Energy transfer between the B850 (LH2) and B875 (LH1) antenna complexes of a mutant strain of Rhodobacter sphaeroides lacking reaction centers is investigated by femtosecond pump-probe spectroscopy at room temperature. Measurements are made at wavelengths between 810 and 910 nm at times extending to 200 ps after selective excitation of either B850 or B875. Assignments of the spectroscopic signals to the two types of antenna complex are made on the basis of measurements in strains that lack either LH1 or LH2 in addition to reaction centers. Energy transfer from excited B850 to B875 proceeds with two time constants, 4.6 +/- 0.3 and 26.3 +/- 1.0 ps, but a significant fraction of the excitations remain in B850 for considerably longer times. The fast step is interpreted as hopping of energy to LH1 from an associated LH2 complex; the slower steps are interpreted as migration of excitations in the LH2 pool preceding transfer to LH1. Transfer of excitations from B875 to B850 could not be detected, possibly suggesting that the average number of LH2 complexes in contact with each LH1 is small.
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