1. Chemical Biology Laboratory, National Cancer Institute, Frederick, Maryland 21702, United States
2. Chemistry and Synthesis Center, National Heart Lung and Blood Institute, Rockville, Maryland 20850, United States
3. Protein Characterization Laboratory, Frederick National Laboratory for Cancer Research, Leidos Biomedical Research, Frederick, Maryland 21701, United States
4. Centre for Medicines Discovery, Nuffield Department of Medicine, University of Oxford, Oxford OX3 7FZ, U.K.