1. Department of Biology; Institute of Molecular Biology and Biophysics; ETH Zürich; Otto-Stern-Weg 5 8093 Zürich Switzerland
2. Max-Planck-Institute of Molecular Physiology; Department of Structural Biochemistry; Otto-Hahn-Strasse 11 44227 Dortmund Germany
3. Department of Organic Chemistry; University of Konstanz; Universitätsstrasse 10 78464 Konstanz Germany