Heat of the reaction forming the three‐stranded poly (A + 2U) complex

Abstract Calorimetric measurements of the heat of the addition of the second strand of poly U to poly (A + 2U) to form the three‐stranded poly (A + 2U) complex in 0.1 M and 0.5 M NaCl at 24 and 37°C. are reported. A value of Δ H = −3800 cal./mole of poly (A + 2U) formed was found to be fairly insensitive to the experimental conditions employed. The heat of the addition of the third strand to the preformed poly (A + U) helix is considerably less exothermic than the heat of reaction between poly A and poly U to form poly (A + U). The insensitivity of the heat of addition of the third strand (poly U) to changes in salt concentration and temperature lends qualitative support to the earlier hypothesis that the major portion of the variation of the Δ H of the poly A and poly U reaction with experimental conditions arises from differences in the conformation of poly A. Combining the information obtained in this study for the Δ H of the reaction forming poly (A + 2U) with data for the Δ H of the formation of poly (A + U) indicates that the conversion of poly (A + U) to poly (A + 2U) is opposed by an enthalpy change which increases with temperature. Extrapolation of these values to 52°C. where poly (A + U) is transformed to poly (A + 2U) in 0.5 M NaCl leads to a value of Δ H = +3800 cal./mole (A + 2U) formed and Δ S = 11.5 cal./mole of (A + 2U) formed. It is concluded from the calorimetric data that the driving force for the poly (A + U) to poly (A + 2U) conversion reaction is the favorable entropy change.