Abstract
Electron transfer (ET) between neutral and cationic tryptophan residues in the azurin construct [Re-I(H126)(CO)(3)(dmp)](W124)(W122)Cu-I (dmp = 4,7-Me-2-1,10-phenanthroline) was investigated by Born-Oppenheimer quantum-mechanics/molecular mechanics/molecular dynamics (QM/MM/MD) simulations. We focused on W124(center dot+) W122 ET equilibrium, as well as back ET Re-I(CO)(3)(dmp(center dot-)) -> W124(center dot+) that restores *Re-II(CO)(3)(dmp(center dot-)).
Strong electronic coupling between the two indoles (>= 40 meV in the crossing region) makes the productive W124(center dot+) <- W122 ET adiabatic. Energies of the two redox states are driven to degeneracy by fluctuations of the electrostatic potential at the two indoles, mainly caused by water solvation, with contributions from the protein dynamics in the W122 vicinity.
ET probability depends on the orientation of Re(CO)(3)(dmp) relative to W124 and its rotation diminishes the hopping yield. Comparison with hole hopping in natural systems reveals structural and dynamics factors that are important for designing efficient hole-hopping processes.