Enhanced rates of subpicosecond energy transfer in blue-shifted light harvesting LH2 mutants of Rhodobacter sphaeroides.

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.