1. Laboratory of Chemical Physics, Building 5, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0520, Division of Computational Biosciences, Center for Information Technology, National Institutes of Health, Bethesda, Maryland 20892-5624, and Laboratory of Molecular Biology, Building 5, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0530