In biological systems, electron flow often occurs very quickly between distant redox centers in electron-transfer proteins. Researchers have proposed that charge transfer in such proteins is accelerated by redox-active amino acid residues that act as donors or acceptors to relay electrons between the redox centers. But experimental evidence for this proposal has been indirect and inconclusive. A collaborative team based at Cornell University, California Institute of Technology, and the University of London has now obtained the best direct experimental evidence for the proposal so far by assessing the influence of an intervening tryptophan residue on the rate of electron transfer between redox centers in a semisynthetic model protein (Science 2008, 320, 1760). The team found that electron transfer between distant redox centers in an azurin protein mutant occurs about 300 times faster when the tryptophan residue is present than when it is absent. Researchers believe such expedited electron transfers can potentially be exploited in the design of new energy-yielding devices, among other applications.