Study of triplet states in single crystals of bacterial reaction centers by high-frequency magnetic resonance spectroscopy and quantum chemistry
Final Report Abstract
Photosynthesis is an essential process for life on earth. For our sociely, in a time where fossil fuels are being depleted at alarming rates, the search for alternative sources of energy even becomes of crucial importance for our own survival. A natural source of energy which is available in great abundance, but only scarcely used, is sunlight. Nature, however, has become very adept in using sunlight as an energy source. For this reason, many details of the mechanism of photosynthesis have been elucidated in the last 40 years by a plethora of techniques and by a multitude of research groups. The initial processes of photosynthesis involve the creation of a charge separated slale (like in a battery) after light absorption. The proteins that catalyze the charge separation are called reaction cenler proteins. Invariably, the photosynthetic bacterial reaction centers contain a dimer of bacteriochlorophylls (P865), which acts as the primary electron donor and a quinone cofactor (QB) as the final electron acceplor. The excited electron in principle can travel along two seemingly idenfical paths (A, B branch) of cofactors in order to reach the quinone cofactor. However, optical measurements indicate that only one of these two pathways is photosynthetically active. The charge separation is found to derive from radical-pair and intersystem crossing mechanisms. The former mechanism is operative for Rb. sphaeroides wild type, R-26.1, mutant GD(M203)/AW(M260) and Bl. viridis wild type in the measured temperature range 10 K - 100 K, indicating effective A-branch separation at these temperatures. The latter mechanism is operative for bacteriochlorophylls in vitro and for Rb. sphaeroides mutants LH(M214)/AW(M260) and LDHW. An intersystem crossing triplet stale indicates that no long-lived radical pairs are formed upon direct excitation of the primary donor and that virtually no charge separation at the B-branch occurs at low temperatures. When the temperature is raised above 30 K, B-branch charge separation is observed. Its yield is al most one percent of the A-branch charge separation. B-branch radical pair formation can be induced at 10 K with low yield by direcl excitation of the bacleriopheophytin of the B-branch at 537 nm. The spin density distributions of the triplet slale of bacteriochlorophyll a and b in vitro were found to be similar except for the presence of additional spin density on carbon 8l in bacteriochlorophyll b. The electron spin density in Pses was found to be almost evenly delocalized over both dimer halves in Rb. sphaeroides (3P865) and Bl. viridis (3P960). The spin density distribution found for 3P865 and 3P960 is essentially the same, except for the elhylidene groups in 3P960 that carry addilional spin density. It thus seems that Nature has optimized only one out of two branches for the charge separation process.
Publications
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Low Temperature Pulsed EPR Study al 34 GHz of the Triplet States of the Primary Electron Donor P865 and the Carotenoid in Native and Mutant Bacterial Reaction Centers of Rhodobacter sphaeroides. Biochemistry 46, 14782-14794, 2007
A. Marchanka, M. Paddock, W. Lubitz, M. van Gastel
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Triplet states in photosynthetic reaction centers of Rb. sphaeroides. Photosynth. Res. 91, 152-153, 2007
A. Marchanka, M. van Gastel, W. Lubiiz
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Triplet States in Photosynthetic Reaction Centers of Rb. sphaeroides Photosynthesis: Energy from the Sun, J.F. Allen, E. Gantl, J.H. Golbeck, B. Osmond (eds.), Springer Dordrecht, 133-136, 2008
A. Marchanka, W. Lubitz, M. Paddock, M. van Gastel
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Spin Density Distribution of the Exciled Triplet State of Bacteriochlorophylls. Pulsed ENDOR and DFT Studies. J. Phys. Chem. B. 113, 6917-6927, 2009
A. Marchanka, W. Lubitz, M. van Gaslel
