Pump-probe polarization anisotropy study of femtosecond energy transfer within the photosynthetic reaction center of Rhodobacter sphaeroides R26

The energy transfer from the accessory bacteriochlorophylls (B) to the special pair (P) in the photosynthetic reaction center has been time resolved with pump-probe polarization anisotropy measurements using 20-25 fs duration pulses near 800 nm. The experiments were carried out at low pulse energies (500 pJ in a 34 mu m spot), low repetition rates (5 kHz), and high sample flow velocities (100 cm/s) to avoid artifacts from saturation and photoexcitation of incompletely relaxed reaction centers. The pump excitation corresponds to 1.4 x 10(6) photons/mu m(2): the ''saturation intensity'' for the charge separation quantum yield is 3 x 10(7) photons/mu m(2). Magic angle pump-probe transients can be satisfactorily fit as biexponential, with an similar to 120 fs bleach decay followed by a 2.8 ps bleach rise. (An similar to 400 fs bleach decay seen in several previous experiments arises from unrelaxed reaction centers.) The initial pump-probe anisotropy is 0.4 and decays with an similar to 80 fs time constant, which we attribute to dipole reorientation by electronic energy transfer. Simultaneous kinetic modeling of the parallel, perpendicular, and magic angle pump-probe transients using the reaction center structure and dipole orientations is consistent with energy transfer proceeding in two steps: similar to 80 fs electronic energy transfer from the accessory bacteriochlorophylls to the upper exciton component of the special pair (B --> P-+) followed by an similar to 150 fs internal conversion from the upper exciton component to the lower exciton component of the special pair (P-+ --> P--). Finally, charge separation after electron transfer from P-- to H causes an electrochromic (Stark) shift of B and produces the 2.8 ps bleach rise, The two-step energy transfer model is supported by the observation of weak quantum beat oscillations (125 cm(-1) and 227 cm(-1)) with near-zero anisotropy in the pump-probe signals. The near-zero anisotropy is only consistent with pump-probe signals from P-+ species created by energy transfer from B. The similar to 80 fs B --> P-+. energy transfer is so rapid that it sets vibrational wave packets in motion on the special pair. Because B --> P energy transfer is more rapid than conventional energy transfer rates, it may be more appropriate to think of energy transfer between pigments in the reaction center as an intermediate case between energy transfer among separate pigments and internal conversion within a single supermolecule.