1. Department of Biochemistry, University of Cambridge, 80 Tennis Court Road, Cambridge CB2 1GA, U.K., Molecular Biophysics Group, CCLRC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, U.K., Astbury Centre for Structural Molecular Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, U.K., and Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.