Jiangbo Wei1,2, Chuan He3,4. 1. Department of Chemistry, Department of Biochemistry and Molecular Biology, and Institute for Biophysical Dynamics, The University of Chicago, Chicago, IL, USA. 2. Howard Hughes Medical Institute, The University of Chicago, Chicago, IL, USA. 3. Department of Chemistry, Department of Biochemistry and Molecular Biology, and Institute for Biophysical Dynamics, The University of Chicago, Chicago, IL, USA. chuanhe@uchicago.edu. 4. Howard Hughes Medical Institute, The University of Chicago, Chicago, IL, USA. chuanhe@uchicago.edu.