Theory and Simulation of the Environmental Effects on FMO Electronic Transitions

Abstract

Long-lived quantum coherence has been experimentally observed in the Fenna-Matthews-Olson (FMO) light-harvesting complex. It is much debated which role thermal effects play and if the observed low-temperature behavior arises also at physiological temperature. To contribute to this debate we use molecular dynamics simulations to study the coupling between the protein environment and the vertical excitation energies of individual bacteriochlorophyll molecules in the FMO complex of the green sulphur bacterium Chlorobaculum tepidum. The so-called spectral densities, which account for the environmental influence on the excited state dynamics, are determined from temporal autocorrelation functions of the energy gaps between ground and first excited states of the individual pigments. Although the overall shape of the spectral density is found to be rather similar for all pigments, variations in their magnitude can be seen. Differences between the spectral densities for the pigments of the FMO monomer and FMO trimer are also presented.

Vibration-assisted resonance in photosynthetic excitation-energy transfer

“…Indeed, by strongly mixing a vibrationally excited level on one site with the vibrational ground state of another site, vibration-assisted resonance creates an even greater degree of coherent delocalisation across the network than excitonic coupling alone. More significantly, the microscopic mechanism presented here may be able to conceptually reconcile the large reorganisation energies found in atomistic numerical studies [17,18] with experimental observations showing the persistence of coherent oscillations up to physiological temperatures [37]. Further work on the full FMO complex with multiple vibrational modes and finite temperatures will address this question.…”

Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria

“…We use MD simulations to generate R(t) and TDDFT excited-state calculations to obtain ǫ(R), which is consistent with the models proposed previously [35][36][37][38][39] Thus, in contrast to many studies based on a quantum master equation, this approach can describe the system-bath coupling in complete atomistic detail.…”

“…A similar approach has been previously applied to small lightharvesting systems such as the FMO complex [37][38][39] and light-harvesting complexes II (LHII) of purple bacteria [35,36]. For a large number of BChls present in the chlorosome, it is computationally unfeasible to calculate site energies of all pigment molecules along an MD trajectory.…”

“…The spectral densities for the syn and the anti monomers are shown in Figure 2. The spectral densities were calculated as a weighted cosine transform of the correlation functions [35,37,65]:…”

Nonclassical energy transfer in photosynthetic FMO complex

“…Characteristics of the system are obtained by calculating the chromophore populations and the system entropy. We also compare the results calculated using the Debye (or the over damped Brownian oscillator) spectral density [9] and the one obtained from FMO molecular dynamics simulations [10].…”