1. Department of Biochemistry, University of Washington, Seattle, WA 98195, USA.
2. Biomolecular Structure and Design Program, University of Washington, Seattle, WA 98195, USA.
3. Department of Chemistry, University of Washington, Seattle, WA 98195, USA.
4. Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, USA.
5. Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA 98109, USA.
6. Laboratory of Organic Chemistry, Eidgenössische Technische Hochschule (ETH) Zürich, 8093 Zürich, Switzerland.
7. Howard Hughes Medical Institute (HHMI), University of Washington, Seattle, WA 98195, USA.