Crystallographic structure of an RNA helix: [U(UA)6A]2.

The crystallographic structure of the synthetic oligoribonucleotide, U(UA)6A, has been solved at 2.25 A resolution. The crystallographic refinement permitted the identification of 91 solvent molecules, with a final agreement factor of 13%. The molecule is a dimer of 14 base-pairs and shows the typical features of an A-type helix. However, the presence of two kinks causes a divergence from a straight helix. The observed deformation, which is stabilized by a few hydrogen bonds in the crystal packing, could be due to the relatively high (35 degrees C) temperature of crystallization. The complete analysis of the structure is presented. It includes the stacking geometries, the backbone conformation and the solvation.