A calorimetric study of the helix-coil transition of poly(cytidylic acid) in acid solution.

In this study we show how the thermal stability of the helical structure of poly(C) is dependent on the degree of protonation. deltaH values for the thermal denaturation are measured directly by means of an adiabatic microcalorimeter in the pH range from pH 5.8 to 3.7. The maximum transition temperature T-m is found at pH 4.5 where poly(C) is semiprotonated. At the same pH value a maximum value of deltaH (deltaH equals 5.27 Kcal/mol base pairs) was obtained. deltaH decreases slightly if the pH changes to lower or higher pH values. No transition was observed either at pH 6.0 or at pH 3.4. The deltaGtheta(25 degrees C) values show a similar dependence on the degree of protonation of poly(C).