1. Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, the University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China, Department of Anatomy, the University of Hong Kong, Sasson Road, Hong Kong, P. R. China, and Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, P.R., China