Photoreaction center

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

Pi and P2 are the photochemical reaction center serving also as light-harvesting unit. They can be two kinds of compounds or a single compound (P) such as a metal complex. The photoreaction center must have a strong absorption in the visible region. Tt and T2 are the electron mediators which take out photochemically separated charges rapidly to prevent back reactions. C and C2 are the reduction and oxidation... 

Fig. 3. A model system for photochemical conversion of solar energy. R2 Reducing agent, R, Oxidizing agent C2 Oxidation catalyst Cj Reduction catalyst T2, T, Electron mediators P2-Pj = P Photoreaction center...

For a model of the photoreaction center (P) to photolyze water, one requires ... 

The incorporation of 1,3-dibutylalloxazine (DBA) as an electron carrier in the vesicle wall increased the accumulation rate of the reduced disodium 9,10-anthraqui-none-2,6-disulfate (AQDSH2) by 7 times. In the system of Fig. 6, the electron is pumped up in two steps at the inside as well as outside of the vesicle wall, however, since the photoreaction centers (surfactant ZnP) are the same for both the walls, the amount of energy acquired by the two steps excitation is equal to that by one step excitation. The incorporation of different photoreaction centers inside and... 

Ru(bpy)j+ was covalently linked to viologen units to give a model of photoreaction center (11-13)35). 

Thus, the important question of the secondary structure of the transmembrane elements can only be addressed with models and by structural comparison with other transmembrane proteins for which the structure has been resolved. Detailed information on the structure of transmembrane elements is available for the photoreaction center of Rhodopseudomonas viridis (review Deisenhofer and Michel, 1989), cytochrome c oxidase (Iwata et al., 1995) and the OmpF porin of E. coli (Cowan et al., 1992 Fig. 5.3), amongst others. In addition, high resolution electron microscopic investigations and X-ray studies of bacteriorhodopsin, a light-driven ion pump with seven transmembrane elements, have yielded valuable information on the structure and configuration of membrane-spaiming elements (Henderson et al., 1990 Kimura et al., 1997 Pebay-Peyrula et al., 1997 Fig. 5.4). With the successful crystallization of the photoreaction center of Rhodopseudomonas viridis, a membrane protein was displayed at atomic resolution for the first time (Deisenhofer et al., 1985). The membrane-... 

FIGURE 19-45 Organization of photosystems in the thylakoid membrane. Photosystems are tightly packed in the thylakoid membrane, with several hundred antenna chlorophylls and accessory pigments surrounding a photoreaction center. Absorption of a photon by any of the antenna chlorophylls leads to excitation of the reaction center by exciton transfer (black arrow). Also embedded in the thylakoid membrane are the cytochrome bkf complex and ATP synthase (see Fig. 19-52). 

Structural studies of the reaction center of a purple bacterium have provided information about light-driven electron flow from an excited special pair of chlorophyll molecules, through pheophytin, to quinones. Electrons then pass from quinones through the cytochrome bci complex, and back to the photoreaction center. 

Cyanobacteria and plants have two different photoreaction centers, arranged in tandem. 

Fig. 19.19 (a) Stereoview of the special pair in the photoreaction center. Rings I of the chlorophyll molecules are stacked upon each other, and the magnesium atom of each chlorophyll is coordinated by an acetate group from the other molecule, (b) Close-up of the nearer chlorophyll molecule from part (a). The unattached acetate group is from the other chlorophyll molecule. (Modified from Deisenhofer, J. Epp. O. Miki, K. Huber, R. Michel. H. J. Mol. Biol. 1984, 180, 385-398. Reproduced with permission.]... 

Fig. 16.10 The electron transfer time (inverse rate) in a chemically modified photoreaction center of bacteriochlorophyll, showing a crossover from thermally activated sequential hopping behavior at high temperature to a superexchange tumieling behavior at low temperature. (Open circles are experimental data from M. E. Michel-Beyerle et al., unpublished fall and dashed lines are theoretical fits from the articles by M. Bixon and J. Jortner cited at the end of this chapter.)...

Boucher F, van der Rest M and Gingras G (1977) Structure and function of carotenoids in the photoreaction center from Rhodospirillum rubrum. Biochi m Biophys Acta 461 339-357... 

PROBABLE FATE photolysis . C-Cl bond photolysis can occur, not important in aquatic organisms, photooxidation half-life in air 9,24-92.4 hrs, reported to photodegrade in water in spite of the lack of a photoreactive center oxidation-, not an important process hydrolysis . very slow, not important, first-order hydrolytic half-life 207 days, reaction with hydroxyl radicals in atmosphere has a half-life of 2.3 days volatilization may be an important process, however, information is contradictory, volatilization half-life from a model river 6 days, half-life from a model pond considering effects of adsorption 500 days, slow volatilization from water is expected with a rate dependent upon the rate of diffusion through air sorption important for transport to anaerobic sediments biological processes biodegradation is important occurs slowly in aerobic conditions, occurs quickly and extensively in anaerobic conditions... 

A more elegant way to combine the advantages of PhCs on the basis of dispersed semiconductors with those of membrane-structured systems seems to be the inclusion of semiconductor nanoparticles into microscopic vesicular systems with bilayer lipid membranes (vesicles are the microscopic bubbles, see Section II and Fig. 4). It is anticipated that semiconductor nanoparticles in such systems can serve the role of very efficient and stable integral photoreaction centers mimicking completely the spatially well-organized... 

When considering long-range electron transfer in proteins, there has been some discussion of the influence of the intervening medium and in particular of whether the electron passes along a particular path or makes significant use of particular protein side chains. For instance it was postulated that aromatic side chains of proteins play an important role. However a recent study by Moser et of electron transfer in the bacterial photoreaction center reveals no such effects in that case. 

PHOTOTRAPPING OF MONO- AND DI- ANIONIC BACTERIOPHEOPHYTIN IN THE PHOTOREACTION CENTER OF ECTOTHIORHODOSPIRA SP. 

Photoreaction center was prepared from Ectothiorhodospira sp. according to the method of Lefebvre et. al. (4). Absorbance changes were measured with a Cary 14R spectrophotometer equipped with a cross-illumination attachment. Low temperature measurements were carried out with an Air Products Corp. Joule-Thomson cryostat, e.p.r. spectra were measured in a E231 cavity with a Varian E-104A spectrometer operating at 9.01 GHz. Temperature was controlled with a Varian E257 accessory. 

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