Despite very similar chemical compositions, the ds-DNA and ds-RNA 18-mers adopt strikingly different helical conformations (Figure and S1). Consistently with previous structural studies, the simulated DNA B-form helices have a very narrow minor groove (width around 7.5 Å), while the major groove is much wider (see SI Figures and Table ). In contrast, for the RNA A-helix, the major groove is narrow (width around 6 Å) and very deep (typical depth of about 10 Å), whereas the minor groove is wide (width around 10 Å) and shallow. The surface of the nucleic acid is colored according to the τ reor value computed at each site. Snapshots were prepared using the VMD software. Following a strategy previously successfully applied to proteins and DNA, we performed a site-resolved analysis of water dynamics in the hydration shell of these RNA and DNA 18-mers. Water dynamics in biomolecular hydration shells can be monitored in different ways. Commonly used residence times focus on the time spent by water molecules in the vicinity of a given solute site. However, such times can only be probed indirectly in the experiments and very sensitively depend both on the exact site definition and on the time allowed for transient excursions outside the site. Here, we rather focus on water reorientation dynamics, which is measurable by NMR. This rotational dynamics is very closely linked to the reorganization dynamics of the H-bond network, and thus reports on the lability of the hydration shell.