Photosynthesis antenna system

The study of artificial photosynthesis has been the subject of ongoing attention for many years now due to the need for sustainable energy resources. In natural photosynthesis a lightharvesting antenna system with a large optical cross-section (for example the LH2 complex) absorbs a photon that is funneled by energy transfer (ET) to the reaction centre [1-3]. Excellent candidates to mimic the natural antenna system are molecules that efficiently absorb light and are able to transfer the captured energy to other parts of the molecule. Molecules based on Zn and free-base porphyrins are examples of compounds that can be used as models for the LID complex [4]. 

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. 

In the photosynthesis of green plants, photosystems I and II (PS I, PS II) contain chlorophyll a, a Mg(II)-porphyrin, as an antenna system for light absorption and energy transfer to the reaction centers of PS I and PS II. PS II consists of a dimeric chlorophyll a as reaction center, pheophytin a, a metal-free chlorophyll a as electron transfer system to PS I and - on the other side - a water-oxidizing Mn cluster. The electron connection between PS II and PS I is carried out by a cyth/f complex (heme complexes and an FeS protein). The reaction center of PS I is also a dimeric chlorophyll (perhaps together with other chlorophylls), and chlorophyll and several FeS proteins for electron transfer. 

Syntheses of oligoporphyrins have targeted the construction of model systems to miinic the assembly of chlorophylls in photosynthesis of green plants and bacteria. In earlier times, a chlorophyll dimer was considered the key unit of antenna systems which gather sunlight for production of carbohydrate. Many model systems that elucidate the mechanism of photoinduced electron-transfer reaction of photosynthesis have been reported and well documented in many reviews. Syntheses of covalently linked electron donors and acceptors have been extended to model compounds appended to successive electron acceptors such as quinone derivatives. 

T p y+l)/k0j, where k j is the RC trapping rate and V is the ratio of the probabilities of finding the exciton in the antenna system and RC. It can be shown that from nine experimental observables (fluorescence and phosphorescence intensities and quantum yields, 7) qj) only one is independent. In all the transfer regimes, the observables depend only on V which is in general a function of time, intramolecular rate constants, size of the photosynthetic unit and initial conditions. Therefore, V (t) is the maximum information obtainable from the observables. These and further results representing general theoretical answers to problems l)-5) were illustrated on the case of the bacterial photosynthesis (Rhodopseudomonas viridis) where they are valid for the whole range of the physically acceptable values of the Forster radius. 

Mimuro M, Kikuchi H. Antenna systems and energy transfer in cyanophyta and rhodaphyta. Advances in Photosynthesis and Respiration. Light-Harvesting Antennas in Photosynthesis (Green BR, Parson WW, eds). 2003 13 281-306. Kluwer Academic Publishers, Dordrecht. 

Carotenoids are primarily synthesized in plants and micro-organisms (19,20). A major function of carotenoids clearly relates to the absorbance of light in the process of photosynthesis (12,19,20). Carotenoids serve as ancillary pigments in the antenna systems of photosynthetic organisms. Thus, the energy in the excited states of the carotenoids is ultimately transferred to the chlorophyll molecule at the reaction center of the antenna system (12,20). 

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