Thorsten Ritz, Ana Damjanović, and Klaus Schulten.
Light-harvesting and photoprotection by carotenoids:
Structure-based calculations for photosynthetic antenna systems.
In G. Garab, editor, Photosynthesis: Mechanisms and Effects
(Proceedings of the XIth International Congress on Photosynthesis),
volume 1, pp. 487-490, Dordrecht, 1998. Kluwer Academic Publications.
RITZ98A
Carotenoids contribute to light harvesting and protect photosynthetic systems through triplet quenching. Both functions involve excitation transfer between carotenoids and chlorophylls (Chls). This transfer has been investigated computationally on the basis of the x-ray structures for LH2 of Rs. molischianum and Rps. acidophila. The electronic states of Chls and carotenoids have been described by means of semiempirical PPP-SCF-CI calculations. In case of the B850 band in LH2 involving a ring of tightly coupled bacteriochlorophylls (BChls) this description was extended to account for the excitonic character of the respective excitations. The transfer rates between the carotenoid and BChl states have been determined. The results show that efficient singlet excitation transfer occurs through the Coulomb (multipole) mechanism and not through the Dexter electron-exchange mechanism. In the case of triplet excitation transfer, based solely on the Dexter mechanism, the calculated transfer times are short enough to ensure efficient photoprotection of one third of the BChls in LH2. A further third of the BChls is not directly protected, but can transfer triplet excitation efficiently to one of the protected BChls. A second set of carotenoids is necessary for the protection of the remaining BChls.
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