1. Department of Chemistry and Biochemistry and Department of Pharmacology, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0365, Infectious Diseases Laboratory, The Salk Institute for Biological Studies, La Jolla, California 92037, Chemistry Department, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, and Department of Biology and Biochemistry, Houston Science Center, Room 402D, University of Houston, Houston, Texas 77204-5513