1. Department of Chemistry, Centre for Research in Mass Spectrometry, and Department of Biology, York University, 4700 Keele Street, Ontario, Canada M3J 1P3, MDS SCIEX, 71 Four Valley Drive, Concord, Ontario, Canada L4K 4V8, and Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong