We report a method for the structure-based calculation
of the spectral
density of the pigment–protein coupling in light-harvesting
complexes that combines normal-mode analysis with the charge density
coupling (CDC) and transition charge from electrostatic potential
(TrEsp) methods for the computation of site energies and excitonic
couplings, respectively. The method is applied to the Fenna–Matthews–Olson
(FMO) protein in order to investigate the influence of the different
parts of the spectral density as well as correlations among these
contributions on the energy transfer dynamics and on the temperature-dependent
decay of coherences. The fluctuations and correlations in excitonic
couplings as well as the correlations between coupling and site energy
fluctuations are found to be 1 order of magnitude smaller in amplitude
than the site energy fluctuations. Despite considerable amplitudes
of that part of the spectral density which contains correlations in
site energy fluctuations, the effect of these correlations on the
exciton population dynamics and dephasing of coherences is negligible.
The inhomogeneous charge distribution of the protein, which causes
variations in local pigment–protein coupling constants of the
normal modes, is responsible for this effect. It is seen thereby that
the same building principle that is used by nature to create an excitation
energy funnel in the FMO protein also allows for efficient dissipation
of the excitons’ excess energy.
The intermolecular contribution to the spectral density of the exciton-vibrational coupling of the homotrimeric Fenna–Matthews–Olson (FMO) light-harvesting protein of green sulfur bacteria P. aestuarii is analyzed by combining a normal mode analysis of the protein with the charge density coupling method for the calculation of local transition energies of the pigments. Correlations in site energy fluctuations across the whole FMO trimer are found at low vibrational frequencies. Including, additionally, the high-frequency intrapigment part of the spectral density, extracted from line-narrowing spectra, we study intra- and intermonomer exciton transfer. Whereas the intrapigment part of the spectral density is important for fast intramonomer exciton relaxation, the intermolecular contributions (due to pigment-environment coupling) determine the intermonomer exciton transfer. Neither the variations of the local Huang–Rhys factors nor the correlations in site energy fluctuations have a critical influence on energy transfer. At room temperature, the intermonomer transfer in the FMO protein occurs on a 10 ps time scale, whereas intramonomer exciton equilibration is roughly two orders of magnitude faster. At cryogenic temperatures, intermonomer transfer limits the lifetimes of the lowest exciton band. The lifetimes are found to increase between 20 ps in the center of this band up to 100 ps toward lower energies, which is in very good agreement with the estimates from hole burning data. Interestingly, exciton delocalization in the FMO monomers is found to slow down intermonomer energy transfer, at both physiological and cryogenic temperatures.
We
present a microscopic theory for the description of fluctuation-induced
excitation energy transfer in chromophore dimers to explain experimental
data on a perylene biscarboximide dyad with orthogonal transition
dipole moments. Our non-Condon extension of Förster theory
takes into account the fluctuations of excitonic couplings linear
and quadratic in the normal coordinates, treated microscopically by
quantum chemical/electrostatic calculations. The modulation of the
optical transition energies of the chromophores is inferred from optical
spectra of the isolated chromophores. The application of the theory
to the considered dyad reveals a two to three order of magnitude increase
in the rate constant by non-Condon effects. These effects are found
to be dominated by fluctuations linear in the normal coordinates and
provide a structure-based qualitative interpretation of the experimental
time constant for energy transfer as well as its dependence on temperature.
Photosynthetic green sulfur bacteria are able to survive under extreme low light conditions. Nevertheless, the light-harvesting efficiencies reported so far, in particular for Fenna-Matthews-Olson(FMO) protein-reaction center complex (RCC) supercomplexes, are...
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