Aza analogues of nucleic acid bases: experimental determination and computational prediction of the crystal structure of anhydrous 5-azauracil

The crystal and molecular structure of 5-azauracil, C(3)H(3)O(2)N(3) M(r) = 1 13.07 Da, was determined from X-ray diffraction data. The material crystallizes in the orthorhombic space group Pbca with eight molecules of 5-azauracil in a cell of dimensions a = 6.5135(3), b = 13,5217(4), c = 6.5824(4) Angstrom, crystal density D(c) = 1.779 g cm(-3). The structure was determined using direct methods and refined by full-matrix least-squares to a conventional R index of 0.0337 for 763 observed reflections and 86 parameters. Two strong hydrogen bonds, N1H1...O4 and N3H3...N5, and several weaker intermolecular interactions produce a crinkled sheet structure. This crystal structure was independently predicted by a search for minima in the lattice energy, as calculated using an ab initio optimised molecular structure and a distributed multipole model for the electrostatic interactions. Indeed, the global minimum in the search corresponded to the same Pbca space group, with rms errors in the cell lengths of 3.7%. There is a larger energy gap separating the observed hydrogen bonding motif structure from alternative structures, with different hydrogen bonds and connectivity, for 5-azauracil than for 6-azauracil and uracil. (C) 1999 Elsevier Science B.V. All rights reserved.