Nonlinear effects and thermal expansion as expressed in self-consistent phonon calculations on the temperature dependence of a phase change: Application to the B to Z conformation change in DNA

The temperature and salt concentration dependence of the B to Z conformation phase change observed in some DNA polymers is calculated using the modified self-consistent phonon approximation theory. The principal modification from more standard self-consistent phonon theory is the incorporation of thermal expansion in the initial determination of the effective force constant. It is this modification that has allowed application of the method to melting temperatures and led to the theory of DNA melting. The temperature dependence of the B to Z transition is shown to depend entirely on the incorporation of the thermal expansion into the theory. The excellent agreement between the predictions of the modified theory with observation indicates the importance of including thermal expansion effects.