Abstract: In photosynthesis, light is harvested by antenna proteins that can transfer the electronic excitation energy to a reaction center with near unit quantum efficiency.  The remarkable efficiency of these energy transfer processes has been a mystery for over 50 years, and recent two-dimensional (2D) spectroscopy experiments have found signatures attributed to electronic coherence that persist for picoseconds.  In theoretical treatments of electronic energy transfer, an adiabatic (Born-Oppenheimer fast-electron/slow-vibration approximation) framework has been used (even for non-adiabatic processes).   We have recently shown that the delocalized, anti-correlated component of pigment vibrations can drive non-adiabatic electronic energy transfer in photosynthetic light harvesting antennas. This mechanism drives the excitation of anti-correlated vibrational wavepackets on the ground state of the antenna that generate all of the reported 2D signatures. Additional properties of these vibrational wavepackets reveal the design principle for the antenna.  Such photosynthetic energy transfer steps resemble molecular internal conversion through a nested intermolecular funnel.  Photosynthetic pigments have characteristics that make this intermolecular funnel unavoidable, so that the adiabatic framework breaks down everywhere in the vibrational coordinate space accessible to the antenna.  There are indications that this mechanism may be operative in a variety of antenna proteins using 5 different photosynthetic pigments.

Reference:  PNAS vol. 110, p.1203-1208 (2013)