1. Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, Uppsala 75120, Sweden
2. Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520, United States
3. Departments of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6059, United States
4. Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States
5. Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843-2128, United States