1. Biophysics Graduate Group, University of California, Berkeley, Berkeley, CA 94720, USA.
2. Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA.
3. Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720, USA.
4. Howard Hughes Medical Institute, University of California, Berkeley, Berkeley, CA 94720, USA.
5. Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
6. Department of Physics, University of California, Berkeley, Berkeley, CA 94720, USA.