1. Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA
2. Laboratory of Single Molecule Biophysics, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892, USA
3. Center for Theoretical Biological Physics, Rice University, Houston, TX 77005, USA
4. Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
5. Department of Molecular Biosciences, Northwestern University, Evanston, IL 60208, USA