Solvation of the left-handed hexamer d(5BrC-G-5BrC-G-5 BrC-G) in crystals grown at two temperatures.

ABSTRACT

Crystals of the hexadeoxyoligomer d(5BrC-G-5BrC-G-5BrC-G) were grown at different temperatures (5 degrees C, 18 degrees C and 37 degrees C) in the absence of divalent cations. The crystals grown at 5 degrees C did not diffract X-rays, while those grown at 18 degrees C and 37 degrees C did. The oligomer adopts the left-handed ZI conformation in both crystals. The main difference resides in a more extensive hydration shell in the crystal grown at high temperature than in the crystal grown at low temperature. The high-temperature crystal displays a spine of hydration running deep in the minor groove and linking exocyclic O-2 atoms of the pyrimidine rings. In both crystalline forms, a hydrated sodium ion bound to the N-7 of a guanine ring was found. Strings of water molecules bridging phosphate anionic oxygen atoms are found along the backbone. The absolute values of the propeller-twist are also different in both structures although the values of the twist are very similar. The results point to the importance of the crystallization conditions when analysing fine structural details like solvation properties of oligomers.