On B800→B800 energy transfer in the LH2 complex of purple bacteria

The B800 Qy-states of the light harvesting 2 (LH2) photosynthetic complex of purple bacteria exhibit two relaxation channels. One is B800→B850 excitation energy transfer that occurs in ∼2 ps in low-temperature limit. This channel is dominant for excitation wavelengths located to lower energy of the B800 absorption band maximum. The second “additional” channel sets in for excitation on the high-energy side of the maximum. The mechanism for the additional channel has been controversial. That it is due to relaxation of Bacteriochlorophyll a (Bchl a) vibrational modes has already been excluded. This left intra-band B800→B800 energy transfer and relaxation of mixed B800–B850 exciton levels as possible mechanisms. High-pressure hole burning results for LH2 of Rhodopseudomonas acidophila are presented, which, together with excitonic calculations, indicate that the latter mechanism is relatively unimportant. Intra-band B800→B800 excitation energy transfer is considered from a probabilistic viewpoint. It appears that the B800 zero-phonon hole action spectrum corresponds to those of Bchl a sites on the C9 ring that cannot undergo downward energy transfer to a nearest-neighbor site. One-third of the sites fall into this category. It is concluded that the additional decay channel is most likely due to above intra-band transfer.