1. Contribution from the Department of Pharmaceutical Chemistry, University of California, San Francisco, San Francisco, California 94143, Computational Chemistry Branch, NASAAmes Research Center, Mail Stop 230-3, Moffett Field, California 94035-1000, Laboratoire de Chimie Théorique, Unité de Recherche Associée au CNRS No. 510, Université Henri Poincaré-Nancy I, BP. 239, 54506 Vandoeuvre-lès-Nancy Cedex, France, and Vertex Pharmaceuticals Incorporated, 40 Allston Street, Cambridge, Massachusetts 02139-4211