1. Contribution from the Department of Molecular Biology and The Skaggs Institute of Chemical Biology, The Scripps Research Institute, MB 33, 10550 North Torrey Pines Road, La Jolla, California 92037, Bruker Biospin GmbH, Rheinstetten, Germany, Department of Biochemistry and Molecular Biology, Medical University of South Carolina, 173 Ashley Avenue, P.O. Box 250509, Charleston, South Carolina 29425, and Cassia, LLC, 4045 Sorrento Valley Boulevard, San Diego, California 92121