Pii: 0014-5793(69)80196-1

Thermal denaturation of protonated polynucleotides is accompanied by a change in protonation [l-5] . The thermal transition at a given pH can thus be directly related to the pk’ of the proton-induced structural transition at a given temperature. For poly (A’).poly (A+) a strict relationship has been shown to hold over a large range of conditions [4-61. In fig. 1 the results in the literature and those obtained recently in this laboratory [7] are plotted. Although the thermodynamics of denaturation of helical polynucleotides has been treated extensively [5,8-131 yielding rather simple expressions, the transitions of protonated polynucleotide structures [5,9,1 l] showed rather complex behaviour. This is due to the fact that not only the electrostatic contribution of the phosphate-phosphate repulsion has to be considered, but also the ion pair attraction. In contrast to poly (C+).poly (C) which can be either deprotonated or protonated upon thermal denaturation depending on the conditions [3] the thermal denaturation of the acid double helical form of poly A is always accompanied by deprotonation. The reaction can be written