1. Contribution from Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, Department of Chemistry, Swarthmore College, Swarthmore, Pennsylvania 19081-1397, Department of Pharmaceutical Chemistry, University of California, San Francisco, San Francisco, California 94143, Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, and Department of Chemistry, University of California, Berkeley, Berkeley, California 94720...