1. Laboratory of Organic Chemistry, ETH Zürich, 8093 Zürich, Switzerland.
2. Department of Biochemistry, University of Zürich, 8057 Zürich, Switzerland.
3. Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, NC 27599-7365, USA.
4. Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, NC 27599, USA.