1. Division of Medicinal Chemistry, College of Pharmacy, P.O. Box 210207, University of Arizona, Tucson, Arizona 85721, Department of Chemistry, P.O. Box 351700, University of Washington, Seattle, Washington 98195, and the Structural Biology Laboratory, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, California 92037