1. Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan 48824, United States
2. Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States
3. Biochemical and Biophysical Systems Group, Lawrence Livermore National Laboratory, Livermore, California 94550, United States
4. Scuola Normale Superiore and CSGI, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
5. Istituto Nazionale di Fisica Nucleare (INFN) sezione di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy