We report studies of energy transfer from the 800-nm absorbing pigment (B800) to the 850-nm absorbing pigment (B850) of the LH2 peripheral antenna complex and from LH2 to the core antenna complex (LH1) in Rhodobacter (Rb.) sphaeroides. The B800 to B850 process was studied in membranes from a LH2-reaction center (no LH1) mutant of Rb. sphaeroides and the LH2 to LH1 transfer was studied in both the wild-type species and in LH2 mutants with blue-shifted B850. The measurements were performed by using approximately 100-fs pulses to probe the formation of acceptor excitations in a two-color pump-probe measurement. Our experiments reveal a B800 to B850 transfer time of approximately 0.7 ps at 296 K and energy transfer from LH2 to LH1 is characterized by a time constant of approximately 3 ps at 296 K and approximately 5 ps at 77 K. In the blue-shifted B850 mutants, the transfer time from B850 to LH1 becomes gradually longer with increasing blue-shift of the B850 band as a result of the decreasing spectral overlap between the antennae. The results have been used to produce a model for the association between the ring-like structures that are characteristic of both the LH2 and LH1 antennae.

A combination of Fourier-Transform (FT) resonance Raman spectroscopy and site-directed mutagenesis has been used to examine the function of two highly conserved aromatic residues, alpha-Tyr-44 and alpha-Tyr-45, in the light-harvesting 2 (LH2) complex of the photosynthetic bacterium Rhodobacter sphaeroides. In LH2 complexes, aromatic residues located at positions alpha-44 and alpha-45 are thought to be located near the putative binding site for bacteriochlorophyll, and alterations at these positions are known to produce blue shifts in bacteriochlorophyll absorbance. In the present work, mutant LH2 complexes carrying the alterations alpha-Tyr-44-->Phe, alpha-Tyr-45-->Phe and alpha-Tyr-44,-45-->Phe,Leu were examined. FT resonance Raman spectroscopy of the resulting complexes shows the breakage of a hydrogen bond to the 2-acetyl carbonyl group of one of the B850 bacteriochlorophylls in the LH2 complex; in the double mutant, breakage of a second bond is probable. These results suggest that one of these hydrogen bonds is to alpha-Tyr-44, placing this residue in close proximity to ring I of one of the B850 bacteriochlorophyll a pigments. The breakage of one, then two, 2-acetyl carbonyl hydrogen bonds correlates well with the shift in the absorbance of the B850 pigments of 11 nm then 26 nm at 77 K. Thus a consistency between literature theoretical calculations and the observations from both absorption and FT resonance Raman spectroscopy is demonstrated.