1Clemens Schöpf Institute of Chemistry and Biochemistry, Darmstadt University of Technology, Petersenstrasse 22, 64287 Darmstadt, Germany, Fax: (+49) 6151-163278
2Max Planck Unit for Structural Molecular Biology c/o DESY, Notkestrasse 85, 22607 Hamburg, Germany
3Emmy Noether Research Group, Institute of Physiological Chemistry and Pathobiochemistry, Johannes Gutenberg University Mainz, 55128 Mainz, Germany
4F. Hoffmann–La Roche Ltd., Pharmaceuticals Division, Preclinical Research CNS, Building 70/345, 4070 Basel, Switzerland
5Bayer Schering Pharma AG, Molecular Imaging Research, S109, 06, 614A, Müllerstrasse 178, 13353 Berlin, Germany