Proton transfer long range

The mechanism of long-range proton transfer processes in solutions is complex becanse seqnential proton hops from initial proton donors to proton acceptors are mediated by water (or solvent) molecules or other groups capable of ionization. 

Intramolecular interaction is a powerful factor that controls molecular architecture, particularly in the case of geometrically flexible molecular systems. The existence and energies of intramolecular classical hydrogen bonds and their role in chemistry and biochemistry are well known. They stabilize molecular conformations, promote short- and long-range proton transfers, participate in the creation of three-dimensional structures of large molecules and play a fundamental role in the phenomenon of molecular recognition. 

Of much significance is the realization of long-lived photo generated tautomeric states and long-range proton transfer (LRpT) processes. The latter could lead to proton transfer charge separated states to be put in parallel with the extensively studied charge separation by photoinduced electron transfer (see Section 8.2.3). A number of systems present photochromism on the basis of photoinduced proton transfer [8.229]. 

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...

A useful example that illustrates the difference between MEP and PMF results is the long-range proton transfer in carbonic anhydrase II (CAII), which is a zinc-enzyme that catalyzes the interconversion of CO2 and HCO [95], The rate-limiting step of the catalytic cycle is a proton transfer between a zinc-bound water/hydroxide and the neutral/protonated His64 residue close to the protein/solvent interface. Since this proton transfer spans at least 8-10 A depending on the orientation of the His64 sidechain ( in vs out , both observed in the x-ray study [96]), the transfer is... 

From the thermodynamics of such dynamical hydrogen bonds , one may actually expect an activation enthalpy of long-range proton diffusion of not more than 0.15 eV, provided that the configuration O—H "0 is linear, for which the proton-transfer barrier vanishes at 0/0 distances of less than 250 pm. However, the mobility of protonic defects in cubic perovskite-type oxides has activation enthalpies on the order of 0.4—0.6 eV. This raises the question as to which interactions control the activation enthalpy of proton transfer. 

Stubbe J, Nocera DG, Yee CS, Chang MCY. Radical initiation in the class I ribonucleotide reductase long-range proton-coupled electron transfer Chem Rev 2003 103 2167-201. 

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