in DNA, the two oxygen atoms point outside the groove, thus resulting in similar hydration dynamics for the two phosphate oxygens due to similar environments, whereas in RNA, O R strongly points towards the inside of the groove (see Figure with very small values of α P R ). This logically results in larger excluded volumes (larger ρ T SEV ) at O R . It also imposes stronger restraints on the elongation of the initial water-phosphate H-bond-the H-bonded water in this case being confined inside the groove-thus leading to a larger ρ T SHB , and overall to a significantly more pronounced slowdown.