1. Contribution from the Department of Biological Sciences, Vanderbilt University, Nashville, Tennessee 37235, Argonne National Laboratory, DND-CAT Synchrotron Research Center, Sector 5, Advanced Photon Source, Argonne, Illinois 60439, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, LaJolla, California 92037, and Laboratorium für Organische Chemie, Eidgenössische Technische Hochschule, CH-8093 Zürich, Switzerland