1. Institute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100050, P. R. China
2. School of Pharmacy, Jining Medical College, Jining, Shandong 276800, P. R. China
3. School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, U.K.