CRYSTAL PACKING EFFECTS ON A-DNA HELIX PARAMETERS - A COMPARATIVE-STUDY OF THE ISOFORMS OF THE TETRAGONAL AND HEXAGONAL FAMILY OF OCTAMERS WITH DIFFERING BASE SEQUENCES

The helix and base pair parameters of A-DNA octamers have been compare d having different base sequences both in the tetragonal and hexagonal crystal systems. For the eight structures in the tetragonal family, t he twist, rise, slide, inclination and tilt are essentially the same, influenced by the similarity in crystal packing. The propeller twist a nd the base pair buckle display small sequence dependent variations. B ut the base pair roll appears to be changed by the specific intermolec ular hydrogen bonding interactions. For four of the five octamer struc tures in the hexagonal family, the base pair rise, slide, inclination, tilt, as well as the propeller twist and buckle are all very similar, while the twist angle and the base pair roll are not The intermolecul ar hydrogen bonding interactions seem to be primarily responsible for the differences in the roll angle in the tetragonal structures but for both the roll and twist angles in the hexagonal structures. These res ults demonstrate that the majority of the observed helix base pair par ameters for A-DNA octamers in crystals are affected by the crystal pac king environment, while a few parameters, like propeller twist and bas e pair buckle display some base sequence dependence.