1. Laboratory of Macromolecular and Structural Chemistry, Catholic University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee-Leuven, Belgium; DSM Research, P.O. Box 18, NL-6160MD Geleen, The Netherlands; and European Molecular Biology Laboratory, EMBL c/o DESY, Notkestrasse 85, D-22603 Hamburg, Germany