The 2D 1 H-1 H NOESY spectrum of the ssRNA shows abundant nuclear Overhauser effect (NOE) connectivities expected for a helical conformation, allowing the near complete assignment of base and sugar (H1 0 ) protons at 298 K (Supplementary Figure S3) . Particularly noteworthy are inter-base NOEs observed between adenine H8 protons within the polyadenine tract and between C33-U34 H6 protons, indicating significant base stacking within the polyadenine core at 277 K and decreased at 298 K (Supplementary Figure S3) (54) . Sequential NOEs are only observed for A25, U26, A35, and A36 upon decreasing the temperature from 298 K to 277 K, indicating a higher level of disorder at the terminal ends ( Figure 1C and Supplementary Figure S3 ). Furthermore, homonuclear three bond scalar couplings ( 3 J H1 0 -H2 0 ) indicate that residues within the polyadenine core adopt a C3 0 -endo sugar pucker conformation, consistent with an A-form-like geometry, with the tendency to adopt alternative sugar pucker conformations increasing towards terminal residues (Supplementary Figure S2) .