Nature Chemistry 1, 573–577 (2009); published online: 30 August 2009; corrected after print 23 October 2009

At roughly the same time, the authors of this Article independently submitted a manuscript to Angewandte Chemie, in which the method presented here was applied to polyamino propylene amine dendrimers. The authors forgot to mention this publication and would like to apologize for their mistake. This missing reference is: Winkler, H. D. F., Weimann, D. P., Springer, A. & Schalley, C.A. Dynamic motion in crown ether dendrimer complexes: A “spacewalk” on the molecular scale. Angew. Chem. Int. Ed. 48, 7246–7250 (2009).