This is the final version of the article. It first appeared from NPG via http://dx.doi.org/10.1038/srep12556 ; Collagens, the most abundant proteins in mammals, are defined by their triple-helical structures and distinctive Gly-Xaa-Yaa repeating sequence, where Xaa is often proline and Yaa, hydroxyproline (Hyp/O). It is known that hydroxyproline in the Yaa position stabilises the triple helix, and that lack of proline hydroxylation in vivo leads to dysfunctional collagen extracellular matrix assembly, due to a range of factors such as a change in hydration properties. In addition, we note that in model peptides, when Yaa is unmodified proline, the Xaa proline has a strong propensity to adopt an endo ring conformation, whilst when Yaa is hydroxyproline, the Xaa proline adopts a range of endo and exo conformations. Here we use a combination of solid-state NMR spectroscopy and potential energy landscape modelling of synthetic triple-helical collagen peptides to understand this effect. We show that hydroxylation of the Yaa proline causes the Xaa proline ring conformation to become metastable, which in turn confers flexibility on the triple helix. ; The authors acknowledge BBSRC grant number BB/G021392/1 (MJD, DGR), EPSRC DTA studentship and Doctoral Prize (WYC), British Heart Foundation RG/09/003/27122 and PG/08/011/24416 (RWF, DB, DAS), Wellcome Trust 094470/Z/10/Z (RWF, DB, DAS), ERC and EPSRC (DJW).