Effects of torsional strain on thermal denaturation of DNA

A class of simple statistical mechanical models for DNA melting, first proposed by Poland and Scheraga, has been demonstrated to exhibit a first or second order thermodynamic singularity, notwithstanding the intrinsic one-dimensional nature of the problem. In the present paper we consider the case of circular DNA and show that the inclusion of twist elastic energy in the Poland-Scheraga models leads either to suppression of the thermodynamic singularity or to a weak, third order singularity. Such behavior may also be present in linear DNA under mechanical influences that preclude the release of torsional strain.