Electron Transport through a Tryptophan Quadruplex in a Dimeric Azurin Construct

A tryptophan quadruplex at a protein-protein interface in a dimeric azurin construct mediates 8-11 ns intramolecular as well as interfacial electron hole transfer (HT) triggered by ultrafast photooxidation by a covalently attached organometallic chromophore (Takematsu et al. J. Phys. Chem. B., 2019, 123, 1578-1591). MM/MD and QM/MM/MD simulations characterized intermediates of through-quadruplex HT (i.e., states with one of the tryptophans oxidized) and assessed the feasibility of individual HT pathways. Simulations demonstrated that the oxidized quadruplex in aqueous solution occurs in four distinct states where the charge is predominantly (≥90%) localized at individual tryptophan indoles. Distributions of indole-indole distances, electronic couplings, as well as electrostatic potentials at indoles indicate kinetic and energetic preferences of interfacial over intramolecular ET. Interfacial indoles are tightly solvated by a chain of quasi-structural water molecules that are shielded from bulk water by protein folds. Solvating water molecules support ET by 0.1-0.2 Å shifts toward positively charged indoles. PDB search revealed that 4-Trp clusters are rather common among naturally occurring oxidoreductases.