1. Department of Chemistry, Haverford College, Haverford, Pennsylvania 19041, United States
2. Department of Biochemistry and Structural Biology, Centre for Molecular Protein Science, Lund University, Lund SE-22100, Sweden
3. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
4. Paulson School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts 02138, United States
5. Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, U.K.