1. Rudolf Boehm Institute for Pharmacology and Toxicology; Leipzig University; Härtelstr. 16-18 04107 Leipzig Germany
2. Department of Chemistry; Ludwig Maximilians University Munich and Center for Integrated Protein Science Munich; Butenandtstr. 5-13 81377 Munich Germany
3. Walther Straub Institute of Pharmacology and Toxicology; Ludwig Maximilians University Munich; Nußbaumstr. 26 80336 Munich Germany
4. Present address: Department of Chemistry, Silver Center for Arts and Science; New York University; 100 Washington Square East New York NY 10003 USA
5. Present address: Department of Chemical Biology; Max Planck Institute for Medical Research; Jahnstr. 29 69120 Heidelberg Germany