Heats of mixing of polyriboadenylic acid and polyribouridylic acid in aqueous NaCl solution

The heats of mixing of polyriboadenylic acid (Poly A) and polyribouridylic acid (Poly U) in various proportions by changing the mole ratio n = [Poly U]/[Poly A] in 0,001, 0,1, and 0,5 molar NaCl solutions containing 0,1 molar tris‐HCl buffer solution were measured. Constant values for the heats of mixing of the (Poly A+ n Poly U) system are found for n ≤ 1 and n ≥ 2. From the results of UV and CD spectra, it may be considered that the mixing of Poly A and n (Poly U) leads to the formation of Poly(A+U) for n ≤ 1, and to that of Poly(A+2U) for n ≥ 2, even at [NaCl] = 0,001 mol/l. The enthalpies of formation of Poly(A+U) and that of Poly(A+2U) depend on the concentration of NaCl. This dependence may be attributed to the differences in conformation of Poly(A+U) in NaCl solution.