title: Energy transfer in spectrally inhomogeneous light-harvesting pigment-protein complexes of purple bacteria.
abstract: 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 approximately 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 approximately 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 approximately 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.
authors.affiliation: Department of Chemical Physics, Lund University, Sweden.
authors.forename: S
authors.initials: S
authors.lastname: Hess
authors.forename: E
authors.initials: E
authors.lastname: Akesson
authors.forename: R J
authors.initials: RJ
authors.lastname: Cogdell
authors.forename: T
authors.initials: T
authors.lastname: Pullerits
authors.forename: V
authors.initials: V
authors.lastname: Sundström
mesh_terms: D020130:Bacterial Chromatophores / Q000378:metabolism; D004735:Energy Transfer; D005454:Fluorescence Polarization / Q000295:instrumentation / Q000379:methods; D007700:Kinetics; D008958:Models, Molecular; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism*; D017433:Protein Structure, Secondary; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D012241:Rhodopseudomonas / Q000378:metabolism*; D013053:Spectrophotometry / Q000379:methods; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins