SEQUENCE-DEPENDENT CONFORMATIONAL VARIATIONS IN THE B-DNA DOUBLE-HELIX OF POLY-D(AATT)CENTER-DOT-POLY-D(AATT)

X-ray diffraction data from well oriented and polycrystalline fibers o f the lithium salt of poly d(AATT).poly d(AATT? are isomorphous with t hose from B-DNA. The double-helix consists of conformationally identic al antiparallel strands and the molecular symmetry is 2 5(2); the asym metric unit is a tetranucleotide, AATT, and 5 tetranucleotides span tw o turns per strand. Two double helices pass through a monoclinic unit cell of dimensions a = 31.05, b = 22.62, c = 33.85 Angstrom (fiber axi s) and gamma = 90 degrees. In each repeating motif, the four nucleotid es have distinct conformations, TpA displays an axial P-O bond and the re is shortening of minor groove in the central region.