Tryptophan to Tryptophan Hole Hopping in an Azurin Construct.

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 ^•+ ← W122 ET, which is the middle step of the photochemical hole-hopping process *Re ^II (CO) ₃ (dmp ^•- ) ← W124 ← W122 ← Cu ^I , where sequential hopping amounts to nearly 10,000-fold acceleration over single-step tunneling ( ACS Cent. Sci . 2019 , 5 , 192-200). In accordance with experiments, UKS-DFT QM/MM/MD simulations identified forward and reverse steps of W124 ^•+ ↔ W122 ET equilibrium, as well as back ET Re ^I (CO) ₃ (dmp ^•- ) → W124 ^•+ that restores *Re ^II (CO) ₃ (dmp ^•- ). Strong electronic coupling between the two indoles (≥40 meV in the crossing region) makes the productive W124 ^•+ ← 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.