1. Department of Applied Chemistry, Zhongshan University, Guangzhou, P. R. China, Department of Biology and Chemistry, City University of Hong Kong, Hong Kong, Fachbereich Chemie und Chemietechnik, Universität Paderborn, Warburger Strasse 100, D-33098 Paderborn, Germany, and Department of Molecular Pharmacology, Institute of Chemistry CRC, H-1525 Budapest, P.O.B. 17, Hungary