1. Center for Theoretical Biological Physics, Houston, Texas 77005, United States
2. Department of Chemistry, Rice University, Houston, Texas 77005, United States
3. Department of Biosciences, Rice University, Houston, Texas 77005, United States
4. Department of Biology and Biological Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden
5. Beckman Institute and Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States