Long distance electron transfer

Abstract

Quantum mechanical models to treat long distance electron transfer are being developed. The model is based on the theory of R.A. Marcus. Our contribution is in the calculation of the electron coupling factor K . Estimations of the latter number, as well as the bond and solvent relaxation energies, A, and A,, respectively, are necessary to be able to calculate the rate constant for a reaction of the conductivity in an electric field. K may be approximately calculated from orbital energy differences at avoided crossings between orbitals localized in different parts of the system. A novel spectroscopic NDO method is suggested in which one may include any atom of the periodic table. Another problem discussed is the inclusion of electronic relaxation effects of the solvent or protein in the calculation. Applications are made to systems where metal ions are connected by organic bridges of different kinds such as dipyridine with coplanar and perpendicular pyridyl groups. As expected the electronic factor depends strongly on the conformation of the bridge. A strong conformational dependence is also obtained for a saturated bridge of the type NH, (CH,), -NH,. In another study we use an a helix as a bridge between two metal ions. If one glycine in this a-helix is substituted by phenylalanine the electronic factor increases by factors of 1.5-10. It is suggested, however, that larger enhancement factors are possible if an aromatic group is positioned in a favorable way. The CNDO/S method is used to study the charge separation process in a bichromophoric molecule and in the reaction center (RC) of Rhodopseudomom viridis. In those cases where the electronic coupling is large enough for the charge transfer states to be seen in the spectrum, the calculated results agree well with the experimental ones, but suggest a novel assignment. The CNDO/S results verify that electron transfer is possible through saturated spacers. In the special pair of RC the S, state is calculated at approximately the correct position. Like the ground state, it has a delocalized character.

The CuA center of cytochrome-c oxidase: electronic structure and spectra of models compared to the properties of CuA domains.

“…The parameterization scheme has been generalized, so that the method may now be applied to molecules containing not only H, C, N, and 0 atoms but any nonrelativistic atom (24). The We have included 100 CI configurations; a calculation using 300 configurations did not change the energy levels to any significant extent.…”

“…For this type of system, there is a large mixing between the atomic orbitals from metal ion and ligand atoms and also a strong mixing between CI configurations. Screened electron repulsion is introduced in our extension (24) in such a way that the method is consistent with local exchange methods. The result is that the CNDO/S method, as well as the multiple scattering Xa or other local exchange methods, predicts ionization energies and electron affinities well.…”

The effect of image force on the efficiency of electron tunneling through macromolecular structures

“…For proteins, apart from the polypeptide chain backbone, the bridged groups can be provided by the aminoacid residues (for details see Petrov (1984); Larsson et al (1988); Sneddon and Brooks (1988); Kharkyanen et al (1978)). If these residues are on the boundary of media with different polarization properties then the formation of the tunneling channel becomes especially advantageous.…”

“…There are numerous papers on the theory of ET where the bridge is the focus of attention (see, e.g., McConnel 1961;De Vault 1980;Petrov 1984;Beratan et al 1987;Da Gama 1990;Goldman 1991;Closs and Miller 1988;Dreyer 1984;Marcus and Sutin 1985;Larsson 1982Larsson , 1983Larsson et al 1988;Finckh et al 1988;Lin 1989;Bertrand 1987;Sneddon and Brooks 1988;Joachim 1987;Heitele et al 1989;Reimers and Hush 1990;Kharkyanen et al 1978;Davydov and Gaididei 1985;Ostapenko and Petrov 1989). Without going into a detailed analysis one may note that if the electron-vibrational states of the quantum system with a finite number of degrees of freedom are in equilibrium with the thermal bath (the environment with an infinite number of degress of freedom), then the ET rate from D to A is given by:…”

Electron self-exchange in azurin: calculation of the superexchange electron tunneling rate.

“…Procedures for calculation of electronic coupling using many-electronic wave functions have been developed for ET systems (19)(20)(21)(22)(23)(24)(25)(26)(27) function corresponds to having the electron on the donor (acceptor) site. These two electronic wave functions were obtained by taking advantage of the possibility of obtaining broken symmetry solutions, in which the odd electron is localized on either the "left" or "right" half of the system.…”