Nature Chemistry 5, 495–501 (2013); published online 19 May 2013; corrected after print 3 June 2013.

In the version of this Article originally published, in Fig 3a, the description for the open triangle should have read: '1 equiv. dye-labelled empty vesicles + vesicles without dipeptide'. This has been corrected in the HTML and PDF versions of the Article.