Energy transfer between carotenoids and bacteriochlorophylls in light-harvesting complex II of purple bacteria

In photosynthetic light-harvesting systems carotenoids and chlorophylls jointly absorb light and transform its energy within about a picosecond into electronic singlet excitations of the chlorophylls only. This paper investigates this process for the light-harvesting complex II of the purple bacterium Rhodospirillum molischianum, for which a structure and, hence, the exact arrangement of the participating bacteriochlorophylls and carotenoids have recently become known. Based on this structure and on CI expansions of the electronic states of individual chromophores (bacteriochlorophylls and carotenoids) as well as on an exciton description of a circular aggregate of bacteriochlorophylls, the excitation transfer between carotenoids and bacteriochlorophylls is described by means of Fermi's golden rule. The electronic coupling between the various electronic excitations is determined for all orders of multipoles (Coulomb mechanism) and includes the electron exchange (Dexter mechanism) term. The rates and efficiencies for different pathways of excitation transfer, e.g., 1¹Bᵤ⁺(carotenoid)→bacteriochlorophyll aggregate and 2¹Ag^-(carotenoid)→bacteriochlorophyll aggregate, are compared. The results show that in LH-II the Coulomb mechanism is dominant for the transfer of singlet excitations. The 1¹Bᵤ⁺→Qₓ pathway appears to be partially efficient, while the 2¹Ag^-→Qy pathway, in our description, which does not include vibrational levels, is inefficient. An improved treatment of the excitation transfer from the 2¹Ag^- state is required to account for observed transfer rates. Exciton splitting of bacteriochlorophyll Qy excitations slightly accelerates the excitation transfer from the 2¹Ag^- state, while it plays a crucial role in accelerating the transfer from the B800BChlQy state. Photoprotection of bacteriochlorophylls through triplet quenching is investigated, too. The results suggest that eight of the $16B850$ bacteriochlorophylls in LH-II of Rhodospirillum molischianum are protected well by eight carotenoids observed in the x-ray structure of the protein. The remaining eight $B850$ bacteriochlorophylls can transfer their triplet excitation energy efficiently to their neighboring protected bacteriochlorophylls. Eight $B800$ bacteriochlorophylls appear not to be protected well by the observed carotenoids.