1. Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599–7295, USA.
2. Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599–7295, USA.
3. Department of Chemistry and Institute for Biophysical Dynamics, University of Chicago, Chicago, IL 60637, USA.
4. Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599–7295, USA.