1. Department of Pharmacology, University of Oxford, Mansfield Road, Oxford OX1 3QT, UK1
2. Department of Chemistry, Box 351700, University of Washington, Seattle, WA 98195-1700, USA2
3. Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE, UK3
4. The Laboratory of Molecular Biophysics, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK4