Base-pairing configurations between purines and pyrimidines in the solid state: IV. Crystal and molecular structure of two 1:1 hydrogen-bonded complexes, 1-methyl-5-bromouracil: 9-ethyl-2-aminopurine and 1-methyl-5-fluorouracil: 9-ethyl-2-aminopurine

Abstract

9-Ethyl-2-aminopurine forms 1:1 hydrogen-bonded complexes with 1-methyl-5-bromouracil and with 1-methyl-5-fluorouracil. Both structures have been determined by three-dimensional X-ray diffraction methods and refined by block diagonal least squares. 9-Ethyl-2-aminopurine: 1-methyl-5-bromouracil crystals are monoclinic, space group P2 1 c , with a = 11.07 A ̊ , b = 12.34 A ̊ , c = 13.84 A ̊ , and β = 127 ° 2′. The aminopurine and bromouracil 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 bromouracil nd an NH … N bond between the ring nitrogens N(1) of 2-aminopurine and N(3) of bromouracil. Adjacent base pairs are related by a center of symmetry and are connected by hydrogen bonds between aminopurine residues. This basic tetramer structure is repeated in the 9-ethyl-2-aminopurine: 1-methyl-5-fluorouracil crystalline complex in a completely different crystal lattice environment. This second structure is monoclinic, space group P2 1 c , with a = 8.56 A ̊ , b = 21.15 A ̊ , c = 7.63 A ̊ , and β = 97 ° 48′. The base pairing configuration is of the Watson-Crick type in both structures, although certain geometric differences have been noted. These structures are discussed with respect to the preceding structures in this series of papers.