Hole Hopping Across a Protein–Protein Interface

We have investigated photoinduced hole hopping in a Pseudomonas aeruginosa azurin mutant Re126WWCuI, where two adjacent tryptophan residues (W124 and W122) are inserted between the CuI center and a Re photosensitizer coordinated to a H126 imidazole (Re = ReI(H126)(CO)3(dmp)+, dmp = 4,7-dimethyl-1,10-phenanthroline). Optical excitation of this mutant in aqueous media (≤40 μM) triggers 70 ns electron transport over 23 A, yielding a long-lived (120 μs) ReI(H126)(CO)3(dmp•–)WWCuII product. The Re126FWCuI mutant (F124, W122) is not redox-active under these conditions. Upon increasing the concentration to 0.2–2 mM, {Re126WWCuI}2 and {Re126FWCuI}2 are formed with the dmp ligand of the Re photooxidant of one molecule in close contact (3.8 A) with the W122′ indole on the neighboring chain. In addition, {Re126WWCuI}2 contains an interfacial tryptophan quadruplex of four indoles (3.3–3.7 A apart). In both mutants, dimerization opens an intermolecular W122′ → //*Re ET channel (// denotes the protein interface, *Re is...