1. Department
of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, United States
2. Department
of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, North Carolina 27599-7260, United States
3. Department
of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706-1544, United States
4. AIDS and
Cancer
Virus Program, Leidos Biomedical Research, Inc., Frederick National
Laboratory for Cancer Research, Frederick, Maryland 21702-1201, United States