1. Boston University, Department of Chemistry, Boston, Massachusetts 02215, United States
2. Carnegie Mellon University, Department of Chemistry, Pittsburgh, Pennsylvania 15213, United States
3. Massachusetts Institute of Technology, Department of Chemistry, Cambridge, Massachusetts 02139, United States
4. Massachusetts Institute of Technology, Department of Biology, Cambridge, Massachusetts 02139, United States
5. Howard Hughes Medical Institute, Cambridge, Massachusetts 02139, United States