1. Drug Discovery Program, Georgetown University Medical Center, 3900 Reservoir Road, NW, Washington, D.C. 20007-2197, Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, Texas 77555-1031, and Laboratory for the Structure of Matter, Code 6030, Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, D.C. 20375-5000