1. Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Center for Theoretical Biological Physics and Department of Chemistry and Biochemistry, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093, and Department of Physics, University of California at San Diego, 9500 Gilman Drive, La Jolla, California 92093