The conversion of two‐stranded poly (A + U) to, three‐strand poly (A + 2U) and poly A by heat

It is shown that the formation of the three‐strand poly (A + 2U) complex is accompanied by a decrease in optical absorbance at 280 mμ, while the format ion of poly (A + U) is not. This difference makes it possible to demonstrate that in dilute solutions (∼10 −4 monomolar), a 1 : 1 mixture of poly A and poly U always forms poly (A + U) at room temperature. At higher temperatures in solutions containing more than about 0.2 M sodium ion, poly (A + U) is converted to poly (A + 2U) and free poly A. These results are consistent with the observations reported by Miles and Frazier, using infrared absorption spectra, at much higher polymer concentrations. It is concluded that most of the physicochemical studies of 1 : 1 mixtures of poly A and poly U in dilute aqueous solution refer to the two‐stranded species. A simple thermodynamic analysis is developed to explain the two‐ to three‐strand conversion, and with the use of a “phase diagram” showing conditions of stability of the various poly A–poly U structures as a function of salt and temperature, it is possible to estimate the value of the enthalpy of formation of poly (A + 2U).