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

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 ...