1. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
2. Department of Chemical and Biomolecular Engineering, University of Melbourne, Parkville, Victoria 3010, Australia
3. Department of Structural and Molecular Biology, University College London, Darwin Building, Gower Street, London WC1E 6BT, U.K.
4. Department of Chemistry, Western Washington University, Bellingham, Washington 98225-9150, United States