Electron tunneling through iron and copper proteins

Iron and copper redox centers facilitate the transfer of electrons through proteins that are part of the respiratory and photosynthetic machinery of cells. Much work has been done with the goal of understanding the factors that control electron flow through these proteins.1-18 The results of many of the key experiments have been interpreted in terms of semiclassical theory. The rate of electron transfer (ET) from a donor (D) to an acceptor (A) held at fixed distance and orientation depends on of temperature (T), reaction driving force (-?Go) a nuclear reorganization parameter (?), and an electronic coupling matrix element (HAB).4,12 The reorganization parameter reflects the changes in structure and solvation that result when an electron moves from D to A. A balance between nuclear reorganization and reaction driving force determines both the transition-state configuration and the height of the barrier associated with the ET process. At the optimum driving force (-?Go = ?), the reaction is activationless, and the rate (kET o) is limited only by the strength of the D/A electronic coupling. When D and A are in van der Waals contact, the coupling strength is usually so large that the ET reaction is adiabatic, that is, it occurs every time the transition-state configuration is formed. In this adiabatic limit, the ET rate is independent of HAB and the prefactor depends only on the frequency of motion along the reaction coordinate. An ET reaction is nonadiabatic when the D/A interaction is weak and the transition state must be reached many times before an electron is transferred. The electronic coupling determines the frequency of crossing from reactants (D + A) to products (D+ + A-) in the region of the transition state. The singular feature of electron transfer is that reactions can proceed at very high rates when D and A are separated by long distances. The electron tunnels through a potential barrier between D and A; for a square barrier, HAB displays an exponential dependence on the distance (R) between the reactants.19 The medium between redox centers potentially can control longrange ET. Owing to a 3.5-Å-1 distance-decay constant (ß), the time required for electron exchange between hydrated ferrous and ferric ions is estimated to be 5×1016 years if the complexes are separated by 20 Å in a vacuum.14 Superexchange coupling via hole and electron states of the intervening medium enhances the D/A electronic interaction and produces a more gradual decrease in rate with distance. Fill the void between hydrated ferrous and ferric ions with water (ß = 1.59 Å-1)20 and the time constant for 20-Å electron exchange decreases dramatically (400 years), but the reaction is still far too slow to support biological activity. If the distance decay factor for ET across a polypeptide is comparable to that found for electron tunneling across hydrocarbon bridges (ß = 0.8-1.0 Å-1),14 then the time for a 20 Åelectron exchange between complexed ferrous and ferric ions in thehydrophobic interior of a protein could be in the millisecond to microsecond range.