Correction to: Nature Communications https://doi.org/10.1038/ncomms1516, published online 18 October 2011

In the version of this article initially published, due to a production error, the small molecules I1 and I2 on the left-hand, upper and lower sides of Fig. 1, respectively, were both labelled I1. Due to the age of the article, the figure cannot be updated directly; the correct Fig. 1 is available as Supplementary information accompanying this amendment. This notice serves to correct the article.