INOR 909 |
| A fundamental process in photosynthesis, respiration and future design of bio-electronic devices is the long-range electron transfer (LRET) between two cofactors in a protein matrix. While experiments have already given valuable insight into LRET theoretical calculations are still at an early stage of development. The reason is that computation of rate determining free energies for LRET is an enormously challenging task requiring explicit electronic structure calculation and sampling of the protein fluctuations over nanoseconds. This is clearly beyond the time scale that can be accessed with straightforward QM/MM methods in the near future. Combining QM/MM with classical MD we present a computational scheme that allows to calculate the relevant thermodynamic and kinetic parameter for diabatic LRET at the QM/MM level but with the statistical accuracy of classical MD. By formulating LRET as a redox process the difficulty of computing reliable excited states is avoided as well as the problem of charge separation in uncorrected density functionals. Our approach is illustrated by the computation of the reorganization free energy for electron self exchange between two Ru-porphyrin cofactors bound to a four-helix bundle protein. The number we obtain, 1.2 eV, is in excellent agreement with experimental estimates for similar systems. We find that 2/3 of the free energy barrier for LRET is due to the solvent and only 1/3 due to the protein while the contribution of the porphyrin cofactors is vanishingly small. In the presentation we will discuss the range of problems that can be addressed with our approach as well as its limitations.
|
|
Computational Chemistry
8:30 AM-12:10 PM, Wednesday, 13 September 2006 Moscone Center -- Room 305, Oral
Division of Inorganic Chemistry |