1. Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong S.A.R., P.R. China, Department of Chemistry, The University of Hong Kong, Pokfulam, Hong Kong S.A.R., P.R. China, and Institut für Physikalische und Theoretische Chemie, Universität Regensburg, D-93040 Regensburg, Germany