1. Department of Molecular Biology & Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom, European Synchrotron Radiation Facility, 6 rue Jules Horowitz, F-38043 Grenoble, France, Department of Chemistry, University of Sheffield, Sheffield S3 7HF, United Kingdom, and Manchester Interdisciplinary Biocentre, University of Manchester, Manchester, M1 7DN, United Kingdom