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

Abstract

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 carotenoids͒ 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 mechanism͒ and includes the electron exchange ͑Dexter mechanism͒ term. The rates and efficiencies for different pathways of excitation transfer, e.g., 1 1 B u ϩ (carotenoid)→bacteriochlorophyll aggregate and 2 1 A g Ϫ (carotenoid)→ bacteriochlorophyll aggregate, are compared. The results show that in LH-II the Coulomb mechanism is dominant for the transfer of singlet excitations. The 1 1 B u ϩ →Q x pathway appears to be partially efficient, while the 2 1 A g Ϫ →Q y pathway, in our description, which does not include vibrational levels, is inefficient. An improved treatment of the excitation transfer from the 2 1 A g Ϫ state is required to account for observed transfer rates. Exciton splitting of bacteriochlorophyll Q y excitations slightly accelerates the excitation transfer from the 2 1 A g Ϫ state, while it plays a crucial role in accelerating the transfer from the B800 BChl Q y state. Photoprotection of bacteriochlorophylls through triplet quenching is investigated, too. The results suggest that eight of the 16 B850 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. ͓S1063-651X͑98͒11007-3͔

A simplified approach for the coupling of excitation energy transfer

“…Nonradiative excitation energy transfer (EET) from a donor to an acceptor chromophore is a fundamental process in photochemically active systems [1][2][3][4][5][6][7][8], such as light-harvesting antenna [2,3] and the complexes of green photosynthetic bacterium [4,5] in the natural world. Furthermore, there has been an escalation in the number of diverse and novel synthetic systems that utilize EET.…”

“…The literature on the calculation of the electronic coupling is voluminous [1][2][3][4][5][6][7][8][9][10][16][17][18][19][20][21][22][23][24][25][26]. However, most of them are focused on singlet-to-singlet (S-S) energy transfer, wherein energy is exchanged between two molecular moieties that have singlet spin multiplicities.…”

Femtosecond dynamics of the forbidden carotenoid S 1 state in light-harvesting complexes of purple bacteria observed after two-photon excitation

“…Despite intensive investigation of the underlying mechanisms (8,10,12,(15)(16)(17)(18)(19)(20), many aspects of the electronic energy transfer (EET) from the Cars are still poorly understood. The electronic states of Cars often are described by analogy to polyenes ( Only the S 0 7 S 2 transition is optically dipole-allowed.…”

“…However, because the S 1 3 S 0 transition is dipole-forbidden it remains a challenge to elucidate the mechanism that promotes Car S 1 to Chl EET. Neither the Förster (22,23) nor Dexter theories (24) provide satisfactory predictions (15,20 (19,25,26). However, the best way to find direct evidence for Car S 1 3 BChl EET and to determine its contribution to the overall Car 3 BChl EET is to excite Car S 1 exclusively and thereby measure ET1 and ET1 directly without any convolution with S 2 state dynamics.…”

Reaction paths based on mean first-passage times

“…We build the transition matrix K xy by using the inter-chlorophyll excitation transfer rates that were calculated in Ref. [22] based on the theory developed in [23] and the recently obtained structure of photosystem I [24]. The MFPT τ x from chlorophyll x to the P700 pair is then [20] …”