Grotthus mechanisms

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...

Thus, in regard to this important part of electrophysiology, the mechanistic details are still fuzzy. What of the conductivity of the axon material Is it ionic Electronic Can there be some kind of Grotthus mechanism such as that illustrated in Figs. 4.121 and 4.123 in Vol. 1 ... 

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 -> prototropic conduction (see also Grotthus mechanism) in acid solutions can be interpreted in a similar way by assuming a proton exchange mechanism (proton transfer or jumping) [xii]. 

Counted among these superionic electrolytes are the solutions of strong acids in water and other hydrogen-bonded solvents (e.g., glycerol, hydrogen peroxide) in which the proton has anomalously high mobility due to the Grotthus mechanism... 

In order to obtain more insight into the dynamics, molecular dynamics (MD) simulations of similar slit systems have been performed [62-64], Since ab initio MD methods are not applicable, an effort was made to incorporate the Grotthus mechanism into the MD simulations via a simplified empirical valence bond (EVB) model [65],... 

From these data is also possible to calculate the activation energy in pores of given water content from an Arrhenius representation of the calculated diffusion coefficients [63], The results show no indication of a substantial increase in activation energy at water levels as low as A = 4, contrary to the results in [23] and more in line with results from Kreuer (see Fig. 6 and [50]). Rather, the activation energy for proton transfer remains low at all studied water contents. Thus, it appears that in an individual pore the Grotthus mechanism remains rate-determining whereas the surface mechanism does not dominate the transport behavior. Similar results as in slab pores were also found in cylinder pores [63],... 

The protons migrate mainly as lone protons, and jump from lattice site to lattice site by the Grotthus mechanism. The protonic conductivity of zirconates (Ca,Sr, Ba) ZrOj] is approximately one order of magnitude louver than that of the cerates ho vever, their chemical and mechanical stabilities are superior. In contrast to cerates, zirconates barely react vith carbon dioxide gas. The stability of barium cerates can be increased vhen cerium atoms are partially substituted by zirconium atoms. 

Polarisation of the substrate carbonyl group appears to be achieved in yet a third way by mammalian glucose 6-phosphate isomerase, which interconverts glucose and fructose 6-phosphates, The crystal structure of the rabbit enzyme in complex with the reactive intermediate analogue o-arabinohydroxamic acid Ki = 0.2 pM) (in its hydroximic form) reveals a cluster of four water molecules hydrogen bonded to each other, to the counterparts of 01 and 02 of the enediolate intermediate and to an active site arginine. Grotthus mechanisms of proton transfer between the two tautomers of the enediolate probably occur. 

No dedicated acid catalyst which donates a proton to 04 has as yet been found in any pectin or pectate lyase transfer from solvent or from more remote protein acids and bases via a Grotthus mechanism appears to be all that is required. 

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