1. Department of Chemistry, University of California Berkeley, Berkeley, California 94720, United States
2. Howard Hughes Medical Institute, University of California, Berkeley, California 94720, United States
3. Institute for Quantitative Biosciences, University of California, Berkeley, California 94720, United States
4. Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, United States
5. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States