Phase-Transfer Theory and Its Application to Long Chain Molecules: DNA

Phase-transfer theory is applied to study two unrelated phenomena in DNA molecule: (a) the electronic structure of DNA and (b) the helix–coil transition, also known as denaturation. It is shown from theory that some spectral lines and gaps are always there in all DNA molecules irrespective to the arrangements of the four fundamental groups: adenine (A), thymine (T), guanine (G), and cytosine (C). This explains the fact that optical spectroscopy shows a lot of over-all similarities in the various DNA molecules. To study the helix–coil transition, the usual Ising model is used with periodic boundary condition at the two infinite ends of the molecule. Exact and explicit expressions for the transitions are obtained for any periodically arranged molecule. An exact expression can also be written down for the random molecule, if the order of the A–T and G–C pairs can be specified. The width of the transition is shown to be the maximum in the 50% A–T pairs and 50% G–C pairs. Strong interactions between the neighboring pairs are indicated by the fact that steplike transition curves are not observed in experiments.