Theoretical Analysis of the Energy Gap Dependence of the Reconstituted B800 → B850 Excitation Energy Transfer Rate in Bacterial LH2 Complexes

Theoretical analysis was carried out for a remarkable energy gap dependence of the rate of excitation energy transfer (EET) from reconstituted B800 to B850 in LH2 whose experimental data were published recently (Herek, J. L.; Fraser, N. J.; Pullerits, T.; Martinsson, P.; Polivka, T.; Scheer, H.; Cogdell, R. J.; Sundstrom, V. Biophys. J. 2000, 78, 2590). This energy gap dependence could not be explained by calculations using the generalized Förster theory. As an alternative method, we solved the generalized master equation for the population of the excited BChla molecules that are coupled inside the B850 ring and between B850 and B800 by adopting second-order perturbation theory for the memory function. Making use of the experimental data of optical absorption and fluorescence spectra of monomers and proper site energies in LH2, we could theoretically reproduce the above experimentally observed energy gap dependence of the EET rate very well. The result of the present analysis indicates that the rate-limiting step for the EET of B800 → B850 is the EET from the localized state of B800 mostly to the nearby BChla molecules with a site energy of about 810 nm in the B850 ring. A physical explanation of this result is discussed.