1. Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
2. Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
3. Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA