1. Department of Chemistry, National University of Ireland, Galway, Ireland, European Molecular Biology Laboratory, c/o DESY, Notkestrasse 85, D-22603 Hamburg, Germany, York Structural Biology Laboratory, Department of Chemistry, University of York, Heslington YO10 5DD, U.K., Department of Biochemistry, University College Dublin, Dublin 4, Ireland, and Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada