1. Contribution from the Center for Theoretical Biological Physics and Department of Physics, University of California at San Diego, La Jolla, California 92093, Department of Chemistry and W. M. Keck Center for Computational and Structural Biology, Rice University, 6100 Main Street, Houston, Texas 77005, and Structural and Computational Biology and Molecular Biophysics, Baylor College of Medicine, One Baylor Plaza, Houston, Texas 77030