1. Department of Molecular and Cellular Biology, The University of Arizona, Life Sciences South Building, Tucson, Arizona 85721, Stanford Synchrotron Radiation Laboratory, SLAC, P.O. Box 20450, MS 69, Stanford, California 94309, Department of Chemistry, The University of Arizona, Tucson, Arizona 85721, ExxonMobil Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801, and Department of Pharmacology and Toxicology, College of Pharmacy, The University of Arizona, Tucson, Arizona 85721