All NMR experiments were performed on a Avance Bruker 600 MHz NMR spectrometer equipped with a triple-resonance 5-mm cryogenic probe. NOESY experiments were performed at 277 K and 298 K using a mixing time of 350 ms (39) . 13 C spin relaxation experiments were performed at natural abundance and 298 K (40) . Relative order parameters were calculated by normalizing (2R 2 -R 1 ) to either A31 (C8) or C33 (C6). Relaxation parameters were computed using HydroNMR (41, 42) , assuming an idealized A-form structure, to obtain diffusion tensor parameters (t m and D ratio ), and in-house written software was used to compute R 2 /R 1 values as previously reported (33, 40) . Motionally averaged bond lengths of 1.104 Å were used for both C8 and C6 moieties as previously described (40, 43) . The following experimentally derived CSAs (s xx , s yy , s zz ) were used in the analysis: (89, 15, À104); (80, 5, À85); and (98.4, 9.2, À107.5) for C2, C8 and C6 moeties (43, 44) . IP-COSY experiments were performed at 277 K and 298 K to observe relative 3 J H1 0 -H2 0 scalar coupling crosspeak intensities (45) . Base and sugar 1 H-13 C splittings were measured from the difference between the upfield and downfield components of the 1 H-13 C doublet along the 1 H component using the narrow transverse relaxation-optimized spectroscopy (TROSY) component in the 13 C dimension as implemented in 2D 1 H-13 C S 3 CT-heteronuclear single quantum correlation (HSQC) experiments (46) . 2 H splittings were 71 and 69 Hz for WT and A29C, respectively. Idealized A-form structures were constructed using Insight II (Molecular Simulations, Inc.) correcting the propeller twist angles from +15 to À15 using an in-house program, as previously described (47) . The complementary strand was removed and the resulting ssRNA used in NMR data analysis. B-form helices were constructed using W3DNA (48) .