1. School of Chemistry, University of Leeds, Leeds LS2 9JT, UK.
2. Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, UK.
3. Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695-7905, USA.
4. School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK.