Department of Pharmaceutical Chemistry, University of California, San Francisco, San Francisco, California, United States of America.|Graduate Group in Biophysics, University of California, San Francisco, San Francisco, California, United States of America.|Department of Bioengineering and Therapeutic Sciences, University of California, San Francisco, San Francisco, California, United States of America.|Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, California, United States of America.|Department of Pharmaceutical Chemistry, University of California, San Francisco, San Francisco, California, United States of America , Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, California, United States of America.