1. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge
CB2 1EW, U.K.
2. Institut
de Biologie et Technologies
de Saclay, URA 2096 CNRS, CEA Saclay, 91191
Gif-surYvette, France
3. Division
of Molecular Biosciences, Imperial College London, London SW7 2AZ, U.K.