FIGURE 7.

Structural dynamics of P4 in the presence of Mg²⁺. (A) J_NN-COSY spectrum of P4 in Mg²⁺. The ^2hJ_NN coupling constants (in hertz) are shown next to each cross peak (Dingley et al. 1999). (B) Correlation plot between P4 RDCs measured in 20 mM Mg²⁺ and values back-calculated when independently fitting order tensors to an idealized A-form geometry for domains I (red) and II (blue). Error bars represent measurement uncertainty (2.4 Hz as for free, Fig. 3A). RDCs from residues C8 (open squares) and G23 (open diamonds) are shown but were not included in the analysis. (C) Orientation of order tensor frames (S_xx, S_yy, and S_zz) for domains I (in red) and II (in blue) determined in the presence of Mg²⁺. Results are shown relative to the free P4 interhelical conformation determined using RDCs in the absence of Mg²⁺. (D) ¹⁵N R ₂/R ₁ values for guanine (circles) and uridine (diamonds) residues measured in P4 in the presence of 15 mM Mg²⁺. Average values for each domain are shown as dotted lines. (E) One-bond C–H RDCs in P4 (left) and normalized intensities in E-P4 (right) in the bulge and flanking residues measured in the absence (filled symbols) and presence (open symbols) of MgCl₂. (F) Schematic diagram of local and global motions in P4 and Mg²⁺ association (yellow residues).