The First Events in Photosynthesis: Electronic Coupling and Energy Transfer Dynamics in the Photosynthetic Reaction Center from Rhodobacter sphaeroides

The rapid electronic state dynamics that occur prior to charge separation in the photosynthetic reaction center of Rhodobacter sphaeroides R-26 are investigated by “two-color” wavelength-resolved pump-probe and anisotropy measurements. A narrow band (40 fs duration transform limited) pump pulse is used to selectively excite reaction center pigments: the accessory bacteriochlorophyll (B), the upper excitonic state of the special pair (Py+), or the lower excitonic state of the special pair (Py-). Population dynamics are then measured with a 12 fs duration probe pulse across the entire Qy absorption spectral region as a function of time, wavelength, and polarization. Excitation of either Pyor B results in the formation of a distinct optical band at 825 nm exhibiting polarization characteristics consistent with those expected for Py+; the band appears instantaneously upon excitation of Pywith a negative anisotropy and appears somewhat delayed after excitation of B. The dynamics observed following direct excitation of the Py+ absorption band, that is identified to occur at 825 nm, suggests that internal conversion between the excitonic states of P is rapid, occurring with a 65 fs time constant. Excitation of the accessory BChl (i.e., populating the excited state, B*) provides a detailed answer for the mechanism of energy transfer within the bacterial reaction center. The process proceeds via a twostep mechanism, flowing sequentially from B* to Py+ to Pywith time constants of 120 and 65 fs, respectively. These results follow from a kinetic model analysis of several pump-wavelength-dependent and polarizationdependent differential probe transmission transients that yield the first spectrum of Py+ at room temperature. The coherent excitonic dynamics of the special pair states, Pyand Py+, are measured and analyzed for coupling strengths and time scales for electronic dephasing and population relaxation. These results, in conjunction with a range of the transient transmission spectra, suggest that the initially excited state of the zeroth order chromophores, i.e., B* and Py+, is delocalized at the earliest times, consistent with a supermolecular picture of the reaction center.