1. Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, U.S.A.
2. Department of Biochemistry and Biophysics and the Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, U.S.A.
3. School of Dentistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, U.S.A.
4. Department of Molecular Pharmacology, St Jude Children's Research Hospital, Memphis, TN 38105, U.S.A.