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.

The structure and function of the LH2 (B800–850) complex from the purple photosynthetic bacterium Rhodopseudomonas acidophila strain 10050

“…If LH2 is excited with a sub 100 fsec pulse at 800 nm, then at room temperature it takes ~0.7 psec for the energy to be transferred from B800 to B850 in R. sphaeroides (Hess et al, 1995;Jimenez et al, 1996;Soo et al, 1996) and -~0.8 psec in R. acidophila (Ma et al, 1996). Energy transfer among the nine B800 BChls during the lifetime of B800* can be explored by looking at the decay of the anisotropy of this excited state population, since each B800 BChl a molecule has a different orientation with respect to the direction of the exciting laser pulse.…”

Low-Intensity Pump-Probe Measurements on the B800 Band of Rhodospirillum Molischianum

“…sphaeroides at 77 K, Hess et al (1995) found biexponential decays with wavelength-dependent time constants. For 790, 800, and 810 nm excitation/detection, fast components of 0.33, 0.46, and 0.60 ps, and slow components of 1.67, 1.80, and 1.90 ps, respectively, were measured.…”

“…This energy transfer within B800-850 complexes of different purple bacteria has been studied by time-resolved measurements (Hess et al, 1995;Monshouwer et al, 1995;Jimenez et al, 1996;Joo et al, 1996;Wu et al, 1996a;Kennis et al, 1997;Pullerits et al, 1997;Herek et al, 2000;Salverda et al, 2000;Agarwal et al, 2001;Ihalainen et al, 2001), hole-burning spectroscopy (van der Laan et al, 1990;Reddy et al, 1991;de Caro et al, 1994;Wu et al, 1996aWu et al, , 1996bMatsuzaki et al, 2001;Zazubovich et al, 2002) and also by singlemolecule experiments (van Oijen et al, 2000); the subject has been reviewed by Sundström et al (1999). For the events taking place in the B800 band, de Caro et al (1994) proposed a two-pool model.…”

The Light Harvesting Process in Purple Bacteria

“…F6rster [21]) model for intraband energy transfer [62]. However, it is thought that for B850 the couplings are of the same magnitude as the disorder (inhomogeneous broadening) and the homogeneous linewidth.…”