Tryptophan-accelerated electron flow across a protein-protein interface.

We report a new metallolabeled blue copper protein, Re126W122CuI Pseudomonas aeruginosa azurin, which has three redox sites at well-defined distances in the protein fold: ReI(CO)3(4,7-dimethyl-1,10-phenanthroline) covalently bound at H126, a Cu center, and an indole side chain W122 situated between the Re and Cu sites (Re-W122(indole) = 13.1 A, dmp-W122(indole) = 10.0 A, Re-Cu = 25.6 A). Near-UV excitation of the Re chromophore leads to prompt CuI oxidation (<50 ns), followed by slow back ET to regenerate CuI and ground-state ReI with biexponential kinetics, 220 ns and 6 μs. From spectroscopic measurements of kinetics and relative ET yields at different concentrations, it is likely that the photoinduced ET reactions occur in protein dimers, (Re126W122CuI)2 and that the forward ET is accelerated by intermolecular electron hopping through the interfacial tryptophan: *Re//←W122←CuI, where // denotes a protein–protein interface. Solution mass spectrometry confirms a broad oligomer distribution with prevalent ...