Protein bridge electron tunneling

The pruning procediure naturally leaves intact donor and acceptor complexes, and a number amino adds that make up the tunneling bridge which connects Ru and Cu ions. In this particular case, two stretches of the protein backbone provide the connection. The two stretches form "molecular wires along which electron tunnels between donor and acceptor. The two wires were identified because for each one the connection to the redox site is strong on one end and weak on the other the Met residue is more weakly coupled than the Cys residue to Cu ion, and the His residue of the Met wire is more strongly coupled to Ru than the Gin residue of the Cys wire to the Ru complex. The relative importance of these two paths can only be established in a more accurate calculation that can quantitatively correctly... 

Xie Q, Archontis G, Skourtis SS. Protein electron transfer A numerical study of tunneling through fluctuating bridges. Chem. 47. Phys. Lett. 1999 312 237-246. 

The prefactor, P, depends on the electronic coupling of donor and acceptor with the bridging orbitals. A correlation between p (cf. Eq. 3) and Ec (Eq. 4) is easily demonstrated An individual strand of a p-sheet protein defines a linear tunneling pathway along the peptide, spanning a distance (r — ro) of 0.34 nm per residue (three covalent bonds). Thus, inserting a p value of 10 nm in Eq. 3, the decay factor, Ec of Eq. 4 becomes 0.6 per covalent bond. 

The ne f challenge that arises in making predictions of nonadiabatic electron transfer rates is to determine the electronic coupling element, Simple orbital tunnelling analysis predicts that if (a) the donor is delocalized over Ajj orbitals and the acceptor is delocalized over orbitals, (b) the average of the donor and acceptor orbital energies are 2 eV removed from the bridging levels (based upon the electronic absorption properties of proteins),... 

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