1. Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218, United States
2. Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States
3. Sealy Center for Structural Biology and Molecular Biophysics and Department of Biochemistry and Molecular Biology, The University of Texas Medical Branch at Galveston, Galveston, Texas 77555, United States
4. T. C. Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, Maryland 21218, United States