Exciton relaxation and energy-transfer processes in the B850 circular aggregate of bacteriochlorophyll a molecules from the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides were studied at temperatures below 18 K. Excitons were selectively excited by a 7 nm spectral bandwidth pump pulse resonant with the inhomogeneously broadened long-wavelength side of the B850 ground-state absorption spectrum (between 860 and 879 nm). The transient spectra were measured over the 786−924 nm spectral range using a white light continuum probe pulse. Characteristic changes of transient spectra were observed over 4 decades of time, from about 10-13 to about 10-9 s. The spectral evolution was pump wavelength-dependent, changes being least notable at far-red excitation. A simple model was put forward to interpret the data, assuming that the sample consists of an ensemble of spectrally disorded excitons, each representing a separate B850 ring. It was found that the exciton coupling and diagonal disorder play almost equally important roles in the formation of the spectral and dynamical properties of light excitations in B850 antenna. The main effects of disorder considered were the spectral shifts, splitting of the degenerate exciton levels, and redistribution of the dipole strength of the transitions. Assuming that the contribution of least disturbed excitons is largest near the peak of the ground-state absorption spectrum and greatest near the edge, most of the known spectroscopic properties of LH2 complexes can be well understood, at least qualitatively. Specifically, the rough 100 fs response time was assigned to interexciton level relaxation; the three time constants, ca. 800 fs, ca. 15 ps, and ca. 150 ps, were attributed to exciton energy transfer, most likely, between the B850 rings. The ca. 1 ns decay time is due to the finite exciton lifetime. The circular LH1 antennas likely possess similar properties.

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.

No abstract

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