is of course only true, if actinic red or NIR-light of comparable wavelength is absorbed by both the natural and artificial photosynthetic systems. 

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