BASE-PAIRING CONFIGURATIONS BETWEEN PURINES AND PYRIMIDINES IN SOLID STATE .2. CRYSTAL AND MOLECULAR STRUCTURE OF 9-ETHYL-8-BROMOADENINE-1-METHYL-5-BROMOURACIL

9-Ethyl-8-bromoadenine and 1-methyl-5-bromouracil form a 1:1 hydrogen-bonded complex in the crystalline state. The structure has been determined by three-dimensional X-ray diffraction methods and refined by block diagonal least squares to a final residual of 8.6%. The crystals are monoclinic, space group P21 c, with a = 10.78 A ̊, b = 13.13 A ̊, c = 13.51 A ̊ and β = 123.1 °. The adenine and bromouracil bases form a planar complex, joined by two hydrogen bonds; an NH ... N bond between the N(3) ring nitrogen on bromouracil and the N(1) ring nitrogen on adenine, and an NH ... O bond involving the adenine amino group interacting with the O(2) carbonyl oxygen on bromouracil. Adjacent base pairs are related by a center of symmetry and are connected by hydrogen bonds between adenine residues, hydrogen-bonding involving the amino group on one adenine residue interacting with the N(7) imidazole nitrogen on the symmetry-related residue. The reversed Watson-Crick type base-pairing configuration found in this study has been seen in previous co-crystallization studies with closely related compounds, and its significance is discussed. © 1969.