Grotthus chain mechanism

They are often about half as fast as related reactions of hydrogen ion this is explicable as a consequence of the abnormal mobility of the hydrogen ion, due to the Grotthus chain mechanism. 

Figure 2.18 Schematic of the Grotthus mechanism of long-range proton transfer in water molecules. In this, hydrogen bonds and covalent bonds interchange, releasing a proton from one end of the chain as a new proton is introduced at the start of the chain...

The energy released by electron transfer can be used in the transport of protons through the membrane. One of the proton conduction mechanisms in proteins is through a chain of hydrogen bonds in the protein, i.e. a Grotthus mechanism (Section 2.9), similar to the mechanism of proton movement in ice. Protons are injected and removed by the various oxidation/reduction reactions which occur in the cell there is no excess of protons or electrons in the final balance, and the reaction cycle is self-sustaining. 

The Grotthus-type diffusion mechanism was used to explain the proton diffusion process in imidazole chains [160]. The protonic defects cause local rather than long-range disorder by forming (... Him - (HIniH) - ImH. ..) and (ImH) configurations. At 117 °C, the proton-transfer step is fast, with a time scale ofO.lps the reorientation step is rate-determining and the corresponding time scale is approximately 30 ps. 

The ease of occurrance of proton transfer reactions is also demonstrated by the apparently high mobilities of the solvent ions. Due to a chain-like mechanism, the essentials of which were first described by Grotthus in 1806, the proton is attached to an associated solvent unit, which in turn may transfer another proton to another bulk of solvent molecules. The high mobilities of the solvent anions... 

Fig. 8.16 A simplified version of the Grotthus mechanism of proton conduction through water. The proton entering the chain at the top is not the same as the proton leaving the chain at the bottom.

---