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

• Published in: The journal of physical chemistry. B 1999-03, Vol.103 (11), p.2014-2032
• Main Authors: Arnett, David C, Moser, C. C, Dutton, P. L, Scherer, N. F
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp984464j

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 (P y +), or the lower excitonic state of the special pair (P y -). Population dynamics are then measured with a 12 fs duration probe pulse across the entire Q y absorption spectral region as a function of time, wavelength, and polarization. Excitation of either P y - or B results in the formation of a distinct optical band at 825 nm exhibiting polarization characteristics consistent with those expected for P y +; the band appears instantaneously upon excitation of P y - 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 P y + to P y - 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 P y + at room temperature. The coherent excitonic dynamics of the special pair states, P y - and P y +, 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 P y +, is delocalized at the earliest times, consistent with a supermolecular picture of the reaction center.