Base-pairing configurations between purines and pyrimidines in the solid state: II. Crystal and molecular structure of 9-ethyl-8-bromoadenine-1-methyl-5-bromouracil

Abstract

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 P2 1 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 NH … N bond between the N(3) ring nitrogen on bromouracil and the N(1) ring nitrogen on adenine, and an NH … 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.