1. Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
2. Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602, United States
3. Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States
4. Department of Biochemistry, University of California, Riverside, Riverside, California 92521, United States
5. Chemistry and Biochemistry Department, University of California, Santa Cruz, Santa Cruz, California 95064, United States