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, 810, and 880 nm were used to selectively excite the 760 nm transitions of the bacteriopheophytins (H), the bacteriochlorophyll monomer (B) transitions near 800 and 808 nm, and the 880 nm bacteriochlorophyll dimer (P) transition (810 nm excitation also presumably excites the upper exciton band of P). While the general features of the kinetic and spectral behavior observed are similar to previous room-temperature measurements, the excitation wavelength dependence is generally more pronounced and much longer-lived. The absorbance changes throughout the 740-1000 nm region are excitation wavelength dependent. These differences are clearly evident after several tens of picoseconds, and some spectral differences persist for hundreds of picoseconds. Previous reports have explained much of the excitation wavelength dependence of reaction centers in terms of formation of charge separation intermediates directly from B* or H* such as P⁺B^- or B⁺H^-. However, it is unlikely that either of these charge-separated states would persist after several tens or hundreds of picoseconds. Though this certainly does not rule out charge separation directly from excited states of B and H, it suggests that other explanations must be put forth to account for at least a large fraction of the excitation wavelength dependence observed. A likely possibility is spectral heterogeneity within the reaction center population, resulting in optical selection by different excitation wavelengths. This could explain much of the excitation wavelength dependent spectral evolution on time scales longer than 1 ps.