BASE-PAIRING CONFIGURATIONS BETWEEN PURINES AND PYRIMIDINES IN SOLID STATE .4. CRYSTAL AND MOLECULAR STRUCTURE OF 2 1-1 HYDROGEN-BONDED COMPLEXES, 1-METHYL-5-BROMOURACIL - 9-ETHYL-2-AMINOPURINE AND 1-METHYL-5-FLUOROURACIL - 9-ETHYL-2-AMINOPURINE

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 P21 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 NH ... O bond involving the amino group of 2-aminopurine to the carbonyl oxygen, O(2), of bromouracil nd an NH ... 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 P21 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. © 1969.