Energy transfer in spectrally inhomogeneous light-harvesting pigment-protein complexes of purple bacteria

Energy transfer within the peripheral light-harvesting antenna of the purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas palustris was studied by one- and two-color pump-probe absorption spectroscopy with similar to 100-fs tunable pulses at room temperature and at 77 K. The energy transfer from B800 to B850 occurs with a time constant of 0.7 +/- 0.05 ps at room temperature and 1.8 +/- 0.2 ps at 77 K and is similar in both species. Anisotropy measurements suggest a limited but fast B800 <----> B800 transfer time (tau similar to 0.3 ps). This is analyzed as incoherent hopping of the excitation in a system of spectrally inhomogeneous antenna pigment-protein complexes, by a master equation approach, The simulations show that the measured B800 dynamics is well described as energy transfer with a characteristic average nearest-neighbor pairwise transfer time of 0.35 ps among similar to 10 Bchl molecules in a circular arrangement, in good agreement with the recent high-resolution structure of LH2, The possible presence of fast intramolecular relaxation processes within the Bchl a molecule was investigated by measurement of time-resolved difference absorption spectra and kinetics of Bchl a in solution and in low-temperature glasses, From these measurements it is concluded that fast transients observed at room temperature are due mainly to solvation processes, whereas at 77 K predominantly slower (>10-ps) relaxation occurs.