1. Laboratory of Macromolecular Structural Chemistry, Catholic University of Leuven, Celestijnenlaan 200F, B3001 Heverlee, Belgium, DSM Research, Postbus 18, 6160 MD Geleen, The Netherlands, and European Molecular Biology Laboratory, Hamburg Outstation, EMBL c/o DESY, Notkestrasse 85, D-22603 Hamburg, Germany