Theory of rapid excitation-energy transfer from B800 to optically-forbidden exciton states of B850 in the antenna system LH2 of photosynthetic purplebacteria

Excitation-energy transfer (EET) has been observed to be very rapid from B8 00 to B850, between two circular aggregates of bacteriochlorophyll molecule s (BChls), in the Light-harvesting antenna complex LH2 of photosynthetic pu rple bacteria. This rapid EET cannot be understood within the framework of Forster' s formula,. since the luminescence spectrum of B800 overlaps littl e with the absorption spectrum of B850. The present work shows that it can be rationalized on the basis of a recently proposed formula for EET between molecular aggregates. The formula differs from Forster"s one when excited stares are excitonic at least in one of the donor and the acceptor aggregat es with a mutual distance not much larger than their physical sizes, as in the present case. Excited states of B850 are regarded as excitonic, while t hose of B800 as monomeric. The exciton-phonon coupling was taken into accou nt over all orders for B800 and within a self-consistent second-order pertu rbation for B850. Calculated rates of the EET are in the range of 10(11)-10 (12) S-1, increasing weakly with temperature, in good agreement with experi ments. We demonstrate that this rapid EET occurs to optically forbidden exc iton slates of B850, without total transition dipole, due to strong interac tion of st transition dipole on a BChl in B800 with those on nearby BChls i n B850.