Tuesday, June 19, 2012

Coupled electron-proton transfer II

I had a nice visit this morning at University of Washington with Jim Mayer who has worked extensively on coupled electron-proton transfer [see this post for an earlier discussion]. Here are a few things I learnt.

CPET is involved in one of the most important processes in biology, whereby we get all of our oxygen! This is the Kok S-state mechanism of Photosystem II: the amino acid tyrosine-Z is oxidised to yield a neutral tyrosyl radical. Specifically, the electron is transferred 14 Angstroms (i.e. a long way!) to a photoexcited chlorophyll radical and the proton is transferred across a hydrogen bond to a nearby histidine residue (e.g. see this 2003 PNAS paper for evidence).

It is important to note that the proton and the electron are spatially separated and "attached" to different atoms. Nevertheless, their motion is concerted, i.e. the transfer is not sequential.

A major question concerns whether this process is adiabatic or non-adiabatic. Uncertainty about the answer is highlighted in a recent issue of Chemical Reviews. One article is by Hammes-Schiffer and Stuchebrukov which advocates a non-adiabatic approach.
A different article by Siegbahn and Blomberg considers DFT based calculations and implicitly assumes an adiabatic approach.

We agreed there is a need for some simple models to describe this fascinating phenomena.

Aside: a recent Science paper from Mayer's group shows that similar chemistry occurs in transition metal oxides which are important in energy research. Titanium dioxide has been the subject of 58,000 papers!

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