Structure of the DNA Octanucleotide d(ACGTACGT)₂

d(ACGTACGT), C₇₈H₈₄N₃₀O₃₂P₇.20H₂O, M_r (DNA) = 2170, tetragonal, P4₃2₁2 (No 96), a = 42.845 (1), b = 42.845(1), c = 24.804 (1) Å, V = 45532.5 (2) ų, z = 8,λ(MoKα) = 0.71069 Å,µ(MoKα) = 0.10 mm⁻¹, T = 295 K, R = 0.18 for 1994 unique reflections between 5.0 and 1.9 Å resolution. The self-complementary octanucleotide d(ACGTACGT)₂ has been crystallized and its structure determined to a resolution of 1.9 Å. The asymmetric unit consists of a single strand of octamer with 20 water molecules. It is only the second example of an octanucleotide having terminal A·T base pairs whose structure has been determined by X-ray crystallography. The sequence adopts the modified A-type conformation found for all octanucleotide duplexes studied to date with the helix bent by approximately 15° and an average tilt angle of 0°. Unusually the data collection was carried out using a 3 kW molybdenum sealed-tube source. The conformational details are discussed in comparison with other closely related sequences.