1. School of Chemistry Sun Yat-sen University Guangzhou 510006 P. R. China
2. Department of Biochemistry Zhongshan School of Medicine Sun Yat-sen University Guangzhou 510080 P. R. China
3. Guangdong Key Laboratory of Natural Medicine Research and Development & College of Pharmacy Guangdong Medical University Dongguan 523808 P. R. China
4. School of Public Health Sun Yat-sen University Guangzhou 510080 P. R. China