Research ArticleBIOPHYSICS

Do photosynthetic complexes use quantum coherence to increase their efficiency? Probably not

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Science Advances  17 Feb 2021:
Vol. 7, no. 8, eabc4631
DOI: 10.1126/sciadv.abc4631
  • Fig. 1 Mechanisms of efficiency-driven evolution and environment-assisted quantum transport.

    (A) Schematic description of the evolutionary progress of photosynthetic complexes toward their current geometry, with efficiency being the evolutionary driving force. As evolution progresses, the structure of the photosynthetic complex evolves toward its current structure [the Fenna-Matthews-Olson (FMO) complex in this example] while increasing efficiency. Whether this is indeed the evolutionary pathway of photosynthetic complexes, and if so, whether quantum coherence is part of the efficiency enhancement is a central question in the field of quantum biology. (B) Schematic depiction of the population uniformization mechanism shown for a uniform chain of six sites (blue lines depict the sites in the chain; yellow arrows show the excitation of first site and extraction from fifth site). The density of the sites is described by blue bars for the quantum regime, ENAQT regime, and classical regime, along with a schematic form for the current versus dephasing curves.

  • Fig. 2 Effect of environment on photosynthetic transfer efficiency in FMO and PC645.

    Calculated exciton current as a function of dephasing for the FMO (A) and PC-645 (B) complexes. The shaded green area indicates the estimated range of physiological dephasing rates. Insets show a schematic description of the exciton complexes (the full Hamiltonians used are provided in section S6).

  • Fig. 3 Exciton density arrangement in the formation of ENAQT.

    (A) Density configuration (i.e., exciton occupation at different sites) of the FMO complex for three different regimes: quantum limit (blue line, γdeph = 10−4 μs−1), biological condition (yellow line, γdeph = 106 μs−1), and classic limit (green line, γdeph = 1012 μs−1). The transition from the quantum regime toward the classical regime is accompanied by a shift in the density configuration, from a wave function–determined configuration to a uniform gradient between the source and the sink, with a uniform configuration in between (26). To more clearly see this, (B), (C) and (D) present the schematic structure of FMO, where each sphere represents a BChl site, and the color brightness reflects its density.

  • Fig. 4 Effect of environment on photosynthetic transfer efficiency in LH2.

    Average LH2 exciton current as a function of dephasing rate (black line), calculated for ≈900 possible paths. Pink curves show the current of arbitrary chosen realizations (i.e., entry and exit sites) in LH2. Shaded green area marks the natural dephasing rate. Inset: Schematic description of LH2 transfer network (the full Hamiltonian used is provided in section S6).

  • Fig. 5 Current versus dephasing rate for 5000 realizations of FMO-like networks.

    Energies were kept fixed, while hopping matrix elements were picked from a range of ±200 cm−1. ENAQT is obtained for almost the same range for all realizations, indicating the independence of efficiency in the ENAQT regime (and the regime itself) on the structure of the system.

Supplementary Materials

  • Supplementary Materials

    Do photosynthetic complexes use quantum coherence to increase their efficiency? Probably not

    Elinor Zerah Harush and Yonatan Dubi

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    This PDF file includes:

    • Sections S1 to S6
    • Figs. S1 to S3

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