1. Department of Chemistry; National University of Singapore; 3 Science Drive 3 Singapore 117543 Singapore
2. Molecular Recognition Research Center; Bio-Med Program of KIST-School UST; Korea Institute of Science & Technology; Hwarangno 14-gil 5, Seongbuk-gu Seoul 136-791 Korea
3. College of Pharmacy; Jinan University; Guangzhou 510632 China