1. Department of Nuclear Medicine, Saarland University−Medical Center, Kirrbergerstrasse, D-66421 Homburg, Germany
2. Department of Medical Biochemistry and Molecular Biology, Saarland University, D-66421 Homburg, Germany
3. Department of Organic Chemistry, Saarland University, D-66123 Saarbrücken, Germany
4. Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland