LEVEL MIXING AND ENERGY REDISTRIBUTION IN BACTERIAL PHOTOSYNTHETIC REACTION CENTERS

We performed ultrafast polarized light experiments in which we pumped the 800 nm band and probed several near- and mid-IR transitions of pho tosynthetic reaction centers from Rhodobacter sphaeroides. Absorption into the upper excitonic level of the special pair (P-Y+) is part of t his band, but it is not known whether P (Y+) behaves as a localized st ate or if it mixes with accessory bacteriochlorophyll (BChl) states. A calculation of the anisotropy of pump-probe signals fails to reproduc e the experimental results if the localized picture is used. In fact, the transition into P-y+ has to be 4-fold intensified over a simple ex citon model prediction in order to give rise to anisotropies which are consistent with the experiment. This substantial intensification is i nconsistent with previous experimental results. Agreement between theo ry and experiment can be achieved if the P-Y+ state is mixed with exci ted states of accessory BChl. Stimulated emission from the lower excit onic level of the special pair (P-Y-), probed at 950 nm after pumping at 800 nm, does not appear instantaneously, but rises with a time cons tant of 110 fs. A novel excited state absorption of the accessory BChl s at 1200 nm, assigned as a monomer transition by comparison with a pu mp-probe experiment on free BChl dissolved in acetone, also decays wit h a similar to 100 fs time constant. Although Forster energy transfer from accessory Bchl states to P-Y- can account for the fast transients , under the delocalized state picture suggested in this paper they wou ld rather correspond to an internal conversion process from the mixed states to the P-Y- state.