1. University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, UK.
2. Department of Structural and Molecular Biology, University College London, London WC1E 6BT, UK.
3. Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands.
4. Centre for Systems Chemistry, Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands.