Friday, June 3, 2011

What do you conclude from this graph?

The graph below (taken from a paper Environment assisted quantum transport by Patrick Rebentrost, Masoud Mohseni, Ivan Kassal, Seth Lloyd and Alán Aspuru-Guzik) shows the calculated efficiency of transport (blue curve) of an exciton through at network in the presence of a dissipative and dephasing environment. The horizontal scale is the magnitude of the dephasing rate. n.b. it is a logarithmic scale and spans 10 orders of magnitude. The vertical line is the estimated dephasing rate at room temperature. 
To me the curve shows that the efficiency is quite insensitive to the environment, i.e. it is greater than 80 per cent for dephasing rates varying by eight orders of magnitude! In terms of biological functionality I would say that the environment matters little to the efficiency, except when the system couples very strongly to the environment.

However, other people interpret this graph in a very different way. In a 2010 paper in PNAS,  Long-lived quantum coherence in photosynthetic complexes at physiological temperature, the authors (led by Greg Engel from University of Chicago) state
theoretical studies incorporating both incoherent and coherent transfer as well as thermal dephasing predict that environmentally assisted quantum transfer efficiency peaks near physiological temperature
we ... observe quantum coherence lasting beyond 300 fs, showing that evolution has had the opportunity to exploit the theorized environmentally assisted quantum transport (EnAQT) mechanism for biological function. 
What do you think?


  1. For an updated ENAQT figure please visit:


  2. The revised figure. Is equally ineffective at demonstrating optimization under natural selection pressure. To demonstrate that this is happening, one would need to compare the genetic frequency of distinct mutations in at least two separate species which experience different selection pressure. That is really the final story.

    YOU CANNOT INFER THE EXISTENCE OF SELECTION PRESSURE BY EXAMINING THE BEHAVIOR OF ONE PROTIEN FROM ONE SPECIES!!! It does not matter how fast it is! If every FMO comples were equally (and uniformly) fast and/or efficient this would still not demonstrate natural selection pressure! What is need is a DIFFERENCE in the DISTRIBUTION of rates in species known to experience DIFFERENT selection pressure!

    This is, to my eye, an example of a bunch of physicists trying to exploit biology's funding pool... without any understanding of the underlying biology!


  3. In retrospect, I can hardly blame any physicist for seeking funding wherever possible to better serve the cause to which we are all so devoted... but...

    The bold claim being made in biomolecular physics is overshadowed (in my mind) by an even bolder suggestion about POPULATION BIOLOGY. This question could be stated and pursued with quantitative methods but I haven't seen much serious discussion of this. The logic seems to be:

    1. We observe a signal that we believe to be a signature of quantum transport. It is longer than we would normally expect, by about an order of magnitude.
    2. We know that the transport is essential to an important process. We presume that the excitation transport rate is important to: A) the rate of water splitting in this protein variant (this would be true if it were rate-limiting) B) the overall photosynthetic efficiency and C) the survivability of the organism.
    3. Nature implements a genetic algorithm (GA). We believe (reasonably) that plays the accepted role in speciation and evolution; we reason that the survivability of the organism is broadly represented with significant weight in the objective function.
    4. Therefore, quantum coherence lifetimes are optimized by evolution!

    Statement 4 either does not follow logically, or it is a tautology! (Genetic algorithms do a global search, so coherence lifetime-determining mutations may be in the domain of the evolutionary objective function; the suggestion is that the coherence lifetime also has significant weight.)