Figure 4.

The strictly antiparallel pairing mode of homo-DNA and p-RNA is the result of highly inclined backbone and base-pair axes. The central ApT base-pair steps in the structures of the (A) homo-DNA (28) and (B) p-RNA (26) octamer duplexes with sequence CGAATTCG. The views are into the major groove and illustrate the strong, positive and negative backbone-base pair inclinations in homo-DNA and p-RNA, respectively, as well as the dominance of cross-strand stacking. In both cases, the quasi-linear local geometry of the backbone renders the directions of backbone and helix axis nearly parallel.