Energy transfer within various LH2 antenna complexes of the photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas acidophila has been studied at 77 K using tunable femtosecond and subpicosecond infrared pulses. The complexes examined include the wild-type B800-850 as well as three different specifically mutated complexes. The site-directed mutant strains were altered at positions 44 and 45 near the C-terminus of the alpha-subunit, which introduces a spectral blue-shift of the 850-nm absorption band. In addition to a constant band at 800 nm, the mutations alpha Tyr44,Tyr45-->Phe,Tyr; -->Tyr,Phe; and -->Phe,Leu have absorption peaks at 838, 838, and 826 nm, respectively. As the spectral overlap between the B800 and the variable bands increases, the rate of energy transfer as measured by the lifetime of the B800 excited state also increases from 2.4 +/- 0.2 to 1.8 +/- 0.2, 1.6 +/- 0.2, and 0.8 +/- 0.1 ps. This correlation between energy-transfer rate and spectral blue-shift of the B850 absorption band is in qualitative agreement with the trend predicted from Förster spectral overlap calculations, although the variation of the experimentally determined rate through the series of mutants is somewhat wider than what is predicted by simulations. In addition to the decay time constants relate to the B800-->B850 energy transfer, the B800 excited state is seen to decay with a faster 150-500-fs component due to energy transfer between spectrally inhomogeneous B800 molecules and possibly also vibrational relaxation and cooling in the bacteriochlorophyll excited state.

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Decay of the bacteriochlorophyll excited state was measured in membranes of the purple bacteria Rhodospirillum (R.) rubrum, Rhodobacter (Rb.) sphaeroides wild type and Rb. sphaeroides mutant M21 using low intensity picosecond absorption spectroscopy. The excitation and probing pulses were chosen in the far red wing of the long wavelength absorption band, such that predominantly the minor antenna species B896 was excited. The decay of B896 was studied between 77 and 177K under conditions that the traps were active. In all species the B896 excited state decay is almost temperature independent between 100 and 177K, and probably between 100 and 300 K. In this temperature range the decay rates for the various species are very similar and close to 40 ps. Below 100 K this rate remains temperature independent in Rb. sphaeroides w. t. and M21, while in R. rubrum a steep decrease sets in. An analysis of this data with the theory of nuclear tunneling indicates an activation energy for the final transfer step from B896 to the special pair of 70cm(-1) for R. rubrum and 30cm(-1) or less for Rb. sphaeroides.

The excited state decay kinetics of chromatophores of the purple photosynthetic bacterium Rhodospirillum rubrum have been recorded at 77 K using picosecond absorption difference spectroscopy under strict annihilation free conditions. The kinetics are shown to be strongly detection wavelength dependent. A simultaneous kinetic modeling of these experiments together with earlier fluorescence kinetics by numerical integration of the appropriate master equation is performed. This model, which accounts for the spectral inhomogeneity of the core light-harvesting antenna of photosynthetic purple bacteria, reveals three qualitatively distinct stages of excitation transfer with different time scales. At first a fast transfer to a local energy minimum takes place (approximately 1 ps). This is followed by a much slower transfer between different energy minima (10-30 ps). The third component corresponds to the excitation transfer to the reaction center, which depends on its state (60 and 200 ps for open and closed, respectively) and seems also to be the bottleneck in the overall trapping time. An acceptable correspondence between theoretical and experimental decay kinetics is achieved at 77 K and at room temperature by assuming that the width of the inhomogeneous broadening is 10-15 nm and the mean residence time of the excitation in the antenna lattice site is 2-3 ps.