1. Department of Chemistry; University of Cambridge, Cambridge CB2 1EW; United Kingdom
2. Biozentrum, University of Basel; Basel CH-4056 Switzerland
3. Dipartimento di Scienze Biomediche, Sperimentali e Cliniche; Università degli Studi di Firenze; Firenze 50134 Italy
4. Division of Molecular Biosciences, Imperial College London; London SW7 2AZ United Kingdom