Light-harvesting complex energy transfer

Light harvesting and energy transfer by the chlorophyll (Chi) and carotenoid molecules of the antenna (A) complexes to the reaction center (RC) of PSII. 

A, Absorption chi, chlorophyll car, carotenoid EET, excitonic energy transfer EF, exoplasmic fracture face EM, electron microscopy FWHM, full width at half maximum lEF, Isoelectric Focusing, LD, linear dichroism LHC, light harvesting complex PAGE, polyacrylamide gel electophoresis PF, protoplasmic fracture face PS, photosystem RC, reaction centre SDS, sodium dodecyl sulphate SSTT, single step transfer time. 

Figure 23-27 Illustration of proposed exciton transfer of the energy of light absorbed by bacteriochlorophyll a of purple bacteria. Energy absorbed by the light harvesting complex LH2 is transferred in steps to another LH2, to LH1 and to the reaction center. The short lines within the circles represent the edges of the BChla chromophores. After Kiihlbrandt300 with permission.

There are some recent application of a mixed quantum classical description to investigate quantum dynamics in large CC. The absorbance of a photosynthetic light harvesting complex caused by electronic Frenkel-exciton formation has been considered in Ref. [23], and Refs. [24,25] focused on excitation energy transfer in a DNA double helix strand. In both cases, however, the considerations have been restricted to an approximate description based on the use of... 

PS1, containing still part of its own light harvesting complex, has provided excellent kinetic results on the energy transfer to the reaction centre, with the subsequent photooxidation of P700, and the time of this whole process. 

These reactions taking place in the RC are the primary chemical reactions of photosynthesis. (A detailed description ot these primary chemical reactions in RCs of purple bacteria is given in Chapter 3.) The primary physical processes of photosynthesis are light absorption and transfer of excitation energy. These processes take place mainly in the light-harvesting complexes (LHCs) described in Chapter 11. 

The main reactions that occur within the reaction centres of illuminated photosynthetic bacteria are energy- and electron-transfer processes between remote but closely-spaced components. A series of light-harvesting complexes collect incident photons and, by a succession of rapid steps, transfer the photon... 

In order to mimic electronic energy transfer from the light-harvesting complex to the special pair of the reaction centre complex, a variety of bichromophoric molecules have been synthesized in which terminal donor and acceptor units are separated by a transparent spacer group. Provided the geometry of the dyad is known, especially the separation distance and mutual orientation, it becomes possible to consider the rates of intramolecular energy transfer in terms of... 

On the other hand, carotenoids in light-harvesting complexes of photosynthetic bacteria assume an all-trans configuration, either twisted (as in Rs. rubrum, Chromatium vinosum, md Rp. palustris) or planar (as in Rb. sphaeroides and Rp. capsulata). Note that chain twisting is not ascribed to the type of carotenoid, but rather to some specific interaction between the carotenoid and the protein environment and apparently affects the efficiency of singlet energy transfer. 

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