1. Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599–3290, USA.
2. Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, NC 27599, USA.
3. AIDS and Cancer Virus Program, SAIC-Frederick, Inc., Frederick National Laboratory for Cancer Research, Frederick, MD 21702–1201, USA.
4. Department of Biology and Carolina Center for Genome Sciences, University of North Carolina, Chapel Hill, NC 27599, USA.
5. Department of Genetics, University of North Carolina, Chapel Hill, NC 27599, USA.
6. Department of Chemistry, Saint Louis University, Saint Louis, MO 63103, USA.