Nature Chemistry 6, 393–403 (2014); published online 20 April 2014; corrected after print 7 January 2015.

In the version of this Article originally published, the middle initial of the co-author Nabil M. Wilf was incorrect, and the Acknowledgements section was missing a reference to the European Research Council; the statement should have read: “We thank the Medical Research Council (U105181009, UD99999908) and the European Research Council (MC-A024-PG0A) for financial support. We thank S-P. Chew (MRC-LMB Mass Spectrometry) for obtaining MALDI data.” These errors have now been corrected in the online versions of the Article.