1. Hefei National Laboratory for Physical Science at Microscale and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026 P. R. China, and Structural Biology Program, Department of Physiology and Biophysics, Mount Sinai School of Medicine, New York University, One Gustave L. Levy Place, New York, New York 10029-6574 zwang3@ustc.edu.cn; ming-ming.zhou@mssm.edu