Photosynthesis primary events

We examine here possible structural effects that may result from or accompany the generation of the primary photoproducts, and speculate about the consequences of concomitant changes in distances,conformations, relative orientations and charges on the electronic profiles of and interactions between the BChls, BPheos and their radicals. Because the primary events in green plant photosynthesis also involve a series of chlorophyll donors and acceptors ( ), similar trends should therefore prevail for chlorophyll radicals as well. Furthermore, radicals of porphyrins and hydroporphyrins (saturated porphyrins such as chlorins and isobacteriochlorins) have been... 

Lorimer, G. H., The carboxylation and oxygenation of ribulose-1,5-bisphosphate The primary events in photosynthesis and photorespiration. Ann. Rev. Plant Physiol. 32 349, 1981. Mechanisms and biochemical significance of the carboxylase and oxygenase reactions. 

The photoinduced chemical transformation that follows the excitation process often activates an enzyme cascade or opens an ion channel. These secondary reactions amplify the primary event of light absorption. In some mechanisms, translocation of electrons (photosynthesis, for example) or of... 

Chlorophyll is at the very heart of the primary events of photosynthesis. It helps convert the sun s radiant energy into chemical free energy that can be stored in various ways. In this section we will represent light absorption, excitation transfer, and the photochemical step as chemical reactions this will serve as a prelude to a further consideration of certain molecular details of photosynthesis. 

Levanon, H. (ed.), 1992, Primary events in photosynthesis problems, speculations, controversies, and future trends. Isr. J. Biochem., 32 369n518. 

Warshel, A., Creighton, S., and Parson, W. W., 1988, Electron-transfer pathways in the primary event of bacterial photosynthesis. J. Phys. Chem., 92 2696n2701. 

The Primary Events of Photosynthesis Take Place in Thylakoid Membranes... 

The moderate visible absorption of Ceo-donor dyads (see above) prompts to C o-chromophore systems as suitable models for the mimicry of the primary events in natural photosynthesis [330-332]. To enhance the absorption of the fullerene containing dyads in the visible region strong chromophoric units, such as (i) ruth-enium(II) polypyridyl complexes or (ii) metalloporphyrins have been implemented as antenna systems. In these C6o-chromophore systems the role of the fullerene changed dramatically, namely, it only accepts an electron or energy from the pho-toexcited chromophore. 

A FIGURE 8-33 Photoelectron transport, the primary event in photosynthesis. After absorption of a photon of light, one of the excited special pair of chlorophyll a molecules in the reaction center left) donates an electron to a loosely bound acceptor molecule, the quinone Q, on the stromal surface of the thylakoid membrane, creating an essentially irreversible charge separation across the membrane right). The electron cannot easily return through the reaction center to neutralize the positively charged chlorophyll a. 

The primary events of bacterial photosynthesis are outlined in Fig. 5. The diagram is drawn such that the energy of each intermediate is represented by its position along the ordinate. From this schematic... 

Fig. 5. Schematic of the primary events of bacterial photosynthesis. An approximate redox scale is shown on the left. The numbers above the arrows are the rate constants for the electron transfer reactions between the components. P is the photoactive bacteriochlorophyll a. 1 is the intermediate acceptor believed to be bacteriopheophytin a. The Q s are ubiquinone molecules.

The reaction center (RC) represents the minimum stmctural unit capable of the primary event in photosynthesis. In this pigment-protein complex the absorbed light energy is used to transfer an elearon against a red-ox potential difference, thus determining a charge separation across the photosynthetic membrane. ... 

The reaction center of photosystem II of the chlor-oplast contains 2-4 Mn atoms. The effect of Mn deficiency on photosynthesis resembles poisoning by DCMU (see), i.e. evolution of O2 by photosystem II is inhibited, while photosystem I is unaffected. It is thought that the Mn is intimately involved in the primary event of the photolysis of water, probably alternating between the III and II states. 

The spectral properties of the Ti-anion radicals of Chi and BChl and their respective magnesium-free analogs pheophytin (Pheo) and bacteriopheophytin (BPheo) are of considerable interest, since spectroscopic methods have detected the formation of the latter three radicals during the primary events of photosynthesis [49]. We have characterized the spectra of the anion radicals of Et-Pheo-a, Et-Chl-a, Et-BPheo-a and Et-BChl-a and have compared the calculated [33, 36, 37] and experimental spectra in Fig. 13. As shown in the figure, once again the absorption spectra display characteristic visible and Soret absorption regions, but the visible bands are generally much more structured than are those of either the neutral or cationic moieties. 

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