Base-pairing configurations between purines and pyrimidines in the solid state: I. Crystal and molecular structure of a 1:2 purine-pyrimidine hydrogen-bonded complex: 9-ethyladenine-1-methyl-5-iodouracil

Abstract

9-Ethyladenine and 1-methyl-5-iodouracil form a 1:2 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 9.2%. The crystals are monoclinic, space group P2 1 c , with a = 15.13 A ̊ , b = 8.60 A ̊ , c = 20.11 A ̊ and β = 119 ° 42′. Two iodouracil residues interact with one adenine residue to form a planar trimer complex containing two different hydrogen-bonded configurations. One configuration is similar to that found in previous co-crystallization studies of adenine and uracil derivatives, and involves a Hoogsteen type base-pairing arrangement. The other base-pairing configuration resembles the Watson-Crick adenine-thymine base-pairing configuration, except that the iodouracil residue is reversed in its orientation, hydrogen-bonding involving the O(2) carbonyl oxygen, rather than O(4) as in the Watson-Crick scheme. The over-all crystal structure consists of an angulated sheet-like arrangement of trimer base pairs, held together by van der Waals' type interactions. The relationship of this structure to previous structures containing adenine and uracil derivatives is discussed.