1. Department of Biosciences, Durham University, Durham DH1 3LE, UK.
2. Department of Chemistry and Biochemistry, Center for Biomolecular Structure and Organization, University of Maryland, College Park, MD 20742, USA.
3. Department of Chemistry, Durham University, Durham DH1 3LE, UK.
4. Biophysical Sciences Institute, Durham University, Durham DH1 3LE, UK.
5. Biophysics Program, Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742, USA.