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Publication Abstract from PubMed

Re126W122CuI Pseudomonas aeruginosa azurin incorporates three redox sites, ReI(CO)3(4,7-dimethyl-1,10-phenanthroline) covalently bound at H126, the W122 indole side chain, and CuI, which are well separated in the protein fold: Re-W122(indole) = 13.1 A; dmp-W122(indole) = 10.0 A, Re-Cu = 25.6 A. In view of the long intramolecular Re-Cu distance, it is surprising that CuI is oxidized in less than 50 ns after near-UV excitation of the Re chromophore. Back electron transfer (BET) regenerating CuI and ground-state ReI takes much longer (220 ns and 6 us). We show that these ET reactions occur in protein dimers, (Re126W122CuI)2, which are in equilibrium with unreactive monomers. In support of this interpretation, ET yields and kinetics are concentration-dependent and solution mass spectrometry (LILBID-MS) confirms the presence of a broad oligomer distribution with prevalent monomers and dimers; in the crystal structure, two Re126W122CuII molecules are oriented in such a way that the redox cofactors Re(dmp) and W122-indole belonging to different monomers are located at a protein-protein interface (//), where the intermolecular ET-relevant distances (Re-W122(indole) = 6.9 A, dmp-W122(indole) = 3.5 A, and Re-Cu = 14.0 A) are much shorter than intramolecular ones. We propose that forward ET is accelerated by intermolecular electron hopping through a surface tryptophan: *Re//<-W122<-CuI; our kinetics analysis indicates that an equilibrium (K = 0.8-0.9) between *Re and charge-separated Re(dmp*-)(W122*+), which is established in a few ns, stores part of the excitation energy. The second ET step, intramolecular CuI oxidation, CuI->W122*+, occurs in 30 ns. The system is well coupled for forward ET but not for ReI(dmp*-)->CuII BET. Our work on interfacial electron hopping in (Re126W122CuI)2 sheds new light on redox-unit placements required for functional long-range charge separation in protein complexes.

Tryptophan-accelerated electron flow across a protein-protein interface.,Takematsu K, Williamson H, Blanco-Rodriguez AM, Sokolova L, Nikolovski P, Kaiser JT, Towrie M, Clark IP, Vlcek A, Winkler JR, Gray HB J Am Chem Soc. 2013 Sep 13. PMID:24032375^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.