1. Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, Institute of Molecular Pharmacology, Robert-Rössle-Str. 10, 13125 Berlin, Germany, Institut für Biochemie, Humboldt-Universität, Monbijoustr. 2, 10117 Berlin, Germany, and Jerini AG, Invalidenstr. 130, 10115 Berlin, Germany