Proton-coupled electron transfer

Important examples include water oxidation in photosynthesis, nitrogen fixation, oxygen reduction reaction, and the function of hydrogenases.

Although it is relatively simple to demonstrate that the electron and proton begin and end in different orbitals, it is more difficult to prove that they do not move sequentially.

The main evidence that PCET exists is that a number of reactions occur faster than expected for the sequential pathways.

[2] SOD2 uses cyclic proton-coupled electron transfer reactions to convert superoxide (O2•-) into either oxygen (O2) or hydrogen peroxide (H2O2), depending on the oxidation state of the manganese metal and the protonation status of the active site.

The amide anion is stabilized by short-strong hydrogen bonds (SSHBs) with the Mn-bound solvent and the nearby Trp123 residue.

"Square scheme" used to discuss PCET (diagonal) vs discrete electron transfer and proton transfers.
The PCETs of SOD2 use PTs between Q143 and a Mn-bound solvent molecule. Deprotonation of Q143 is stabilized with SSHBs shown as yellow-hashed lines. ETs occur with the substrate, superoxide, not shown in the figure.