1. Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
2. Department of Pharmaceutical Chemistry, University of California San Francisco Mass Spectrometry Facility, San Francisco, California 94158, United States
3. Department of Medicine, Beth Israel Deaconess Medical Center, Boston, Massachusetts 02115, United States
4. Department of Systems Biology, Harvard Medical School, Boston, Massachusetts 02115, United States