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Explaining the Efficiency of Photosynthesis: Quantum Uncertainty or Classical Vibrations?

April 4, 2022

Photosynthetic organisms are known to use a mechanism of vibrationally assisted exciton energy transfer to efficiently harvest energy from light. The importance of quantum effects in this mechanism is a long-standing topic of debate, which has traditionally focused on the role of excitonic coherences. Here, we address another recent claim: that the efficient energy transfer in the Fenna–Matthews–Olson complex relies on nuclear quantum uncertainty and would not function if the vibrations were classical. We present a counter-example to this claim, showing by trajectory-based simulations that a description in terms of quantum electrons and classical nuclei is indeed sufficient to describe the funneling of energy to the reaction center. We analyze and compare these findings to previous classical-nuclear approximations that predicted the absence of an energy funnel and conclude that the key difference and the reason for the discrepancy is the ability of the trajectories to properly account for Newton’s third law.

Figure 1. Schematic depiction of energy transfer through the FMO complex.

The analysis demonstrates that the energy funnel in photosynthesis requires detailed balance between the quantized electronic states but not necessarily nuclear quantum uncertainty. Although the simulations in this paper concerned the particular example of FMO, the main conclusion, that the commutator ⟨[ξ, ξ]⟩ should not be interpreted as nuclear quantum uncertainty, applies more generally to other light-harvesting complexes. Note, however, that nuclear quantum effects (for instance in the term ⟨ξ2⟩) may still be important for quantitatively describing the dynamics of light-harvesting complexes in certain cases. According to Redfield theory, the relevant vibrations are those that are close to resonance with the interexciton energy gaps. For FMO, these gaps are on the order of kBT, which is why the bath can be well approximated using classical statistics. For complexes with larger energy gaps, such as LH2 (67) or PE545, (68) quantum features such as nuclear zero-point energy are likely to play a more important role. The development of trajectory-based methods that can properly treat nuclear quantum effects is an area of ongoing research.
Reference: Runeson, J.E., Lawrence, J.E., Mannouch, J.R., and Richardson, J.O. (2022) Explaining the Efficiency of Photosynthesis: Quantum Uncertainty or Classical Vibrations? J. Phys. Chem. Lett. 13, 3392-3399 (10.1021/acs.jpclett.2c00538).

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