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J. Phys. Chem. B 1999, 103, 878-883
Energy Relaxation within the B850 Absorption Band of the Isolated Light-Harvesting Complex LH2 from Rhodopseudomonas Acidophila at Low Temperature Simone I. E. Vulto,* John T. M. Kennis,† Alexander M. Streltsov,‡ Jan Amesz, and Thijs J. Aartsma Department of Biophysics, Huygens Laboratory, Leiden UniVersity, P.O. Box 9504, 2300 RA Leiden, The Netherlands ReceiVed: June 8, 1998; In Final Form: October 9, 1998
Energy relaxation on the bacteriochlorophyll B850 aggregate of the isolated light-harvesting antenna complex LH2 of Rhodopseudomonas acidophila at 7 K was examined by means of two-color femtosecond absorption spectroscopy. Upon excitation at 860 nm, at the blue side of the B850 absorption maximum, two kinetic components were observed in the subpicosecond time region: a major component of 100 fs and a minor one of 700 fs. The spectrum of the first component was typical for energy relaxation in an excitonically coupled system. The same two components were observed in the position of the isosbestic wavelength of the absorbance difference spectrum, which shifted to the red by 4 nm with time. Anisotropy decay to a final value of 0.07 mainly occurred within our time resolution of 70 fs. This corresponds to a homogeneous line width of B850 of at least 80 cm-1. In addition, a weak slow phase in the anisotropy decay of 700 fs with an amplitude of 5-10% of the total decay was observed. In the absorbance kinetics, time constants of 100 and 700 fs were also observed upon excitation at 880 nm, which is at the red side of the absorption band, but now the spectrum only shifted by about 0.5 nm with time. In this case the decay of the anisotropy occurred within the time resolution to a final value of 0.12-0.14. The final position of the difference spectrum upon excitation at 880 nm was red-shifted by 1 nm with respect to that excited at 860 nm.
Introduction The study of the physical principles of energy transfer in photosynthetic antenna systems has greatly benefited from the elucidation of the three-dimensional structures of a number of these pigment-protein complexes. One of the most appealing examples is the light-harvesting 2 (LH2) complex of purple bacteria, which shows a perfect circular symmetry with rings of closely spaced bacteriochlorophyll (BChl) molecules,1-3 making it an attractive model system for studies of energy transfer and of the influence of static and dynamic disorder on the electronic structure of strongly coupled pigment systems. A number of spectroscopic techniques has been applied to study the excitonic and energy transfer characteristics of the LH2 complex. Measurements of ultrafast excitation depolarization4 and of nonlinear absorption5 have been interpreted to indicate exciton delocalization over the entire aggregate. This interpretation is in conflict, however, with recent measurements of the oscillator strength of the lowest exciton state and of the shape of the absorption difference spectrum, and most investigators now agree that exciton localization occurs on only 2-4 pigments of the B850 ring.6-11 This article concerns the excitation dynamics of isolated LH2 complexes from Rhodopseudomonas (Rps.) acidophila at low temperature. Fluorescence emission spectra have shown a strong dependence on the wavelength of excitation, suggesting a * Corresponding author. Fax: 31-71-5275819. E-mail: vulto@ biophys.leidenuniv.nl. † Present address: Department of Chemistry, University of California, Berkeley, CA 94720. ‡ Present address: School of Applied and Engineering Physics, 212 Clark Hall, Cornell University, Ithaca, NY 14853.
considerable inhomogeneity of the B850 absorption band,7 but such measurements do not provide information on the dynamics of the energy relaxation. At room temperature, excitation depolarization was found to occur in 100 fs or less in the LH2 complexes of Rps. acidophila12,13 and Rhodobacter (Rb.) sphaeroides4,9,14 and the energy to equilibrate over the B850 aggregate in about 250 fs, as indicated by the time evolution of the absorbance difference spectra upon excitation of B850.12 Measurements with membranes of Rb. sphaeroides at low temperature showed a biphasic depolarization within the B850 absorption band with time constants in the femto- and picosecond time regions.11 Further, hole-burning measurements showed that narrow zero-phonon holes could only be burnt in the red edge of the B850 absorption band, indicating rapid downhill energy transfer,15 while transient spectroscopy indicated a time constant of a few hundred femtoseconds or less for this process.11,16,17 In the isolated LH2 complex from Rps. acidophila little spectral evolution of B850 was seen upon excitation in the B800 band, indicating that energy transfer within the B850 ring was fast compared to the time constant (1.8 ps) for energy transfer from B800 to B850.7 In this paper we present magic angle and polarized femtosecond two-color measurements upon excitation in the blue edge (860 nm) and the red edge (880 nm) of the B850 band of the isolated LH2 complex from Rps. acidophila at 7 K. Biphasic relaxation kinetics were observed with time constants of 100 and 700 fs, independent of the wavelength of excitation and measurement. As judged from its spectrum, the first component represents energy relaxation in a system with strong dipolar interaction. The slower component presumably represents thermal equilibration within the manifold of exciton states. The same time constants were observed in a red shift of the
10.1021/jp9825415 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/15/1999
Energy Relaxation within the B850 Absorption Band
J. Phys. Chem. B, Vol. 103, No. 5, 1999 879 methylamine oxide (LDAO)/30 mM Tris buffer (pH 8.0) and mixed with 2 parts glycerol to obtain a clear glass at low temperature. The sample was transferred to a 0.8 mm cuvette and then cooled to 7 K in a helium flow cryostat (Oxford Instruments). Two-color femtosecond pump-probe measurements were performed with a mode-locked, cavity dumped Ti:sapphire laser setup described earlier.18 Both excitation and probe beams were precompensated for dispersion by double passing of two fusedsilica prisms in the optical path. This configuration resulted in a chirp of less than 10 fs over the probe beam.19,20 The repetition rate of the cavity dumper was set to 45 kHz to avoid accumulation of triplet states in the sample. The pulse energy was adjusted to 0.2 nJ (
