Sequence-Dependent Conformational Variations in theB-DNA Double-Helix of Poly d(AATT)·Poly d(AATT)

X-ray diffraction data from well oriented and polycrystalline fibers of the lithium salt of poly d(AATT).poly d(AATT) are isomorphous with those from B-DNA. The double-helix consists of conformationally identical antiparallel strands and the molecular symmetry is 2 5(2); the asymmetric unit is a tetranucleotide, AATT, and 5 tetranucleotides span two turns per strand. Two double helices pass through a monoclinic unit cell of dimensions a = 31.05, b = 22.62, c = 33.85 A (fiber axis) and gamma = 90 degrees. In each repeating motif, the four nucleotides have distinct conformations, TpA displays an axial P-O bond and there is shortening of minor groove in the central region.