1. Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
2. Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, USA 60637
3. Laboratoire International Associé Centre National de la Recherche Scientifique et University of Illinois at Urbana-Champaign, Unité Mixte de Recherche no. 7019, Université de Lorraine, B.P. 70239, 54506 Vandœuvre-lès-Nancy Cedex, France
4. Theoretical and Computational Biophysics Group, Beckman Institute, Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA