1. Department of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E California Blvd MC 127-72, Pasadena, California 91125, United States
2. Department of Chemistry, University of California Davis, One Shields Avenue, Davis, California 95616, United States
3. Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States