Quantum coherence in photosynthesis?

In the latest PNAS Peter Wolynes has a nice Commentary, Some quantum weirdness in physiology on a paper by Ishizaki and Fleming, Theoretical examination of quantum coherence in a photosynthetic system at physiological temperature.

Wolynes rightly cautions about invoking quantum effects to explain biological functions but seems convinced by work from Flemings group.

Here, I just explain what I believe is the essential physics in the theoretical paper since it can lost in all the detail. The key time (energy) scales in the calculation are:

• relaxation time of the environment (50 fsec)
• period of coherent oscillations (150 fsec)
• reorganisation energy of the environment , E_R (35 cm-1 ~ 1 psec)
• thermal energy, kB T (200 cm-1 ~ 150 fsec)

A relatively simple calculation using an independent boson model (see for example this paper) shows that there is initially a Gaussian decay of quantum coherence on the time scale, tau_g


Using the estimate E_R ~ 35 cm-1 gives a time scale of about 300 fsec for the decoherence time comparable to that in the detailed simulations of Ishizaki and Fleming. Decreasing the temperature by a factor of 4 will result in a two-fold increase in the decoherence time.

It should also be stressed that this reorganisation energy is an order of magnitude smaller than that estimated for other chromophores in proteins (see Table 2 in this paper). This will only be possible if the chromophore is well isolated from water in the environment.