1. a Krebs Institute, Departments of Chemistry and Molecular Biology, Sheffield University , Sheffield , S3 7HF
2. b Ludwig Institute for Cancer Research, University College London , London , W1P 8BT
3. c Department of Chemistry , Southampton University , Southampton , S017 1BJ
4. d Department of Biochemistry and Molecular Biology , University College London , London , WC1E 6BT
5. e Department of Crystallography , Birkbeck College, University of London , London , WC1E 7HX , UK
6. f Department of Chemistry , E-mail: