Vibrational Beatings Conceal Evidence of Electronic Coherence in the FMO Light-Harvesting Complex

In biological light harvesting, solar energy is captured by photosynthetic antennae for subsequent storage into chemical bonds. The remarkable efficiency reached in transferring the energy between the collection and storage events recently has been attributed to long-lived electronic coherence present in such antennae systems. We present numerical simulations indicating that the spectroscopic transients that supported this hypothesis are not induced by electronic coherence but instead are caused by vibrational (nuclear) motion in the electronic ground state potential. Besides emphasizing the significance of such nuclear modes, our findings stimulate a reconsideration of the role of electronic coherence in promoting energy transfer in natural photosynthesis. Furthermore, they require us to rethink how energy transfer efficiency is reflected in spectral signals.