1. Max-Planck-Institute of Biochemistry, 82152 Martinsried, Germany;2. Department of Pharmaceutical Sciences, University of Pittsburgh, Biomedical Science Tower 3, 3501 Fifth Avenue, Pittsburgh, PA 15261, USA;3. Department of Chemistry, University of Pittsburgh, Biomedical Science Tower 3, 3501 Fifth Avenue, Pittsburgh, PA 15261, USA;4. Department of Drug Design, University of Groningen, A. Deusinglaan 9, 9713 Groningen, the Netherlands;5. Institute of Structural Biology, Helmholtz Zentrum München, Ingolstädter Landstrasse 1, 85764 Neuherberg, Germany;6. Organic Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland