Mortaza Aghtar, Johann Strümpfer, Carsten Olbrich, Klaus Schulten, and
Ulrich Kleinekathoefer.
On the different types of vibrations interacting with electronic
excitations in the PE545 and FMO antenna systems.
Journal of Physical Chemistry Letters, 5:3131-3137, 2014.
AGHT2014
The interest in the phycoerythrin 545 (PE545) photosynthetic antenna system of marine
algae and the Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria has
drastically increased since long-lived quantum coherences were reported for these
complexes. For the PE545 complex, this phenomenon is clearly visible even at ambient
temperatures, while for the FMO system it is more prominent at lower temperatures. The
key to elucidate the role of the environment in these long-lived quantum effects is the
spectral density. Here, we employ molecular dynamics simulations combined with quantum
chemistry calculations to study the coupling between the biological environment and the
vertical excitation energies of the bilin pigment molecules in PE545 and compare them to
prior calculations on the FMO complex. It is found that the overall strength of the resulting
spectral densities for the PE545 system is similar to the experiment-based counterpart but
also to those in the FMO complex. Molecular analysis, however, reveals that the origin for
the spectral densities in the low frequency range, which is most important for excitonic
transitions, is entirely different. In the case of FMO, this part of the spectral density is due
to environmental fluctuations, while, in case of PE545, it is essentially only due to internal
modes of the bilin molecules. This finding sheds new light on possible explanations of the
long-lived quantum coherences and that the reasons might actually be different in
dissimilar systems.
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