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) have been measured, and energy transfer rates within these complexes have been determined by spectra hole-burning. The systems studied were membranes of a wild-type strain of Rhodobacter sphaeroides, membrane samples from four LH2-only strains containing specifically mutated LH2 complexes of the same bacterium, and the isolated B800-820 complex of Rhodopseudomonas acidophila (strain 7050). The mutants exhibit blue-shifted B850 absorption bands with their spectral positions depending on the specific amino acid residues replaced in the a-polypeptide sequence. Energy transfer rates from B800 to B850 (or to their respectively blue-shifted bands) have been obtained by hole-burning experiments in the B800 band. The mutants of Rb. sphaeroides and the LH2 complex of Rps. acidophila yielded transfer times similar to those of the B800-850 complex of Rb. sphaeroides. These values, which for the various complexes vary between 1.7 and 2.5 ps in the wavelength region from 798 to 805 nm, do not decrease monotonically with the spectral distance between the bands. Various models based on Forster's energy transfer mechanism are discussed, of which only one is consistent with the results. In this model the energy is assumed to be transferred not directly from the Q(y) 0-0 band of B800 to that of the (blue-shifted) B8 50, but indirectly through the excitation of a vibrational mode.