EXCITATION WAVELENGTH-DEPENDENT SPECTRAL EVOLUTION IN RHODOBACTER-SPHAEROIDES R-26 REACTION CENTERS AT LOW-TEMPERATURES - THE Q(Y) TRANSITION REGION

The spectral evolution associated with energy and electron transfer in quinone-depleted reaction centers from Rhodobacter sphaeroides strain R-26 was investigated at low temperatures using femtosecond transient absorbance spectroscopy as a function of excitation wavelength. Laser pulses of 150 fs duration and 5 nm spectral bandwidth at 760, 800, 81 0, and 880 nm were used to selectively excite the 760 nm transitions o f the bacteriopheophytins (H), the bacteriochlorophyll monomer (B) tra nsitions near 800 and 808 nm, and the 880 nm bacteriochlorophyll dimer (P) transition (810 nm excitation also presumably excites the upper e xciton band of P). While the general features of the kinetic and spect ral behavior observed are similar to previous room-temperature measure ments, the excitation wavelength dependence is generally more pronounc ed and much longer-lived. The absorbance changes throughout the 740-10 00 nm region are excitation wavelength dependent. These differences ar e clearly evident after several tens of picoseconds, and some spectral differences persist for hundreds of picoseconds. Previous reports hav e explained much of the excitation wavelength dependence of reaction c enters in terms of formation of charge separation intermediates direct ly from B or H* such as P+B- or B+H-. However, it is unlikely that ei ther of these charge-separated states would persist after several tens or hundreds of picoseconds. Though this certainly does not rule out c harge separation directly from excited states of B and H, it suggests that other explanations must be put forth to account for at least a la rge fraction of the excitation wavelength dependence observed. A likel y possibility is spectral heterogeneity within the reaction center pop ulation, resulting in optical selection by different excitation wavele ngths. This could explain much of the excitation wavelength dependent spectral evolution on time scales longer than 1 ps.