1. Molecular Design and Synthesis Center, Vanderbilt Institute of Chemical Biology, Nashville, Tennessee 37232-0142, United States
2. Chemical Biology Laboratory, Center for Cancer Research, National Cancer Institute, Frederick, Maryland 21702-1201, United States
3. Leidos Biomedical Research, Frederick National Laboratory for Cancer Research, Frederick, Maryland 21701-4907, United States
4. Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37232-0142, United States