1. Contribution from the Departments of Chemistry and Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, Institut für Organische Chemie und Chemische Biologie, Center for Biomolecular Magnetic Resonance, Johann Wolfgang Goethe-Universität Frankfurt, Max-von-Laue-Strasse 7, 60438 Frankfurt/M, Germany, and Department of Physiology and Biophysics, Boston University School of Medicine, 715 Albany Street, Boston, Massachusetts 02118