1. Department of Chemical Biology; Max Planck Institute of Molecular Physiology; Otto-Hahn-Str. 11 D-44227 Dortmund Germany
2. Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Department of Chemical Biology, College of Chemistry and Molecular Engineering, Synthetic and Functional Biomolecules Center, and Peking-Tsinghua Center for Life Sciences; Peking University; Beijing 100871 China