1. Contribution from the Pharmaceutical Chemistry Department IV, Schering AG, 13342 Berlin, Germany, The Salk Institute for Biological Studies, P.O. Box 85800, San Diego, California 92186, and The Center for Biomolecular Recognition, IMBG, Biochemistry B, The Panum Institute, Blegdamsvej 3c, DK-2100 N, Copenhagen, Denmark