1. Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720–1460, USA.
2. Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720–1460, USA.
3. Department of Chemistry, Stanford University, Stanford, CA 94305, USA.
4. Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA.
5. Department of Biochemistry, Stanford University, Stanford, CA 94305, USA.