1. Institute for Organic Chemistry and Chemical Biology, Center for Biomolecular Magnetic Resonance, Johann Wolfgang Goethe-University of Frankfurt, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany
2. Departments of Chemistry and Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
3. Department of Chemistry, Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215, United States