1. Department of Chemistry and Biochemistry and Center for Biomolecular Structure and Organization, University of Maryland, College Park, Maryland 20742, Centro de Biología Molecular Severo Ochoa, Universidad Autónoma de Madrid, 28049 Madrid, Spain, and Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892