For the CCCG RNA, NMR studies revealed a well folded top and bottom helix, with a characteristic peak at ~10.6 ppm consistent with the formation of the GNRA tetraloop (Figure ). Of note, cytosine residues lack imino protons and hence are not observable in 2D imino proton spectra. However, residues neighboring the 2×2 CC/CC internal loops, G7 and G21, had sharp line widths and were well resolved, indicating a stable internal loop within the neighboring helical context (Figure ). To study the dynamic nature of the loops, we completed both 1 H- C HSQC and 1 H -1 H D2O NOESY experiments (Figures , and Table ). We also characterized the GCCC RNA by NMR (Figure ). In the 2D imino proton spectrum, the GCCC also appeared to fold well, with sharp line widths, off-diagonal peaks for the top and bottom helices, and a characteristic peak at ~10.6 ppm consistent with the formation of the GNRA tetraloop (Figure ). Both 1 H - C HSQC and 1 H -1 H D2O NOESY experiments were also completed on this construct (Figures , and Table ), which revealed sharp, well resolved peaks indicating a stable single population. Taken together, the NMR data suggests that differences in neighboring base pairs of the 2×2 CC/CC internal loops give rise to very minor changes in structure. These minor changes in structure were not distinct enough to discern unique structural differences in 2D 1 H - C HSQC, 1 H -1 H H2O NOESY or 1 H -1 H D2O NOESY spectra (Figures and).