1. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
2. Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
3. Department of Biochemistry and Structural Biology, Lund University, Lund 22100, Sweden
4. Department of Chemistry, Haverford College, Haverford, Pennsylvania 19041, United States
5. Department of Physics and School of Engineering and Applied Science, Harvard University, 29 Oxford Street, Cambridge, Massachusetts 02138, United States