High-resolution structures of DNA fragments determined using X-ray crystallography or NMR have provided descriptions of a veritable alphabet of conformations. They have also shown that DNA is a flexible molecule, with some sequences capable of adopting two different structures. Here, the first example is presented of a DNA fragment that can assume three different and distinct conformations in crystals. The decanucleotide d(GCATGCATGC) was previously reported to assume a single-stranded double-fold structure. In one of the two crystal structures described here the decamer assumes both the double-fold conformation and, simultaneously, the more conventional B-type double-helical structure. In the other crystal the sequence assumes the A-type double-helical conformation. These results, taken together with CD spectra, which were recorded as the decamer was titrated against four metal ions and spermine, indicate that the molecule may exist as a mixed population of structures in solution. Small differences in the environmental conditions, such as the concentration of metal ion, may decide which of these crystallizes out. The results also support the idea that it may be possible for DNA to change its structure to suit the binding requirements of proteins or drugs.

The title compound, C18H18F2N2O3, crystallizes with two independent molecules (A and B) in the asymmetric unit. They differ essentially in the orientation of the pyridine ring with respect to the benzene ring; these two rings are inclined to each other by 53.3 (2)° in molecule A and by 72.9 (2)° in molecule B. The 3-(cyclopropylmethoxy) side chain has an extended conformation in both molecules. The two molecules are linked by a pair of C—H...O hydrogen bonds and two C—H...π interactions, forming an A–B unit. In the crystal, this unit is linked by N—H...O hydrogen bonds, forming a zigzag –A–B–A–B– chain along [001]. The chains are linked by C—H...N and C—H...F hydrogen bonds to form layers parallel to the ac plane. Finally, the layers are linked by a third C—H...π interaction, forming a three-dimensional structure. The major contributions to the Hirshfeld surface are those due to H...H contacts (39.7%), followed by F...H/H...F contacts (19.2%).