1. Faculty of Biosciences, Pharmacy and Psychology; Leipzig University; Institute of Biochemistry; Brüderstr. 34 04103 Leipzig Germany
2. Center for Structural Biology; Vanderbilt University; 465 21st Avenue South Nashville TN 37203 USA
3. Department of Chemistry; The Scripps Research Institute; 10550 North Torrey Pines Road La Jolla CA 92037 USA