1. Departments of Biopharmaceutical Science, Pharmaceutical Chemistry, and Cellular and Molecular Pharmacology and Institute for Quantitative Biomedical Research, University of California, San Francisco, 1700 4th Street, Byers Hall, Box 2550, Room 508E, San Francisco, California 94158, and Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, 2575 Sand Hill Road, MS99, Menlo Park, California 94025