Yi Hu1, Grace Y J Chen2,1, Shao Q Yao2,1. 1. Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117543 (Republic of Singapore). 2. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 (Republic of Singapore), Fax: (+65) 6779-1691.