1. Leiden/Amsterdam Center for Drug Research (LACDR), Division of Medicinal Chemistry, Department of Pharmacochemistry, Faculty of Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, Laboratory for Crystallography, Institute for Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, and Department of Organic Pharmaceutical Chemistry, Uppsala Biomedical Center, Box 574, Uppsala University, S-751 23 Uppsala, Sweden