TEMPORALLY AND SPECTRALLY RESOLVED SUBPICOSECOND ENERGY-TRANSFER WITHIN THE PERIPHERAL ANTENNA COMPLEX (LH2) AND FROM LH2 TO THE CORE ANTENNA COMPLEX IN PHOTOSYNTHETIC PURPLE BACTERIA

We report studies of energy transfer from the 800-nm absorbing pigment (B800) to the 850-nm absorbing pigment (B850) of the LH2 peripheral a ntenna complex and from LH2 to the core antenna complex (LH1) in Rhodo bacter (Rb.) sphaeroides. The B800 to B850 process was studied in memb ranes from a LH2-reaction center (no LH1) mutant of Rb. sphaeroides an d the LH2 to LH1 transfer was studied in both the wild-type species an d in LH2 mutants with blueshifted B850. The measurements were performe d by using approximate to 100-fs pulses to probe the formation of acce ptor excitations in a two-color pump-probe measurement. Our experiment s reveal a B800 to B850 transfer time of approximate to 0.7 ps at 296 K and energy transfer from LH2 to LH1 is characterized by a time const ant of approximate to 3 ps at 296 K and approximate to 5 ps at 77 K. I n the blue-shifted B850 mutants, the transfer time from B850 to LH1 be comes 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 L H1 antennae.