Quest for Spatially Correlated Fluctuations in the FMO Light-Harvesting Complex

Abstract

The light absorption in light-harvesting complexes is performed by molecules such as chlorophyll, carotenoid, or bilin. Recent experimental findings in some of these complexes suggest the existence of long-lived coherences between the individual pigments at low temperatures. In this context the question arises if the bath-induced fluctuations at different chromophores are spatially correlated or not. Here we investigate this question for the Fenna-Matthews-Olson (FMO) complex of Chlorobaculum tepidum by a combination of atomistic theories, i.e., classical molecular dynamics simulations and semi-empirical quantum chemistry calculations. In these investigations at ambient temperatures, only weak correlations between the movements of the chromophores can be detected at the atomic level and none at the more coarse-grained level of site energies. The often employed uncorrelated bath approximations indeed seems to be valid. Nevertheless, correlations between fluctuations in the electronic couplings between the pigments can be found. Depending on the level of theory employed, also correlations between the fluctuations of site energies and the fluctuations in electronic couplings are discernable.

Laser-Limited Signatures of Quantum Coherence

“…In contrast, the measured spectral linewidths predict electronic dephasing times at 77 K hardly exceeding 100 fs [11], owing to rapid, stochastic variations in the environment, and coherences between excitons are expected to decay roughly on this timescale. The long coherence times could be explained by correlations between the fluctuating electronic site transition energies, but these are not concordant with recent molecular dynamics simulations [12,13].…”

Influence of environment induced correlated fluctuations in electronic coupling on coherent excitation energy transfer dynamics in model photosynthetic systems

“…Recent molecular dynamics (MD) simulation studies on the Fenna-Mathews-Olsen (FMO) excitation energy transfer network, 11,12 the light harvesting complex 2 (LHII) photosynthetic light harvesting complex 13 and the reaction center complex, 14 however, show no significant correlation in site energy fluctuations after averaging, and similar findings have been reported from calculations for the PE545 15 light harvesting system. However, Kleinekathöfer and co-workers, for example, have found that in FMO, 11 there is evidence of correlations in fluctuations of site energy and inter site electronic couplings and electronic coupling-electronic coupling correlations that are more significant compared to the apparently uncorrelated site energy fluctuations.…”

“…Several theoretical groups [11][12][13][14][15] have begun exploring detailed microscopic simulation models looking for different types of environment driven correlated fluctuations in chromophore electronic properties. The ubiquitous finding from the various photosynthetic energy transfer systems studied so far is that fluctuations in site energies seem to be such that on average they are uncorrelated.…”

“…However, Kleinekathöfer and co-workers, for example, have found that in FMO, 11 there is evidence of correlations in fluctuations of site energy and inter site electronic couplings and electronic coupling-electronic coupling correlations that are more significant compared to the apparently uncorrelated site energy fluctuations. There have been questions raised in recent work about the magnitudes of the parameters used, and the assumptions underlying some of these model calculations.…”

“…The possible origin of such correlations could be bath modes that influence the relative distances, and angles between two transition dipoles of different chromophores. 27 Some MD simulations suggest that correlated fluctuations in electronic coupling between chromophore dimer pairs that share a common chromophore, 11 for example, correlation between 4-5 coupling and 5-7 coupling in the FMO complex (i.e., inter-site coupling-inter-site coupling correlations) may be significant and one of the primary goals of this paper is to explore the effects of such correlation on the quantum dynamics of model chromophore networks. The simplified systems we explore in this paper thus include both a two state model where we can single out the effects of site energy-inter-site coupling correlations, and a three state example that is a minimal model in which inter-site couplinginter-site coupling correlation can be explored.…”