Structure of poly d(A) o poly d(t)

On the basis of the x-ray data from polycrystalline and well oriented fibers of the sodium salt of poly d(A) o poly d(T) (Arnott et al, Nucl. Acids Res. 11, 4141-4155 (1983)), a revised fi’-DNA model incorporatingiMike adenine and thymine strands is shown to give a much better x-ray agreement (R = 0.25) than the previously assigned model consisting of mixed sugar conformations in the two strands. The narrowing of the minor and the widening of the major grooves are promiscuous features off?’-DNA which are common to all poly d(purine) o poly d(pyrimidine) duplexes with two hydrogen bonded base-pairs and are in marked contrast with classical 5-DNA. Due to modest propeller (-15°), the cross strand diagonal hydrogen bonds (0.37 nm) in this duplex are not as strong as those in AT-rich oligonucleotide crystal structures.