1. Centre for Crystallographic Studies, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark, European Synchrotron Radiation Facility, B.P. 220, 38043 Grenoble, France, and Department of Biological Chemistry, Institute of Molecular Biology, University of Copenhagen, Sølvgade 83H, DK-1307 Copenhagen K, Denmark