We are grateful to Dr. Masood (Institute of Biophysics, Chinese Academy of Sciences) for providing language help. This project was supported by the National Basic Research Program (973 Program), China (Nos. 2010CB912303 and 2013CB910103 ), the National Natural Science Foundation of China (Grant No. 21275044 ), the Natural Science Foundation of Hunan Province, China (Grant No. 12JJB003 ), and the Beijing Natural Science Foundation, China ( 7131011 ).