Ana Damjanović, Ioan Kosztin, Ulrich Kleinekathoefer, and Klaus Schulten.
Excitons in a photosynthetic light-harvesting system: A combined
molecular dynamics, quantum chemistry and polaron model study.
Physical Review E, 65:031919, 2002.
(24 pages).
DAMJ2002
The dynamics of pigment-pigment and pigment-protein
interactions in light-harvesting complexes is studied
with a novel approach that combines molecular
dynamics simulations with quantum chemistry
calculations and a polaron model analysis. The
molecular dynamics simulation of light-harvesting
complexes was performed on an 87,055 atom system
comprised of an LH-II complex of Rhodospirillum
molischianum embedded in a lipid bilayer and
surrounded with appropriate water layers. For each of
the 16 B850 BChls we performed 400 ab initio
quantum chemistry calculations on geometries that
emerged from the molecular dynamical simulations,
determining the fluctuations of pigment excitation
energies as a function of time. From the results of
these calculations we
construct a time-dependent Hamiltonian of the B850
exciton system from which we determine within linear
response theory the absorption spectrum. Finally, a
polaron model is introduced to describe both the
excitonic and coupled phonon degrees of freedom by
quantum mechanics. The exciton-phonon coupling that
enters into the polaron model, and the
corresponding phonon spectral function are derived
from the molecular dynamics and quantum chemistry
simulations. The model predicts that excitons in the
B850 BChl ring are delocalized over five pigments at
room temperature. Also, the polaron model permits
the calculation of the absorption and circular dichroism
spectra of the B850 excitons from the sole knowledge
of the autocorrelation function of the excitation energies
of individual BChls, which is readily available from the
combined molecular dynamics and quantum chemistry
simulations. The obtained results are found to be in
good agreement with the experimentally measured
absorption and circular dichroism spectra.
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