1. Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305, USA.
2. Biophysics Program, Stanford University, Stanford, CA 94305, USA.
3. Department of Chemistry and Biochemistry, Biomolecular Science and Engineering Program, University of California at Santa Barbara, Santa Barbara, CA 93106, USA.
4. Department of Physics, Stanford University, Stanford, CA 94305, USA.