Figure 1.

NMR structures and U-turn characteristics of SLV. (a) The MgFree most representative structure determined by NMR (1TBK). Bases of the five-membered U-turn are labeled and shown in color. (b) The MgBound most representative structure determined by NMR (1YN2). Bases of the five-membered U-turn are shown in the same color as part a, and hexahydrated Mn²⁺ are shown as blue spheres. The primary sequence is shown with the bases of the U-turn in black. Close ups of the characteristic U-turn hydrogen bond between the 2′ OH of U₆₉₆ (yellow) and the N7 of A₆₉₈ (red) (c) in the absence of Mg²⁺ and (d) in the presence of Mg²⁺. Close ups of the characteristic U-turn hydrogen bond between the 3′ phosphate of A₆₉₈ (shown in purple here as the 5′ phosphate of C₆₉₉) and the N3 of U₆₉₆, and stacking of 5′phosphate of A₆₉₈ with the U₆₉₆ base in the (e) absence and (f) presence of Mg²⁺. Solid lines indicate a favorable interaction (stacking or hydrogen bonding) is present and dashed lines indicate absence of interaction.