Does electronic coherence in pigment-proteins facilitate energy transfer in photosynthesis?

Recent two-dimensional electronic spectroscopic experiments revealed that electronic energy transfer in photosynthetic light harvesting involves long-lived quantum coherence among electronic excitations of pigments. These findings have led to the suggestion that quantum coherence might play a role in achieving the remarkable quantum efficiency of photosynthetic light harvesting. Further, this speculation has led to much effort being devoted to elucidation of the quantum mechanisms of the photosynthetic excitation energy transfer.

The observation of long-lived electronic quantum coherence in a light harvesting protein (the Fenna-Matthews-Olson (FMO) complex) by Fleming and coworkers (Brixner 2005 Nature, Engel 2007 Nature) stimulated a huge burst of activity among theorists and experimentalists. Much of the interest arose because the finding of electronic quantum coherence is a “warm, wet, and noisy” biological system was considered very surprising. The initial experiments were carried out at 77 K, but more recent studies by two groups have detected coherence lasting at least 300 fs at physiological temperatures (Engel 2010 PNAS, Scholes 2010 Nature). In addition extensive quantum coherence was observed in the most important light harvesting complex on Earth, the light harvesting complex II, or LHCII (Calhoun 2009 JPCB).

However, these 2D measurements have been over an ensemble of FMO complexes with a static, in-homogenous distribution of energies. The result of experimental averaging over this ensemble is a decay in quantum beating signal due loss of phase coherence between FMO complexes. However, it remains possible that an individual complex of FMO may retain electronic coherence for a longer time than that suggested by the ensemble measurement.

To better determine the microscopic dynamics of electronic states in FMO, we propose an experiment that probes electronic coherence in individual complexes of FMO. Using coherent spectroscopy on an individual FMO complex, we intend to bypass the averaging over an ensemble of FMO complexes and probe electronic coherence within a single FMO complex. Because fluctuation of energy levels causes a decay of electronic coherence, the observed life time of electronic coherence will determine the intensity of energy fluctuations in individual complexes of FMO.

Helpful Background Reading:

1. Two-dimensional spectroscopy of electronic couplings in photosynthesis, T. Brixner, J. Stenger, H. M. Vaswani, M. Cho, R. E. Blankenship and G. R. Fleming, Nature, 434, 625-628 (2005).

2. Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems. G. S. Engel, T. R. Calhoun, E. L. Read, T.-K. Ahn, T. Mancal, Y.-C. Cheng, R. E. Blankenship, and G. R. Fleming, Nature, 44, 782–786 (2007).

3. Quantum Coherence Enabled Determination of the Energy Landscape in Light-Harvesting Complex II, T. R. Calhoun, N. S. Ginsberg, G. S. Schlau-Cohen, Y.-C. Cheng, M. Ballottari, R. Bassi, G. R. Fleming, J. Phys. Chem. B, 113, 16291-16295 (2009).

4. Quantum coherence and its interplay with protein environments in photosynthetic electronic energy transfer. A. Ishizaki, T. R. Calhoun, G. S. Schlau-Cohen, and G .R. Fleming, Phys. Chem. Chem. Phys., 12, 7319–7337 (2010).

5. Quantum coherence in photosynthetic complexes, T. R. Calhoun, G. R. Fleming, Phys. Status B, 248, 833-838 (2011).