1. Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
2. Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, California 94720, United States
3. Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
4. Departments of Chemistry and of Molecular and Cell Biology, University of California, Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States