PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Carotenoid singlet excited state energy transfer mechanism in purple bacterial reaction centers. Su Lin^*,1, Evaldas Katilius¹, Hadi Tabbara¹, Robielyn Ilagan², George Gibson³, Harry Frank², Neal Woodbury¹, ¹ Department of Chemistry & Biochemistry, Tempe, AZ, USA² Department of Chemistry, University of Connecticut, Storrs, CT, USA³ Department of Physics, University of Connecticut, Storrs, CT, USA

ABSTRACT- Carotenoid singlet excited state energy transfer in wild type and mutant purple bacterial reaction centers from Rhodobacter (Rb.) sphaeroides has been studied using ultrafast transient laser spectroscopy. The pathway and efficiency of energy transfer has been studied as a function of the energies and structures of the donor and acceptor molecules. In the H(M182)L mutant, the bacteriochlorophyll (B_B) molecule located in between the carotenoid and the reaction center special pair (P), was replaced with a bacteriopheophytin (_B). As a result, the energy levels of the Q_X and Q_Y bands of _B increased by 1780 cm^-1 and 347 cm^-1 respectively, relative to those of B_B. These band shifts associated with _B in the H(M182)L mutant significantly alter the spectral overlap between the carotenoid and _B, as well as between _B and P, resulting in a change in the overall efficiency of carotenoid-to-P energy transfer. Carotenoids with various extents of -electron conjugation were also examined. Anaerobically grown WT RCs contain the carotenoid, spheroidene, which has ten conjugated carbon-carbon double bonds. RCs from the GA strain contain neurosporene, hydroxyneurosporene and methoxyneurosporene, all of which have nine conjugated double bonds. RCs from the mutants and aerobically grown WT cells have the carbonyl-containing spheroidenone which has eleven conjugated double bonds. By varying the acceptor energies and the structure of the carotenoid, insight can be gained into the mechanism of energy transfer reaction between the carotenoid and P.

KEY WORDS: energy transfer , Carotenoid singlet excited state , purple bacterial reaction center, ultrafast transient laser spectroscopy