1. Department
of Chemistry, University College London, Christopher Ingold Laboratories,
20 Gordon Street, London, WC1H 0AJ, U.K.
2. Howard Hughes
Medical Institute
and Department of Molecular Biology and Center for Computational and
Integrative Biology, Massachusetts General Hospital, 185 Cambridge Street, Boston, Massachusetts 02114, United States
3. Harvard X-ray Crystallography Centre, 12 Oxford Street, Cambridge, Massachusetts 02138,
United States