Thermodynamic Instability in DNA

The exact Hill's treatment of the Ising model describing DNA has been developed to make it applicable to the study of environment-coupled instabilities in DNA. Thermodynamic properties of DNA close to a co-operative order-disorder melting point have been investigated in terms of the developed Ising model. Assuming a weak coupling between the environment and Watson-Crick hydrogen bonds in DNA, it has been shown that the partition function can be broken down into a product of an environmental part, random coil part, and a helix part, the last one being dependent on w^/kT and waP/kT only, where H £ and wnls are DNA energy melting parameters. If the energy parameters depend on the volume V only, then the specific heat at a constant volume Cv.p tends to approach very large values along the melting curve; however as may be deduced, the Ising-DNA model is unstable in the immediate neighbourhood of its melting point and undergoes denaturation. A suitable experimental measure for the stability of the native double-helical structure of DNA was formulated. Equations were constructed which permit the prediction of the typical thermodynamic behaviour of helix-coil transition under weak interactions with the environment. Instability in DNA has been shown to occur very close to the melting curve only, and Cv,n>0 (thermal stability) and the isothermal compressibility j3e > 0, j3a > 0 (mechanical stability) — are all positive definite quantities, may be expected to parallel each other much from the melting point.