Figure 23.3.4.8 

Representative base-pair steps from B-DNA single-crystal X-ray analyses. (a) Pyrimidine-purine C-A step from C-C-A-A-G-A-T-T-G-G (B22, B46) (roll/slide/twist = −7.4°/2.6 Å/49.9°). Note the lack of ring-on-ring stacking, replaced by the stacking of pyrimidine O2 and purine N6 or O6, on aromatic rings of the adjacent base pair. This stacking opens up the twist angle to an unusual 50°. Note also the large +2.6 Å slide, which positions pyrimidine O2 over the six-membered rings of the neighbouring purines. (b) Pyrimidine-purine T-A step from C-G-A-T-A-T-A-T-C-G (B62) (roll/side/twist = 3.8°/−0.2 Å /39.5°). The stacking is similar to C-A, except that a near-zero slide positions pyrimidine O2 over the five-membered rings of purines. (c) Purine-purine A-A step from C-C-A-A-C-G-T-T-G-G (B46, B50) (roll/slide/twist = 8.8°/0.5 Å /28.7°). Ring-on-ring overlap now predominates, with consequently lowered twist angle and essentially zero slide. Note that purines are more extensively stacked than pyrimidines, which appear to be approaching the O2-on-ring stacking of Y-R steps. (d) Purine-pyrimidine A-T step from C-G-A-T-A-T-A-T-C-G (B62) (roll/slide/twist = 5.2°/0.0 Å/25.2°). Ring-on-ring stacking again lowers the twist angle and keeps slide around zero. Now there is no stacking of exocyclic N or O on neighbouring rings.