to 11 Å. Not only does each bacteriochlo-
rophyll experience different interactions
with the other bacteriochlorophylls but
the protein environment surrounding
each bacteriochlorophyll is unique.
One goal of both experimental and
theoretical research on this complex
was to understand the contribution of
each bacteriochlorophyll in the process
of energy transfer in the cell. Despite
the difference observed in the structural
features for each bacteriochlorophyll, the
optical absorption spectrum observed in
the FMO complex is relatively feature-
less at room temperature, with all of the
bacteriochlorophylls absorbing light at
the same wavelength. When the samples
are cooled to low temperature, the
contributions of the different bacterio-
chlorophylls is more distinctive, with
the broad band observed at room temperature having some resolved under-
lying bands. Several theoretical calculations have been performed with
the goal of assigning these partially resolved absorption bands to indi-
vidual bacteriochlorophylls, but the calculations consistently show that
the pigments are highly interacting and hence difficult to distinguish.
Time-resolved optical spectroscopy has helped to distinguish the relative
contributions, at each point in time, of the different underlying bands but
a unique temporal assignment could not be made unambiguously as the
interactions could only be inferred indirectly. These experimental diffi-
culties have been overcome with the development of two-dimensional (2D)
femtosecond infrared spectroscopy (Brixner et al. 2005). The diagonal
peaks in the 2D traces correspond to the positions of the peaks in the
absorption spectrum. The off-diagonal peaks reveal which molecules are
interacting and hence the couplings (Figure 14.17). The technique of 2D
optical spectroscopy makes use of a series of ultrashort laser pulses in a
manner analogous to the pulse sequences used in NMR (Chapter 16). The
initial pulse excites the sample and all of the transitions of interest. After
a delay time corresponding to the coherence time in NMR, the sample
is illuminated with a second laser light pulse. Depending upon the delay
time used, the signal at each frequency either increases or decreases.
After another time interval corresponding to the population time, a third
pulse probes the optical state of the sample. The signal arises from a fourth
spontaneous pulse known as the photon echo that arises from the inter-
actions of the three previous pulses with the transition dipole moments of
the pigments. Each 2D spectrum shows the couplings a specific time after308 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY
Figure 14.16
A schematic
representation of
the FMO complex,
showing the
extensive presence of
βstrands organized
into a βsheet, which
surround seven
bacteriochlorophylls.