Exciton photosynthesis

The approach for this system is the mimicry of the highly efficient photosynthesis process in biological systems, by which an antenna device collects the light energy before a series of exciton, energy, and electron transfers, which lead to the synthesis of the plant s fuel.70-73... 

It is especially timely to review the subject of exciton effects because, with the advent of the X-ray structural model of the Rhodopseudomonas viridis RC [9-12], it is becoming apparent that analyses of exciton effects exhibit a dichotomy. On the one hand there are analyses based on incomplete structural information, on the other there are those based on X-ray structural models. The former generally seem theoretically straightforward and consistent with all experimental data, while the latter tend to be theoretically involuted and inconsistent with at least some of the data. Because the underlying interactions are quite important in photosynthesis, it is worthwhile exploring this situation and trying to understand what underlies it. In Section 2 basic theoretical concepts are briefly summarized. Exciton analyses based on partial structural information are discussed in Sections 3 and 4, and those based on X-ray models are considered in Sections 5-7. 

Standard exciton analyses (Ref. 16 also unpublished calculations by R.E. Fenna and independently by L,L. Shipman) based on the atomic coordinates published by Fenna et al. [44] produce calculated absorption and CD spectra with multiline features that resemble observed spectra in multiplicity and splitting energies. However, the observed overall spectral red-shift is not reproduced theoretically, nor is the pattern of intensity borrowing in the Qy absorption spectrum or the magnitudes and signs of Qy CD bands. These are the aiionialies that continue to plague all exciton-analytical efforts in photosynthesis. 

Knox R. Exciton energy transfer and migration theoretical considerations. In Bioenergetics of Photosynthesis. Govindjee ed. 1975. Academic Press, New York. pp. 183-221. 

Lee H, Cheng Y-C, Fleming GR. Coherence dynamics in photosynthesis protein protection of excitonic coherence. Science 2007 316 1462-1465. 

RM Pearlstein (1982) Chlorophyll singlet excitons. In Govindjee (ed) Photosynthesis, Vol. I Energy Conversion by Plants and Bacteria, pp 294-330. Acad Press... 

Fig. 15. Conceptual development of a membane vesicle subjected to voltage pulses to create a potential difference across the membrane. (A) A1 pm-dlameter sphere of water is Imagined placed between two platinum electrodes 1 mm apart (B) The water sphere is replaced by a sphere of lipid (C) The Interior of the lipid sphere is replaced by a sphere of water, resulting in a lipid shell surrounding an aqueous medium to form the equivalent of a membrane vesicle. See text for details. (D) A schematic representation of a chloroplast thylakoid membrane containing ATP synthase to be subjected to voltage pulses and then the amount of ATP formed determined. Plots of actually measured ATP formation by voltage pulses (E) or light pulses (F) as a function of the number of pulses. (A), (B), (C), (E) and (F) from Witt (1987) Examples for the cooperation of photons, excitons, electrons, electric fields and protons in the photosynthesis membrane. Nouveau Journal deChimie 11 97 (D) adapted from Bauermeister, Schlodderand Graber(1988) Electric field-driven ATP synthesis catalyzed by the membrane-bound ATP-synthase from chloroplasts. Ber Bunsenges Phys Chem 92 1037.

Lately it was found, that the same mechanism is realized also in bacteria41 b. Exciton fusion processes observed by excitation of photosynthesis with very intensive ps- or ns-flashes41c,d practically do not occur under natural excitation conditions (s. Ref.41b ). 

It has long been questioned whether energy transport in photosynthesis is coherent (exciton-like) or stochastic (wave packet-like). Before going into detail, I address the characteristic physical features of these two limiting cases and the problem of how they can experimentally be distinguished. 

Energy Transfer and Excitons are, as we have already mentioned, perhaps the most interesting and in any case the most characteristic photophysical processes in molecular crystals. The investigation of these processes began in 1934, when A. Winterstein, U. Schon and H. Vetter [5] were able to explain the green fluorescence radiation from anthracene crystals, which had been described as due to the effect of an unknown chrysogen , in terms of sensitised fluorescence. This fluorescence is emitted by tetracene molecules which are present at very low concentration in the anthracene. Pure anthracene fluoresces in the crystalline phase just as in solution with a blue-violet colour. This observation set off a large number of spectroscopic studies of the sensitised emission from mixed crystals. Very soon, J. Franck and E. Teller [6] pointed out in a summary report of this field that there was an important cormection here to the primary processes of photosynthesis and other biophysical processes. 

The study of exciton transfer and primary charge separation in photosynthesis requires excitation by picosecond flashes. If a significant fraction of the reaction centers (RCs) shall be closed by a flash, the excitation density must be chosen such high that more than one exciton resides at the same time in the pool of antenna pigments. Then excitons can be lost by singlet-singlet annihilation before they are trapped by the photochemistry in the RC. Annihilation leads to an apparent acceleration of all other reactions connected with the exciton dynamics. The quantitative treatment of this competitive deactivation path allows the bimolecular rate constant of exciton-exciton annihilation to be determined that characterizes a given antenna bed. 

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. 

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