  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
