1. Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong S.A.R., P. R. China, Institute of Modern Physics, Chongqing University of Post and Telecommunications, Chongqing 400065, P. R. China, and CEMES-CNRS, 29 rue Marvig BP 94347, 31055 Toulouse Cedex 4, France