Interaction between polynucleotides in aqueous NaCl solution, 4. Poly(riboadenylic acid) — poly(2-riboinosinic acid) system

Solution properties of poly(A) and poly(I) solution at a mole ratio of poly(A) to poly(I) of 1/2 were studied by means of calorimetry and a modified differential scanning calorimetry (DSC) with the help of circular dichroism (CD) and ultraviolet (UV) spectral methods. It was found that the heat of mixing, ΔH^M, depends sharply on the ionic strength. At [NaCl] < 10^?2 mol · dm^?3 ΔH^M is nearly zero. At higher ionic strength ΔH^M decreases and reaches a minimum value at [NaCl] ≈? 2 · 10^?2 mol ? dm^?3. Above this ionic strength ΔH^M increases again. This behavior is explained on the basis of the formation of a triplex poly(A) · poly(2I) and the transition of poly(I) from a disordered to an ordered structure with increasing ionic strength. The heat of formation, ΔH, of the poly(A) · poly(21) triplex was estimated to be ca ?41 kJ per mole of base triplet. Above an NaCl concentration of 2 · 10^?1 mol · dm^?3 the triplex formation may be represented by the following scheme: