1. Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Building 8A, Room B1A-17, Bethesda, Maryland 20892-0810, Department of Pharmacology, University of North Carolina, School of Medicine, Chapel Hill, North Carolina 27599-7365, and AK Neurochemie, Biozentrum der J. W. Goethe-Universität, Marie-Curie-Strasse 9, D- 60439 Frankfurt am Main, Germany