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 i n the photosynthetic reaction center of Rhodobacter sphaeroides R-26 an inv estigated by "two-color" wavelength-resolved pump-probe and anisotropy meas urements. A narrow band (40 fs duration transform limited) pump pulse is us ed to selectively excite reaction center pigments: the accessory bacterioch lorophyll (B), the upper excitonic state of the special pair (Py+), or the lower excitonic state of the special pair (Py-). Population dynamics are th en measured with a 12 fs duration probe pulse across the entire Q(y) absorp tion spectral region as a function of time, wavelength, and polarization. E xcitation of either Py- or B results in the formation of a distinct optical band at 825nm exhibiting polarization characteristics consistent with thos e 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+ absorpti on band, that is identified to occur at 825 nm, suggests that internal conv ersion between the excitonic states of P is rapid, occurring with a 65 fs t ime constant. Excitation of the accessory BChl (i.e., populating the excite d state, B*) provides a detailed answer for the mechanism of energy transfe r 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 a nalysis of several pump-wavelength-dependent and polarization-dependent dif ferential probe transmission transients that yield thr first spectrum of Py + at room temperature. The coherent excitonic dynamics of the special pair states, Py- and Py+, are measured and analysed for coupling strengths and t ime scales for electronic dephasing and population relaxation. These result s, in conjunction with a range of the transient transmission spectra, sugge st that the initially excited state of the zeroth order chromophores i.e., B* and Py+, is delocalized at the earliest times, consistent with a supermo lecular picture of the reaction center.