J. Phys. Chem. 1990, 94, 7338-7339
7338
Evidence for Ultrafast Exciton Localization in the Q, Band of Bacteriochlorophyll a -Protein from Prosthecochloris aestuarii Paul A. Lylet and Walter S. Struve* Department of Chemistry and Ames Laboratory-USDOE. (Received: June 5, 1990)
Iowa State University, Ames, Iowa 5001 1
Time-dependent linear dichroism was observed for several wavelengths in the Qyband of the BChl a-protein antenna complex from Prosthecochloris aesruarii, and photobleaching spectra were obtained for evidence of exciton localization. The results indicate that excitation transport in this antenna proceeds largely through incoherent hopping between localized states at times longer than 2 ps.
Introduction The bacteriochlorophyll a-protein antenna complex from the green sulfur bacterium Prosthecochloris aestuarii is one of the very few photosynthetic systems whose threedimensional structure is well-characterized.’** Despite this, the mechanics of its involvement in electronic excitation transport (EET) from the light-harvesting chlorosomes to the reaction centers are still largely unknown, and even the interpretation of its 809-nm Qy absorption spectrum is ~nsettled.~”It has long been recognized6 that the BChl a molecules, which are complexed by trimeric protein subunits into three symmetry-equivalent groups of seven chromophores, must be strongly exciton-coupled with dipole-dipole interaction energies as large as 200 ~ m - ’ .The ~ low-temperature Q,, absorption spectrum exhibits a t least five nonvibronic comp o n e n t ~at~ 825, 814, 805,800,and 793 nm. While the failure of simple exciton models to account for the absorption and CD band intensities has cast doubt on their excitonic origin, spectral hole-burning studies have unequivocally established that they arise in large part from exciton ~ p l i t t i n g s . ~Excitation -~ of the BChl a-protein with the burn wavelength 800.0 nm a t 4.2 K, for example, produces satellite holes whose intensities and positions correlate with those of the major bands in the Qyspectrum. These experiments uncovered the existence of at least eight distinct exciton components, indicating that the laser-prepared states may be delocalized over more than one subunit containing seven chromophores. A key mechanistic question is the extent to which the coherence of the laser-prepared exciton states is preserved during the course of EET. van Grondelle suggested in 1985 that vibrational motion in the protein environment should randomize the phases in the coherent states within to s, leading to decoupling and excitation localizati~n.~Johnson and Small observed zerephonon hole widths at 4.2 K corresponding to 100-fs lifetimes7 in all but the two lowest energy exciton components and attributed this to downhill relaxation between exciton components. In this work, we examined polarized photobleaching in the Qy system of the BChl a-protein from P. aestuarii a t 805, 809, and 814 nm. In an earlier pump-probe study,I0 photobleaching depolarization occurred with 4.8-ps decay time in the 603-nm Q, band and was interpreted in terms of excitation hopping between protein subunits organized in the crystallographic P63 hexagonal structure.] In addition to extending the pumpprobe studies to the more familiar Qxsystem, we have obtained Qyphotobleaching spectra for spectral evidence of ultrafast energy redistribution. Experimntal Section The BChl a-protein was dissolved in a mixture of Tris buffer and water, and its optical density at 809 nm was adjusted to 2.0 in a 1-cm path length. Samples were placed between h/4 fused silica flats separated by an 8 W p m Teflon spacer for pump-probe experiments and rotated at I2 Hz to avoid laser photooxidation during scans. ‘William E. Catron Research Fellow.
0022-3654/90/2094-7338$02.50/0
A hybrid mode-locked synchronously pumped Styryl8/IR- 140 dye laser was excited by 532-nm, 70-ps pulses at 76-MHz repetition rate from a CW mode-locked Nd:YAG laser. A cavity dumper slewed the repetition rate to 19 MHz, sharpening the autocorrelation function to 2-3 ps fwhm (depending on wavelength). The pump and probe beams were modulated at 6.5 and 0.5 MHz, respectively, with Isomet 1206C acoustooptic modulators. The pump beam was variably delayed by a translatable BK-7 corner cube prism driven by a Micro-Controle UTlOOl25PP translation stage (0.l-Im steps, 12.5-cm range). The probe polarization was maintained at 45’ from the vertical laser polarization with a calcite Glan-Thompson polarizer, while the pump polarization was alternated among parallel, perpendicular, and 54.7O magic-angle polarizations. The pumpprobe optics reduced the average beam powers incident on the sample to
