1. Center for Biomolecular Magnetic Resonance Institute of Organic Chemistry and Chemical Biology Goethe-Universität Frankfurt am Main Max-von Laue-Straße 7 60438 Frankfurt am Main Germany
2. Department of Molecular Membrane Biology Max Planck Institute of Biophysics Max-von-Laue-Straße 3 60438 Frankfurt am Main Germany
3. Institute for Inorganic Chemistry Goethe-Universität Frankfurt am Main Germany