Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, Center for Molecular Modeling and Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, Computational Bioscience Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, and Department of Biochemistry, University of Chicago, Chicago, Illinois 60615 (At 2007-10-27)