Efficient Energy Transfer in Light-Harvesting Systems, III: The Influence of the Eighth Bacteriochlorophyll on the Dynamics and Efficiency in FMO

Abstract

The most recent crystal structure of the Fenna-Matthews-Olson (FMO) protein complex indicates that each subunit contains an additional eighth chromophore. It has been proposed that this extra site functions as a link between the chlorosome antenna complex and the remaining seven chromophores in FMO [Schmidt am Busch et al, J. Phys. Chem. Lett., 2, 93 (2011)]. Here, we investigate the implications of this scenario through numerical calculations with the generalized Bloch-Redfield (GBR) equation and the non-interacting blip approximation (NIBA). Three key insights into the population dynamics and energy transfer efficiency in FMO are provided. First, it is shown that the oscillations that are often observed in the population relaxation of the dimer composed of sites one and two may be completely suppressed in the eight site model. The presence of the coherent oscillations is shown to depend upon the particular initial preparation of the dimer state. Secondly it is demonstrated that while the presence of the eighth chromophore does not cause a dramatic change in the energy transfer efficiency, it does however lead to a dominant energy transfer pathway which can be characterized by an effective three site system arranged in an equally-spaced downhill configuration. Such a configuration leads to an optimal value of the site energy of the eighth chromophore which is shown to be near to its suggested value. Finally we confirm that the energy transfer process in the eight site FMO complex remains efficient and robust. The optimal values of the bath parameters are computed and shown to be closer to the experimentally fitted values than those calculated previously for the seven site system.

Robustness, efficiency, and optimality in the Fenna-Matthews-Olson photosynthetic pigment-protein complex

“…1(c)) are found to lie close to the chlorosome baseplate, suggesting that initial electronic excitation occurs at one of these sites (or involves an excitonic state delocalised across these chromophores 42 ), while Bchl 3 is the energetic sink in the system, funnelling electronic excitation energy to the RC. While the exact mechanism of EET to Bchl 3 is somewhat ambiguous, in part because the initial excitation in the native environment is similarly ambiguous, recent experimental and theoretical work has shown that FMO exhibits at least two different pathways for EET to site 3; [5][6][7]42 as we show below, these multiple transport paths, as well as the inherent heterogeneity of connectivity between different Bchl sites, are important features in maintaining robust EET.…”

“…1 Much of the contemporary experimental and theoretical interest in photosynthetic PPCs has focussed on the role of quantum coherence in the mechanism and efficiency of EET. 1,[3][4][5][6][7][8][9][10][11][12] Time-resolved non-linear spectroscopic studies suggest the existence of long-lived quantum "beats" in the a) Electronic mail: S.Habershon@warwick.ac.uk population dynamics of PPCs demonstrating that quantummechanical effects may play a role in EET, 1,7,9,10,13 contrary to the textbook idea that such coherence should be "washed-out" by the thermal environment. In contrast, far less emphasis has been placed on answering a more fundamental question: why are PPCs built the way they are?…”

“…(1)), the experimental site energies are known from fluorescence studies, 42,44 while the off-diagonal couplings have been determined by the transition dipole-dipole cube method. 6 In our treatment of the quantum dynamics of FMO-like systems, the electronic Hamiltonian of Eq. (1) is augmented by anti-Hermitian terms describing excitation energy loss by exciton recombination at each Bchl site, as well as excitonic trapping at the sink site (Bchl 3 in Fig.…”

“…[21][22][23][24][25][26] Similar studies of photosynthetic systems have investigated EET efficiency optimisation with respect to environmental model parameters such as temperature or exciton trapping rates. 6,20,21,27,28 The effect of removing a chromophore on EET efficiency has also been investigated, and it has been suggested that photosynthetic EET networks display a robustness to the loss of a chromophore in the network through the utilisation of multiple EET pathways. 6,[29][30][31][32] In contrast to these previous investigations, which have focussed on the role of site energy fluctuations, environmental fluctuations, and exciton trapping rates, this article focusses predominantly on evaluating how the electronic Hamiltonian of the EET system governs the robustness and optimal EET characteristics of a biological photosynthetic system.…”

“…6,20,21,27,28 The effect of removing a chromophore on EET efficiency has also been investigated, and it has been suggested that photosynthetic EET networks display a robustness to the loss of a chromophore in the network through the utilisation of multiple EET pathways. 6,[29][30][31][32] In contrast to these previous investigations, which have focussed on the role of site energy fluctuations, environmental fluctuations, and exciton trapping rates, this article focusses predominantly on evaluating how the electronic Hamiltonian of the EET system governs the robustness and optimal EET characteristics of a biological photosynthetic system. Specifically, we adopt a network-based view of photosynthetic EET systems and focus on investigating quantum transport dynamics in a prototypical PPC, the FMO complex.…”