Electron Tunneling Across the Membrane Core

The appearance of electron transfer across membranes via electron exchange reactions seems to result from the necessity of overcoming the difficulties for an ion to pass through the highly hydrophobic region of the boundary which separates the two membrane monolayers. This boundary corresponds to the maximum of potential energy profile for the ion motion in the membrane (see Fig. 6b). If the ion could reach the top of the potential barrier, it would be able to diffuse with 

Unfortunately, the experimental data concerning the distances at which electron exchange reactions in the membranes take place are very scarce. Tsuchida et al. have shown [147], that even when the photoexcited Zn porphyrin embedded in the membrane cannot approach the membrane // water interface closer than 12 A, the electron transfer is still possible to MV2+ located in the water phase outside the membrane. However, when the distance of the closest approach of these reactants is increased up to 17 A, the electron transfer is totally stopped. Examples of electron transfer proceeding presumably via electron tunneling across molecular layers about 20 A thick, which separate electron donor and acceptor molecules, can be found in papers by Mobius [230, 231] and Kuhn [232, 233]. Note, that in 

According to the data from Smalley [234] the quenching of the fluorescence of magnesium octaethylporphyrin proceeds via electron tunneling in statistical pairs. The distances between the reactants and the characteristic time of electron transfer in the pairs are 10-18 A and 5 ns, respectively. 

Guarr et al. [235] reported the long distance (up to 12 A from edge to edge) electron tunneling from the excited polypyridine Ru complexes (with lifetime about 2 ps) to aromatic amines fixed in a rigid polymer. 

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