1. Department of Pharmacy, School of Medicine, Hangzhou City University, No. 48, Huzhou Road, Hangzhou 310015, P. R. China
2. College of Pharmaceutical Sciences, Zhejiang University, Hangzhou 310058, P. R. China
3. Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, USA