1. Department of Physics, Centre for Materials Physics, University of Durham, Durham, DH1 3LE, UK
2. School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
3. Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, UK
4. School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, LS2 9JT, UK