1. Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0820, United States
2. Center for Biomolecular Science and Engineering, Naval Research Laboratory, Code 6930, Washington, D.C. 20375, United States
3. Health Science Platform, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, P.R. China