Proton hopping - a.k.a. the Grotthuss Mechanism has been said to be â€śthe fastest known chemical reactionâ€ť. 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 its opposite, a proton-deficit) 'tunnels' through the hydrogen bond network of water molecules (or other hydrogen-bonded liquids). Leading to the formation, and simultaneous breaking, of covalent bonds with 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.â€ś
Source : J Phys Chem B. 111(17): 4300â€“4314.
There are currently two hypotheses :
1) Eigen to Zundel to Eigen (Eâ€“Zâ€“E), on the basis of experimental NMR data,
2) Zundel to Zundel (Zâ€“Z), on the basis of molecular dynamics simulation.
Editor's clarification : The so-called 'Eigen' cation is H9O4+ and the 'Zundel' ion is H5O2+
It's also likely that some quantum physics effects are operating :
Despite its widespread importance, the quantum character of proton transfer has not been satisfactorily elucidated.â€ť
Source : Science, Vol. 275, Issue 5301, pp. 817-820
Note: 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 Current Computer Aided Drug Design, 2016;12(4): 255-258.
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