Saturday, August 20, 2011

Do photosynthetic proteins protect quantum coherence?

A 2007 paper in Science Coherence Dynamics in Photosynthesis: Protein Protection of Excitonic Coherence by Lee, Cheng, and Fleming has attracted considerable interest, particularly from people enthusiastic about "quantum biology."
However, some recent papers based on molecular dynamics simulations cast doubt on the main claims of that paper.

The conclusion of the paper, Quest for Spatially Correlated Fluctuations in the FMO Light-Harvesting Complex by Carsten Olbrich, Johan Strumpfer, Klaus Schulten, and Ulrich Kleinekathofer
The comparison between present results and the reported experimental findings is difficult. It seems to be clear, though, that site correlations do not play a role at physiological conditions and that the biological function of the FMO complex is not affected by spatial site energy correlations. A similar conclusion has already been drawn for the light-harvesting II complex of Rhodospirillum molischianum in a similar study.


  1. It is worth noting that the Lee, Cheng and Fleming experiment was on a bacteria reaction center, not the FMO or LH II systems studied in these molecular dynamics simulations. I'm not sure why there have not yet been MD simulations of the bacterial reaction center system, but the comparison is not apples to oranges. In my view, the primary significance of that paper was showing quantum beating, not necessarily ascribing it to protein protection, and that claim is not disputed.

    I would not be at all surprised if proteins protect coherence in some systems but not others. There is a very wide diversity of photosynthetic antennas and it may only be possible to engineer correlated fluctuations to protect coherence in certain structures.

    Whether the proteins protect the coherence or whether the coherent beating is what we should expect from simple models (as you have argued previously on this blog), the coherent beating is there. There is now what I consider fairly compelling evidence for electronic quantum beats in a wide variety of photosynthetic complexes, even at room temperature (at least in FMO, PC645, PE545, LH II* and bacterial reaction centers*). Regardless of how you explain the microscopic origins of the quantum beating it is there and could conceivably make a non-negligible contribution to dynamics.

    * The quantum beating at room temperature in LH II and in bacterial reactions center at room temperature are not yet published results out of Greg Engel's group in center.

  2. The statement that proteins "protect" coherence may be deep or trivial. It would seem that the claim that it is deep comes from the assumption that this behavior a result of evolutionary selection, and qualitatively different than would be expected for multiple chromophores in a similar geometry bound in some random heteropolymeric organic glass of similar composition. This question cannot be answered by a few more studies on selected variants - particularly unpublished ones!