Abstract
For more than 50 years we have known that photosynthetic systems harvest solar energy with almost unit quantum efficiency. However, recent experimental evidence of quantum coherence during the excitonic energy transport in photosynthetic organisms challenges our understanding of this fundamental biological function. Currently, and despite numerous efforts, the causal connection between coherence and efficiency is still a matter of debate. We show, through extensive simulations of quantum coherent transport on networks, that three dimensional structures characterized by centro-symmetric Hamiltonians are statistically more efficient than random arrangements. Moreover, a strong correlation of centro-symmetry with quantum efficiency is also observed under the coherent transport dynamics induced by experimentally estimated electronic Hamiltonians of the Fenna–Mathew–Olson complex of sulfur bacteria and of the cryptophyte PC645 complex of marine algae. The application of a genetic algorithm results in a set of optimized Hamiltonians only when seeded from the experimentally estimated Hamiltonian. These results suggest that what appears to be geometrically disordered complexes may well exhibit an inherent hidden symmetry which enhances the energy transport between chromophores. We are confident that our results will motivate research to explore the properties of nearly centro-symmetric Hamiltonians in realistic environments, and to unveil the role of symmetries for quantum effects in biology. The unravelling of such symmetries may open novel perspectives and suggest new design principles in the development of artificial devices.
Highlights
The apparatus used by photosynthetic organisms to harvest the Suns energy is both complex and highly efficient
Random networks Inspired by the structure of the Fenna–Mathew–Olson (FMO) network—seven (FMO7) [31, 32] or eight (FMO8) [33] chromophores that are connected through dipolar interactions— we study the simplest possible random model which can grasp its essential ingredients: a small random network with N identical sites, which completely neglects the individual chromophores’ couplings to their local environments, and where coherent transport of a single excitation is generated by the Hamiltonian
A careful analysis of the reported coupling matrix elements of the biologically relevant light harvesting complexes FMO8 and PC645, together with the associated error margins, suggests that centro-symmetry may be a design principle used by Nature: when neglecting on-site energy shifts induced by the coupling to background degrees of freedom, the available structure data are in a statistical sense close to centro-symmetric conformations
Summary
The apparatus used by photosynthetic organisms to harvest the Suns energy is both complex and highly efficient. A well established strategy to simulate the quantum excitation transport in such molecular complexes is the dynamical propagation of the electronic excitation in the presence of the many degrees of freedom of the environment [8,9,10,11,12,13] This line of thought may suggest that proper tuning of the interaction between system and environment is responsible for the long coherence times and efficient transport. On the other hand, simplified models [23,24,25,26,27,28] that mimic the excitonic dynamics by those of two coupled two level systems interacting with the environment may help to identify some minimal ingredients that must be included into a general theory Because of their simplicity, they disregard many of the structural features of the light harvesting complexes–possibly those that are most relevant for the transport efficiency in realistic systems. We rather seek to import an alternative, statistical perspective, and to unveil possible design principles which do not immediately emerge, or even may be masked by competing effects, in ab initio calculations
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
