1. Section on Structural and Chemical Biology, Neurosciences and Cellular and Structural Biology Division, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20817, United States
2. Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States
3. The Chemistry and Synthesis Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Rockville, Maryland 20850, United States