1. Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, MS 69, 2575 Sand Hill Road, Menlo Park, California 94025, Department of Chemistry, Arizona State University, Tempe, Arizona 85287, Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia, V6T 1Z3, Canada, and ExxonMobil Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801