Saturday, May 26, 2012

Are reaction speed and efficiency linked?

There is an interesting JACS communication
Backbone Modification of Retinal Induces Protein-like Excited State Dynamics in Solution from a group at Oxford.
Here is some of the abstract:
The drastically different reactivity of the retinal chromophore in solution compared to the protein environment is poorly understood. Here, we show that the addition of a methyl group to the C=C backbone of all-trans retinal protonated Schiff base accelerates the electronic decay in solution making it comparable to the proton pump bacteriorhodopsin.
Contrary to the notion that reaction speed and efficiency are linked, we observe a concomitant 50% reduction in the isomerization yield.
The results are of particular interest because most previous attempts to modify the chromophore and/or solvent have not led to much change in the photo-isomerisation rate.

A point I am not clear on, concerns the significance of the statement:
  "Contrary to the notion that reaction speed and efficiency are linked, we     observe a concomitant 50% reduction in the isomerization yield."

The authors point out that "a common feature of highly efficient light-induced biological processes" is "the correlation between ultra-fast dynamics and high reaction efficiency."
One some level this is not surprising: if one process is much faster than all the other competing processes then that process will have a high quantum yield. However, if we change a variable and increase the speed of a process we should not necessarily expect the quantum yield to increase dramatically since the speed of other competing processes may also increase.
Or am I missing something?

I thank Seth Olsen for bringing this paper to my attention.


  1. The problem may be that the excited state singlet lifetime does not reflect the population branching. Assuming that the photoreaction can be modelled as passage through a conical intersection, it seems that the branching will not be determined by dynamics on the excited state alone.

    Also, if the excited state lifetime is measured by e.g. transient fluorescence decay, this won't probe even the excited state dynamics if the transition is dark in the neighbourhood of the intersection seam.

  2. Actually, there is the "reactivity-selectivity" principle, which suggests that reaction speed and efficiency should work in opposite directions. Viewed from this perspective, the fast and efficient work of biological chromophores are pretty unusual.