Alkyl substituent in place of the thymine methyl group controls the A-X conformational bimorphism in poly(dA-dT).

Circular dichroism studies of a family of poly(dA-y5dU) polynucleotides (y = H, methyl, ethyl, propyl, butyl or pentyl) were conducted in water-alcohol solutions containing sodium or cesium counterions. The polynucleotides denatured or adopted A- or X-DNA double helices depending on the concentration and type of alcohol, type of counterions and the length of the aliphatic substituent in place of the thymine methyl group. Short aliphatic substituents and sodium cations favored A-DNA while long aliphatic substituents and cesium cations promoted X-DNA. This study demonstrates delicacy of the conformational equilibrium of poly(dA-dT) between the A- and X-DNA double helices which depends on both intramolecular and intermolecular factors.