1. Department of Chemistry and Biochemistry, Department of Pharmacology, and NSF Center for Theoretical Biological Physics, University of California at San Diego, La Jolla, California 92093-0365, Biomedical Sciences Graduate Program, University of California at San Diego, La Jolla, California 92093-0365, and Howard Hughes Medical Institute, University of California at San Diego, La Jolla, California 92093-0365