FIGURE 9.

Interaction between the ribose 2′-OH of −1b and the N1 of adenine 1n in Kt-58. The axis of the C helix is approximately vertical on the left side, and the view presents the major groove at the apex of the turn. The bulge of Kt-58 is 2 nt in length, with a single opposed nucleotide on the opposite strand (L_n1). Exceptionally, there is no nucleotide stacked onto the end of the C helix in this K-turn. In this situation, the 2′-OH of the −1b ribose donates a hydrogen bond to N1 of adenine, thus performing the role of L1 in all the other K-turn structures analyzed (see Fig. 5).