PARENT SESSION
Posters P3A Bacteriochlorophyll based antenna systems. Abstracts (219-238)

New mechanism of excitation energy transfer from B800 to B850 in bacterial LH2 complexes. Toshiaki Kakitani^*,1, Akihiro Kimura², ¹ Meijo University, Nagoya, Aichi, Japan² Nagoya University, Nagoya, Aichi, Japan

ABSTRACT- Theoretical analysis was carried out for explaining the remarkable energy gap dependence of the excitation energy transfer (EET) rate from reconstituted B800 to B850 in LH2 whose experimental data were published recently (Hereck et al., Biophys. J. 78 (2000),2590). This energy gap dependence represents that the EET rate gradually decrease as the energy gap is increased and this result could not be explained by calculations using the Forster theory or the generalized forster theory. As an alternative method, we solved the generalized master equation for the population of the excited BCla molecules that are coupled inside the B850 and between B850 and B800 by using the memory function obtained from the experimental data of optical absorption and fluorescence spectra of BChla monomers. As a result, we could theoretically reproduce the above experimentally observed energy gap dependence of the EET rate completely well. The result of the present analysis indicates that the rate-limiting step is the EET from the localized excited state of B800 to the partially delocalized state( mostly dimer excitation) in the B850. In order to confirm this, we made theoretical calculations of solving the generalized master equation in the equilibrium state of B850 and we found that the coherence length is about 2. Let us ask why the fully delocalized exciton state of B850 is observed in the optical absorption while a partially delocalized exciton plays a significant role in the EET from B800 to B850. In the optical absorption, the simultaneous formation of excitonic coherent bond between neighbouring monomers is possible at all the sites and so the formation time of the fully delocalized exciton state is as short as the formation time of dimer exciton and is also shorter than the optical absorption time. As a result, the fully delocalized exciton state is observed by the optical absorption. In the case of the EET, the formation of the excitonic bond must be made successively. Furthermore, it must compete with the exciton destructive time which can be derived from the decay time of the memory function. In the LH2, the decay time of the significant part of the memory function is shorter than the formation time of dimer exciton and longer than trimer exciton, indicating that mostly dimer exciton in B850 plays a significant role in the EET from B800 to B850.

KEY WORDS: excitation energy transfer, generalized master equation, coherence length, energy gap law