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Organization of the Bacterial Light-Harvesting Apparatus Rationalized by Exciton Transport Optimization

Photosynthesis, the process by which energy from sunlight drives cellular metabolism, relies on a unique organization of light-harvesting and reaction center complexes. Recently, the organization of light-harvesting LH2 complexes and dimeric reaction center-light harvesting I-PufX (RC-LH1-PufX) core complexes in membranes of purple non-sulfur bacteria was revealed by atomic force microscopy (AFM)1. Here, we report that the structure of LH2 and its organization within the membrane can be largely rationalized by a simple physical model that relies primarily on exciton transfer optimization. The process through which the light-harvesting complexes transfer excitation energy has been recognized to incorporate both coherent and incoherent processes mediated by the surrounding protein environment. Using the Haken-Strobl model, we show that the organization of the complexes in the membrane can be almost entirely explained by simple electrostatic considerations and that quantum effects act primarily to enforce robustness with respect to spatial disorder between complexes. The implications of such an arrangement are discussed in the context of biomimetic photosynthetic analogs capable of transferring energy efficiently across tens to hundreds of nanometers