1. Department of Chemistry, P.O. Box 870336, University of Alabama, Tuscaloosa, Alabama 35487, Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, Department of Chemistry, P.O. Box 117200, University of Florida, Gainesville, FL 32611, and The Department of Biology, Darwin Building, University College London, Gower Street, London WC1H 9EW, United Kingdom