After primary absorption of a photon by either the B800 BChls or carotenoids its excitation energy is fueled into the B850 ring system. This role is rather straightforward in the case of the eight B800 BChls. These BChls absorb light near 800 nm and transfer the resulting electronic excitation of the individual BChls through the Fö rster mechanism to the B850 BChl ring. Quantum mechanical calculations predict that this transfer should proceed within 700 femtoseconds, a time which agrees with spectroscopic observation. More intricate is the role of carotenoids that absorb at 500 nm. The corresponding excited state, labelled S2, decays within 200 fs into an optically forbidden electronic state, labelled S1, which is in resonance with the BChl excitations in the 800-850 nm range. The state is well known from the physics of highly correlated one-dimensional electron systems and involves two triplet magnons coupled to an overall singlet state. The optically forbidden character of the S1 state of spheroidene precludes its coupling to the B850 ring through the Fö rster mechanism, thus limiting potential mechanisms to coupling through Coulomb interaction including higher-order multipoles or coupling through electron exchange (Dexter mechanism ). Both transfer routes require a close proximity between carotenoids and BChls. Carotenoids and BChls are indeed found in close contact. Calculations based on the geometric arrangement of carotenoids and BChls in LH-II and on CI expansions of the electronic states of carotenoids and chlorophylls, however, suggest that the exchange mechanism is largely ineffective since the respective coupling strength is very small. These calculations showed that the Coulomb mechanism results in a transfer of singlet excitations through the S1 (carotenoid) -> B850 (exciton states) pathway with a time constant of 260 fs. This transfer is also strongly accelerated by the splitting of the B850 exciton levels. Without the exciton splitting, the calculated transfer time would be as slow as 2.5 ps. This suggests that purple bacteria have evolved the ring structure of LH-II to improve resonance between acceptor and donor systems.

--> The ring structure enhances coupling between the primary absorbing chromophores (B800 BChls and carotenoids) and the B850 BChl ring.

By the Dexter mechanism the carotenoids also protect the BChls and the entire bacterium against the detrimental effects of BChl triplet states which arise with a small, but finite probability and can generate excited oxygen according to the reaction ground state (triplet) O2 + triplet BChl --> excited state (singlet) O2 + ground state (singlet) BChl. The carotenoids quench the bacteriochlorophylls' triplet states and, as a result, electronic excitation of O2 is prevented and with it the damaging properties of this compound in its reactive singlet form.

Relevant Publications

Publications Database Energy transfer between carotenoids and bacteriochlorophylls in a light harvesting protein. Ana Damjanović, Thorsten Ritz, and Klaus Schulten. Physical Review E, 59:3293-3311, 1999. Excitons and excitation transfer in the photosynthetic unit of purple bacteria. Thorsten Ritz, Xiche Hu, Ana Damjanović, and Klaus Schulten. Journal of Luminescence, 76-77:310-321, 1998. Pigment organization and transfer of electronic excitation in the purple bacteria. Xiche Hu, Thorsten Ritz, Ana Damjanović, and Klaus Schulten. Journal of Physical Chemistry B, 101:3854-3871, 1997.