Proton hopping - a.k.a. the Grotthuss Mechanism has been said to be “the fastest known chemical reaction”. The effects of which were first noted by Theodor Grotthuss in 1805, who was investigating the decomposition of water under electrolysis.
In the process, an 'excess' proton (or proton deficit) 'tunnels' through the hydrogen bond network of water molecules (in water or other hydrogen-bonded liquids) through the formation and simultaneous breaking of covalent bonds involving neighbouring molecules. On a local scale, this can occur in less than 2 picoseconds.
It has not yet been accurately explained.
“Although well studied for over 200 years, excess proton solvation and transport remains to this day mysterious, surprising, and perhaps even misunderstood.”
There are currently two hypotheses :
1) Eigen to Zundel to Eigen (E–Z–E), on the basis of experimental NMR data,
2) Zundel to Zundel (Z–Z), on the basis of molecular dynamics simulation.
Note: The so-called 'Eigen' cation is H9O4+ and the 'Zundel' ion is H5O2+
It's also likely that some quantum effects are operating :
“Despite its widespread importance, the quantum character of proton transfer has not been satisfactorily elucidated.”
Proton hopping can occur in any solution or structure where the water content exceeds 20%. Therefore it's probably in operation in the majority of biological systems.
Example : It has recently been suggested that proton hopping may be an important mechanism for the rapid operation of electro-chemical nerve impulses. Proton Hopping as the Nerve Conduction Message Curr Comput Aided Drug Des. 2016;12(4):255-258. .
Also see: Water clusters