1. Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China, Laboratory of Structural Biology, Tsinghua University, Beijing 100084, P. R. China, and Key Laboratory of Chemical Biology, Guangdong Province, Graduate School of Shenzhen, Tsinghua University, Shenzhen 518057, P. R. China