1. European Bioinformatics Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SD, UK
2. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
3. Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
4. Biomedical Sciences Research Complex and EaStCHEM School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife KY16 9ST, UK