The crystal structure of a purine-pyrimidine hydrogen-bonded complex: 9-ethyl-2-aminopurine and 1-methyl-5-fluorouracil

Abstract

9-Ethyl-2-aminopurine and l-methyl-5-fluorouracil form a 1:1 complex in the solid state. The structure has been determined by three-dimensional X-ray diffraction methods using 2211 independent reflections and refined by the method of full matrix least squares. The purine and pyrimidine bases form a planar complex, joined by two hydrogen bonds; an N−H....O bond involving the amino group of 2-aminopurine to the carbonyl oxygen, O (2) , of fluorouracil and an N−H.… N bond between the ring nitrogens N (1) of 2-aminopurine and N (3) of fiuorouracil. The base-pairing configuration resembles the Watson—Crick pairing configuration which exists between the naturally occurring base pairs in DNA and has been found in crystalline complexes containing guanine and cytosine derivatives. The over-all crystal structure is an interlocking sheetlike structure, in which adjacent base pairs are related by a center of symmetry and are connected by hydrogen bonds between the aminopurine residues. The effect of the fluorine atom on the hydrogen-bonding interaction as well as the biological implications of this structure are discussed.