Abstract
The enthalpies of the helix‐coil transitions of the ordered polynucleotide systems of poly(inosinic acid)–poly(cytidylic acid) [poly(I + C)], (helical duplex), and of poly (inosinic acid) [poly(I + I + I)], (proposed secondary structure: a triple‐stranded helical complex), were determined by using an adiabatic twin‐vessel differential calorimeter. Measuring the temperature course of the heat capacity of the aqueous polymer solutions, the enthalpy values for the dissociation of the helical duplex poly (I + C) and the three‐stranded helical complex poly(I + 1 + 1), respectively, were obtained by evaluating the additional heat capacity involved in the conformational change of the polynucleotide system in the transition range. The Δ__H__ values of the helix‐coil transition of poly (I + C) resulting from the analysis of the calorimetric measurements vary between the limits 6.5 ± 0.4 kcal/mole (I + C) and 8.4 ± 0.4 kcal/mole (I + C). depending on the variation of the cation concentration ranging from 0.063 mole cations kg H~2~O to 1.003 mole cations/kg H~2~O. The calorimetric investigation of an aqueous poly I solution (cation concentration 1.0 mole/kg H~2~O) yielded the enthalpy value Δ__H__ = 1.9 ± 0.4 kcal/mole (I), a result which has been interpreted qualitatively following current models of inter‐ and intramolecular forces of biologically significant macromolecules. Additional information on the transition behavior of poly(I+ C)Was obtained by ultraviolet and infrared absorption measurements.