Electrostatic effects on polynucleotide transitions. II. Behavior of titrated systems

The theory developed in the previous paper to discuss changes in electrostatic free energies in polynucleotide order–disorder transitions is extended to cases where one or more of the participating species is titrated to some degree α. It is shown that, for any class of transition, the melting temperature T m at constant pH is a linear function of the logarithm of the monovalent counterion concentration M , that at high salt the logarithm of the depression of the melting temperature by pH titration is proportional to the pH change, and that the stability of the ordered form as measured by its melting temperature at neutral pH, is a monotonic function of the quantity pH m – p K , where pH m and p K are the pH of melting and the monomer base p K , both measured under similar conditions of temperature and ionic strength. For the transition from double helix to coil, the dependences of T m and dT m /d log M on pH are determined experimentally and compared with the qualitative predictions of the theory. It is found that dT m /dlog M , a measure of – Δ F̄ el (the negative of the electrostatic free energy change in the transition), decreases with increasing pH. In acid solution, where the coil is more extensively prolonated than the helix, the change in electrostatic free energy in the transition is larger than at neutral pH. Conversely, in alkali the electrostatic five energy change is smaller than at neutral pH. Hence ( dT m / d log M ) acid > ( dT m / d log M neutral ) > ( dT m / d log M ) alkali . At Suffeciently high pH, dT m / d log M is observed to become negative, indicating that the electrostatic free energy change is positive in the transition of this region. Date from the literature on the ionic strength dependence of the melting temperature for the acid helices of poly rA, poly rC, and poly dC are also considered from the standpoint of the theory.