Calorimetric studies of the polynucleotide analog poly(-)2-[2-(thymin-1-yl)propanamido]propenoic acid.

The thermally elicited structural and conformational transitions of the polynucleotide analog, poly(-)-2-[2-(thymin-l-yl)propanamido]propenoic acid, P(-YTDHA, have been studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermogravmetric analysis (TGA). The differential scanning calorimetry curves obtained on solid P(-)TDHA samples exhibited five distinct transitions. The transition occurring at 50°C is attributed to a disruption of interactions involving thymine-thymine stacking. In contrast the transition observed at 83°C is attributed to a hydrogenbonding interaction involving the thymine residues, whereas the transition occurring at 110°C is assigned to hydrogen bonding of the carboxylic acid sidechain groups. The transitions observed at 50, 83, and 110°C are reversible if the heated and quenched sample is allowed to equilibrate in an atmosphere of high humidity. The transition occurring at 127°C is viewed as a structural rearrangement of the polymer backbone that does not involve the participation of water molecules. The transition observed at a temperature above 197°C is attributed to a structural modification of the polymer resulting from decomposition. Solutions of P(-)TDHA in 0.1M phosphate buffer a t pH 7.05 showed only a single transition at 50"C, which is in accord with an observed transition in the solid state assigned to the disruption of base-stacking interactions. The average enthalpy for the transition at 50°C was 0.92 cal/g in the solid state and 1.08 cal/g for the solution, which provides additional support for the assignment.