On the Helix—Coil Transition in Heterogeneous Polymers

A general theory for the helix—coil transition of block copolymers of regularly alternating sequence, applicable in principle to both nucleic acids and polypeptides, is developed. The stability of helical regions of the molecule is permitted to vary depending on the nature of the monomers in that section. Calculations are reported for DNA-like molecules. The approximation is then made that a molecule of random sequence can be treated as a linear combination of block copolymers of varying block size, and on this basis the average length of correlations in the state of neighboring base pairs in a natural DNA which is undergoing transition is estimated to be about 1400 base pairs. An approximate model for DNA melting developed on these grounds yields a transition curve in satisfactory agreement with experiment.