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Determination of Excited-State    Energies and Dynamics in the B Band    of the Bacterial Reaction Center with         2D ...
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Determination of Excited-State Energies and Dynamics in the B Band of the Bacterial Reaction Center with 2D Electronic Spectroscopy

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  1. 1. Determination of Excited-State Energies and Dynamics in the B Band of the Bacterial Reaction Center with 2D Electronic Spectroscopy Gabriela S. Schlau-Cohen†§, Eleonora De Re‡§, Richard J. Cogdell||, and Graham R. Fleming*†‡§ †Department of Chemistry and ‡Graduate Group in Applied Science and Technology, University of California − Berkeley, Berkeley, California, United States § Physical Biosciences Division, Lawrence Berkeley National Lab, Berkeley, California, United States ||University of Glasgow, Glasgow, United Kingdom *grfleming@lbl.govJ. Phys. Chem. Lett. 2012, Vol. 3, 2487-2492 1
  2. 2. Bacterial reaction center (bRC)The site of initial charge separation event in photosynthetic light-harvestingConverts photoenergy to electrons with remarkable near unity quantum efficiency energy transfer electron transfer Two nearly symmetric branches of pigments exhibit similar energy transfer pathways After charge separation, only one branch transfers electrons Differences in excited-state energies and dynamics along the two branches that give rise to the functional asymmetry remain incompletely described Proprietary and Confidential 2 American Chemical Society
  3. 3. Energetically overlapping transitionsprevent study of individual branches Linear absorption spectrum of oxidized bRC at 77 K Each peak contains two closely spaced transitions, where the two transitions correspond to one from each branch. The excited state energies and dynamics of the individual branches have not been separable in linear and nonlinear spectroscopies 2D spectroscopy was applied to the B band centered at 800 nm to investigate the dynamics of the accessory bacteriochhlorophyll (BChl), BA and BB Proprietary and Confidential 3 American Chemical Society
  4. 4. 2D spectroscopy reveals excited stateenergies and energy transfer dynamics ωt CCD spectrometer sample Fourier transform2D spectroscopy is a nonlinear, ultrafast diagonal peak:measurement using four incident laser pulses excited state ωt (emission) energyProbes correlations between excitation andemission energy as a function of a delay time cross peak:between excitation and emission events, known coupling andas the waiting time (T) energy transfer ωτ (excitation) Proprietary and Confidential 4 American Chemical Society
  5. 5. 2D real, nonrephasing spectra of the Bband of the bacterial reaction center 2D nonrephasing spectra reveal two well-separated peaks, corresponding to the two BChl that contribute to the B band (arrows D1 and D2) Separation of peaks enables observation of a cross peak (arrow CP) showing interaction between the BChl within 100 fs Proprietary and Confidential 5 American Chemical Society
  6. 6. Relative intensities of peaks depends onpolarization of incident laser pulses θ2 θ1 θ12 LO Peak scaling Cross-peak-specific (π/3,-π/3,0,0) polarization increases relative weight of cross peaks Scaling based on polarization sequence of incident pulses can reveal molecular process underlying peak Proprietary and Confidential 6 American Chemical Society
  7. 7. Enhancement of cross-peak enablesobservation of cross-peak dynamics Horizontal slices (ωt = 12 325 cm–1) from absolute value, nonrephasing spectra at selected waiting times (0,0,0,0) (π/3,-π/3,0,0) Difference in scaling, as appears in the clear grow-in of the cross-peak (ωτ = 12 450 cm–1) under the cross-peak-specific sequence, indicates existence of energy-transfer pathway Suppression of other features allows appearance at 70 fs and relative increase of cross- peak to be more clearly observed Proprietary and Confidential 7 American Chemical Society
  8. 8. Conclusions Energies of the two distinct, previously inseparable states within the B band were determined, with BA at 12 450 cm–1 and BB at Acknowledgments This work was supported by the Director, 12 325 cm–1. Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy For the first time, transfer of amplitude most under contract DE-AC02-05CH11231 and the Division of Chemical Sciences, simply described as energy transfer between the Geosciences, and Biosciences, Office of two states in the B band was detected. Basic Energy Sciences of the U.S. Department of Energy through grant DE- AC03-76SF000098 (at LBNL). G.S.S.-C. Observed excited-state energies and dynamics thanks the A.A.U.W. American Fellowship can benchmark microscopic modeling of how for support. R.J.C. thanks the BBSRC for small differences in molecular structure (i.e., financial support. We thank A. Ishizaki for differences between the two branches) give rise helpful discussion and A. K. De for experimental assistance. to tuned pigment–pigment or pigment–protein couplings.Proprietary and Confidential 8American Chemical Society