1. Department of Chemistry, Boston University, Boston, Massachusetts 02215, USA
2. National Facility for Protein Science in Shanghai, Zhangjiang Lab, Shanghai Advanced Research Institute, Chinese Academy of Science, Shanghai 201210, China
3. Department of Biomedical Engineering, Boston University, Boston, Massachusetts 02215, USA
4. Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China