Figure 2: RD analysis of excited state in tP5abc.

(a) Chemical exchange with proposed excited state that features partially folded P5c. Exchange parameters are obtained from global fitting the RD profiles. Residues are coloured as follows: RD detected, red; no RD detected, blue; immeasurable due to severe overlap or weak intensity, grey; and all others, black. Residues with imino resonances that undergo changes or disappear on addition of Mg²⁺ are circled. (b,c) Off-resonance RD profiles in the absence (b) and presence (c) of 1 mM Mg²⁺. The complete data set is shown in Supplementary Fig. 3. The data in b and c are globally fitted using two-state exchange model and Laguerre equation⁶¹ (see Supplementary Tables 2 and 3). (d) 2D ¹H-¹⁵N HSQC spectrum of 1 mM WT tP5abc in 10 mM sodium phosphate and 0.01 mM EDTA, pH 6.4, at 10 °C. The residues showing pronounced RD are highlighted by red arrows starting at ¹⁵N chemical shift of GS and ending at ¹⁵N chemical shift of ES, and are labelled with the associated Δω. Also shown is the ellipsoid distribution of ¹⁵N (N1/3) and ¹H (H1/3) chemical shifts of RNA imino groups in different base pair contexts. The chemical shift data are extracted from the BMRB database. The ellipses are centred on the average chemical shift and have size that corresponds to twice the s.d. in the distribution.