Dokument: Triplet states in bacterial reaction centers of Rhodobacter sphaeroides and related systems

Titel:Triplet states in bacterial reaction centers of Rhodobacter sphaeroides and related systems
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URN (NBN):urn:nbn:de:hbz:061-20090715-093909-7
Dokumententyp:Wissenschaftliche Abschlussarbeiten » Dissertation
Autor:Dr. rer. nat. Marchanka, Aliaksandr [Autor]
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Dateien vom 07.07.2009 / geändert 07.07.2009
Beitragende:Prof. Dr. Lubitz, Wolfgang [Gutachter]
Prof. Dr. Kleinermanns, Karl [Gutachter]
Dewey Dezimal-Klassifikation:500 Naturwissenschaften und Mathematik » 530 Physik
Beschreibung:The photoexcited triplet state of the primary donor and the carotenoid in bacterial reaction centers of Rb. sphaeroides, in mutants, in reaction centers of Bl. viridis and in model systems (bacteriochlorophylls in vitro), have been investigated by EPR and ENDOR spectroscopy at cryogenic temperatures with aim to investigate the processes of photosynthetic charge separation and electron and energy transfer. The experiments have been supplemented with DFT calculations.
The triplet state 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 Rb. sphaeroides mutants LH(M214)/AW(M260) and LDHW, indicating that virtually no charge separation at the B-branch occurs at low temperatures. When the temperature is raised above 30 K, low yield B-branch charge separation is observed. B-branch radical pair formation can be induced at 10 K with low yield by direct excitation of the bacteriopheophytin of the B-branch at 537 nm. The formation of a carotenoid triplet state is observed in Rb. sphaeroides wild type and in all mutants with the carotenoid molecule. The thermal activation barrier varies among different mutants. Most probably, the thermally activated transfer step in the B-branch occurs between the primary donor, P865, and the accessory bacteriochlorophyll.
The spin density distributions of the triplet state of bacteriochlorophyll a and b in vitro were found to be similar except for the presence of additional spin density on carbon 81 in bacteriochlorophyll b. The triplet state in bacteriochlorophylls can be satisfactory explained as a mixture of HOMO → LUMO and HOMO1 → LUMO excitations
The electron spin density is almost evenly delocalized over the dimer halves in the special pair of Rb. sphaeroides (3P865) and Bl. viridis (3P960). The spin density distribution found for 3P865 and 3P960 is essentially the same, except for the ethylidene groups in 3P960 that carry additional spin density. In contrary to the triplet state of the bacteriochlorophyll monomer, the triplet state of the dimer can be satisfactorily explained as being a simple HOMO  LUMO excitation. The evenly delocalized spin density distribution indicates that asymmetry of photosynthetic charge separation with respect to the A and B branches of cofactors indeed does not stem from the internal asymmetry in the electronic structure of the special pair. Rather, small differences in the distances and mutual orientation of cofactors in the A- and B-branch are responsible for the large asymmetry in photosynthetic activity of the two branches.
Fachbereich / Einrichtung:Mathematisch- Naturwissenschaftliche Fakultät » WE Chemie
Dokument erstellt am:15.07.2009
Dateien geändert am:07.07.2009
Promotionsantrag am:15.05.2009
Datum der Promotion:26.06.2009
Status: Gast