1. Contribution from the Departments of Molecular and Cellular Biology, Chemistry, Pharmacology and Toxicology, and University Animal Care, University of Arizona, Tucson, Arizona 85721, the Stanford Synchrotron Radiation Laboratory, P.O. Box 4349, MS 69, Stanford, California 94309, and the ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801