1. Department of Chemistry, P.O. Box 30012, Texas A&M University, College Station, Texas 77842-3012, Department of Biopharmaceutical Sciences and Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, 1700 4th Street, San Francisco, California 94158-2550, and Department of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461