Through-space tunneling

Average square barrier models with [3=1.4 A (equation (C3.2.611 would predict the His 72 rate to be 1000 times faster. This equivalency of rates despite the great difference in distances is understood because the strongest pathways in the His 72 derivative contain a through-space tunnelling gap. 

It is beyond the scope of this text to continue the discussion of Marcus theory. Qualitatively, the student should understand that electrons must find a path through the protein from the donor species to the acceptor. This may take place through bonds as outlined above or through electron tunneling events in which electrons travel through space between orbitals of the donor species to the acceptor species. Chapter 6 of reference 13 presents a clear explanation for further reading. 

Fig. 40. Electron transfer from redox systems within the electrolyte across a semiconducting passive layer (n-type) (1) direct tunnelling to CB (2) tunnelling through space charge layer (3) transfer via surface states (4) hopping mechanism via interband states (5) transfer via sub-band and (6) transfer via valence band.

In PCMH, two equally efficient pathways for electron transfer from the flavin N5 to the heme iron were identified in GREENPATH calculations. One path follows the tyrosyl covalent link to FAD at the C8-methyl position, whereby electrons can travel from C8M via the Tyr 384 phenolic ether bond, tunnel through the tyrosine ring atoms and make a through-space jump across the subunit interface to the carbonyl oxygen of Ala 49 in the cytochrome (Figure 16). From there the electrons follow backbone... 

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