1. Department of Pharmaceutical Chemistry and Graduate Group in Biophysics, University of California at San Francisco, San Francisco, California 94143, and Laboratoire de Chimie Théorique, Centre National de la Recherche Scientifique UMR 7616, Université Pierre et Marie Curie-CNRS, Box 137, 4 Place Jussieu, 75252 Paris Cedex 05, France