1. a Center for Theoretical Biological Physics and Department of Physics , University of California , San Diego, La Jolla, California, 92093–0374
2. b Center for Theoretical Biological Physics and Department of Physics , University of California , San Diego, La Jolla, California, 92093–0374 E-mail:
3. c Department of Chemistry and Biochemistry , University of California , San Diego, La Jolla, California, 92093