This presentation has been moved. To view this presentation, please visit http://pubs.acs.org/iapps/liveslides/pages/index.htm?mscNo=jz300841u Determination of Excited-State Energies and Dynamics in the B Band of the Bacterial Reaction Center with 2D Electronic Spectroscopy

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.gov
J. Phys. Chem. Lett. 2012, Vol. 3, 2487-2492 1

2. Bacterial reaction center (bRC)
The site of initial charge separation event in photosynthetic light-harvesting
Converts 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
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3. Energetically overlapping transitions
prevent 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
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4. 2D spectroscopy reveals excited state
energies and energy transfer dynamics
ωt
CCD
spectrometer
sample
Fourier
transform
2D spectroscopy is a nonlinear, ultrafast diagonal peak:
measurement using four incident laser pulses excited state
ωt (emission)
energy
Probes correlations between excitation and
emission energy as a function of a delay time cross peak:
between excitation and emission events, known coupling and
as the waiting time (T) energy transfer
ωτ (excitation)
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5. 2D real, nonrephasing spectra of the B
band 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
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6. Relative intensities of peaks depends on
polarization 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
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7. Enhancement of cross-peak enables
observation 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
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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.
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