1. Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, Grupo de Bioquímica e Biologia Teóricas and Centro de Estudos de Bioquimica e Fisiologia, Lisbon University, P-1749-016 Lisbon, Portugal, Department of Molecular Pharmacology and Toxicology, School of Pharmacy, University of Southern California, Los Angeles, California 90033, and Department of Chemistry, Duke University, Durham, North Carolina 27708