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Science 27 June 2008:
Vol. 320. no. 5884, pp. 1760 - 1762
DOI: 10.1126/science.1158241
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Reports

Tryptophan-Accelerated Electron Flow Through Proteins

Crystal Shih,1 Anna Katrine Museth,1 Malin Abrahamsson,1 Ana Maria Blanco-Rodriguez,2 Angel J. Di Bilio,1 Jawahar Sudhamsu,4 Brian R. Crane,4* Kate L. Ronayne,3 Mike Towrie,3 Antonín Vlcek, Jr.,2* John H. Richards,1* Jay R. Winkler,1* Harry B. Gray1*

Energy flow in biological structures often requires submillisecond charge transport over long molecular distances. Kinetics modeling suggests that charge-transfer rates can be greatly enhanced by multistep electron tunneling in which redox-active amino acid side chains act as intermediate donors or acceptors. We report transient optical and infrared spectroscopic experiments that quantify the extent to which an intervening tryptophan residue can facilitate electron transfer between distant metal redox centers in a mutant Pseudomonas aeruginosa azurin. CuI oxidation by a photoexcited ReI-diimine at position 124 on a histidine(124)-glycine(123)-tryptophan(122)-methionine(121) β strand occurs in a few nanoseconds, fully two orders of magnitude faster than documented for single-step electron tunneling at a 19 angstrom donor-acceptor distance.

1 Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.
2 School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK.
3 Central Laser Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK.
4 Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA.

* To whom correspondence should be addressed. E-mail: bc69{at}cornell.edu (B.R.C.); a.vlcek{at}qmul.ac.uk (A.V); jhr{at}caltech.edu (J.H.R.); winklerj{at}caltech.edu (J.R.W.); hbgray{at}caltech.edu (H.B.G.)

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Science. ISSN 0036-8075 (print), 1095-9203 (online)