1. Department of Pharmaceutical Chemistry & Graduate Group in Chemistry and Chemical Biology, University of California—San Francisco, Byers Hall, 1700 4th Street, San Francisco, California 94158-2550, Mass Spectrometry Facility, Department of Pharmaceutical Chemistry, University of California—San Francisco, 600 16th Street, MC 2240, Genentech Hall, Suite N472A, San Francisco, California 94158-2517