Coupled electron-proton transfer

In many biochemical processes electron transfer and proton transfer are important and have featured in many of my posts. However, another process that is important and fascinating is coupled electron proton transfer. This is the process discussed in a previous post about the enzyme soybean lipoxygenase. Sometimes this can be viewed as a hydrogen atom transfer, but in some cases the electron and proton start or end at different sites on the donor or acceptor molecule. Describing this process (even for the H-atom transfer case) theoretically has proven to be a challenge which has recently attracted significant attention. A recent review is by Sharon Hammes-Schiffer. Basic questions that arise include:

• What is the reaction co-ordinate? Is it the proton (or H-atom) position? Or the solvent (or heavy atoms) configuration? Or both?
• What is the role of proton tunneling?
• Are the dynamics of the electron and the proton both adiabatic or non-adiabatic? 
• Under what conditions is the electron and proton transfer concerted and when is it sequential?

Today I had a nice discussion with James Mayer who has worked extensively on this problem. He has found that a large amount of data for a chemically diverse range of systems can be rationalised within the framework of Marcus-Hush electron transfer theory. [A good review of his work is here].

[The figure above is taken from another review]. The figure below shows some possible mechanisms for the coupled proton-electron transfer between tyrosine and tryptophan, which is important in various biochemical processes. It is taken from here.