The high-resolution structure of d(CATGGGCCCATG) has features that are throughout its length intermediate between A and B ( Ng, Kopka, and Dickerson, 2000), and thus resembles a trapped intermediate in the Aв†”B transition. Structure of the A/B hybrid d(CGCCCGCGGGCG), with the large bend at the center of the structure being apparent ( Malinina et al., 1999). Whether this structure is a consequence of the influence of crystal-packing forces or innate tendencies of this sequence is not clear.ДЇigure 3.15. This structure thus has features akin to an Aв†”B transition, albeit with distinct conformations at different points along the sequence. By contrast the terminal nucleotides have many B-DNA characteristics, such as B-like sugar puckers, backbone conformational angles, and base slide.

This 65В° bending results in very close phosphate groups. The central octamer region has an A-DNA conformation, but with a high degree of bending around the central purine-3вЂІ,5вЂІ-pyrimidine base step ( Fig. The crystal structure of the dodecamer d(CGCCCGCGGGGCG) is remarkable in that parts of the structure demonstrate true hybrid character ( Malinina et al., 1999). However the capacity of DNA structure to surprise is still apparent. It may be thought that there is then little more to learn about A-DNA. There are now over 150 A-DNA crystal structures in the NDB, which provide a very detailed view of this DNA form and the myriad of deviations from canonical ideality (notably in major-groove width) that are possible ( Wahl and Sundaralingam, 1997).

A form b form z form c form of dna