In A-RNA, the close proximity of phosphate groups on both sides of the major groove, as visible on Figure and quantified by the width of the RNA major groove-which is even narrower than the DNA minor groove (see SI Figure )-also leads to a different structure of the ion cloud around RNA and DNA (see Fig. ), as already described in previous works. In particular, we note in RNA a larger density of K + counterions next to the phosphate groups, in the major groove and close the O R atom. In DNA, the cations are mainly at the bottom of the major groove next to the bases and along the helix principal , and that predicted with different components of the Extended Jump Model: ρ T SHB , ρ T SEV or ρ T SEV × ρ T SHB . ρ T SEV is calculated taking only into account the nucleic acid excluded volume (filled circles), or included that coming from the ions (empty circles). Plots were prepared using Matplotlib. axis, with a much smaller density next to the phosphate groups. This explains why inclusion of the excluded volume due to the counterions was necessary to capture the full extent of the slowdown and the split in hydration dynamics between the two phosphate oxygen atoms in RNA, but not in DNA. Note however that the increase of the slowdown due to the ions predicted by our jump model, while noticeable for RNA O R , remains very modest (factor 1.1 and GGGG-RNA (e) (light blue for the angle α P R and blue for the angle α P S ). The plots were prepared using Matlab. on average). In addition, the decay of the jump correlation function is independent of the initial presence of a counterion in the water hydration shell (see SI Figure ), which shows that the memory of the ion presence is lost faster than the typical jump time. Therefore, the presence of slow water molecules cannot be ascribed to that of ions near these locations. Now that we have rationalized the differences in hydration dynamics at the phosphate groups, we use our jump model to investigate why the acceptor sites with slow hydration dynamics in the DNA minor groove (cytosine O2, τ jump ∼24-31 ps), have a much faster hydration dynamics in RNA, where they are on the floor of the shallow groove (τ jump ∼9-10 ps) (see SI Figure ). There, the jump model does not work quantitatively, because it focuses on jumps to (bulk) water, whereas at these confined locations in narrow grooves about half of the jumps occur towards another acceptor site of the biomolecule (see SI for a detailed discussion). However, our model does qualitatively capture the difference in dynamics at these sites between DNA and RNA: