1. Department of Biochemistry, Vanderbilt University , Nashville, TN 37232, USA
2. Center for Structural Biology, Vanderbilt University , Nashville, TN 37232, USA
3. Department of Chemistry, Vanderbilt University , Nashville , TN 37235, USA
4. Neutron Scattering Division, Oak Ridge National Laboratory , 1 Bethel Valley Road , Oak Ridge , TN 37831, USA