EXCITED-STATE DYNAMICS OF MUTATED ANTENNA COMPLEXES OF PURPLE BACTERIA STUDIED BY HOLE-BURNING

Absorption and fluorescence excitation spectra of various LH2 antenna complexes of two purple bacteria at low temperature (1.2 and 4.2 K) ha ve been measured, and energy transfer rates within these complexes hav e been determined by spectra hole-burning. The systems studied were me mbranes of a wild-type strain of Rhodobacter sphaeroides, membrane sam ples from four LH2-only strains containing specifically mutated LH2 co mplexes of the same bacterium, and the isolated B800-820 complex of Rh odopseudomonas acidophila (strain 7050). The mutants exhibit blue-shif ted B850 absorption bands with their spectral positions depending on t he specific amino acid residues replaced in the a-polypeptide sequence . Energy transfer rates from B800 to B850 (or to their respectively bl ue-shifted bands) have been obtained by hole-burning experiments in th e B800 band. The mutants of Rb. sphaeroides and the LH2 complex of Rps . acidophila yielded transfer times similar to those of the B800-850 c omplex of Rb. sphaeroides. These values, which for the various complex es vary between 1.7 and 2.5 ps in the wavelength region from 798 to 80 5 nm, do not decrease monotonically with the spectral distance between the bands. Various models based on Forster's energy transfer mechanis m are discussed, of which only one is consistent with the results. In this model the energy is assumed to be transferred not directly from t he Q(y) 0-0 band of B800 to that of the (blue-shifted) B8 50, but indi rectly through the excitation of a vibrational mode.