1. Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, UMR 8580 du CNRS, “Structures, Proprietes et Modelisation des Solides”, Ecole Centrale Paris, 92295 Châtenay-Malabry Cedex, France, and Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR