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

Abstract

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 Py- or 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 Py- with 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 two-step mechanism, flowing sequentially from B* to Py+ to Py- with time constants of 120 and 65 fs, respectively. These results follow from a kinetic model analysis of several pump-wavelength-dependent and polarization-dependent differential probe transmission transients that yield the first spectrum of Py+ at room temperature. The coherent excitonic dynamics of the special pair states, Py- and 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.