ONE of the most important problems in research on nucleic acid is the understanding of the forces which maintain the helical structure. After the elucidation of the double helix by Watson and Crick1, the helical secondary structure of DNA was thought to derive its stability from hydrogen bonding between the base pair. The discovery that polynucleotides could exist in single strand ordered helical conformation2–6 demonstrated that other types of forces are involved in the stability of this structure. These stacking forces between parallel bases were thought to be of the Van der Waals type. More recent work shows that the presence, absence or modification of the 2′ hydroxyl group plays an important part in the conformational stability of the oligo and poly-nucleotides7–12.