First-order phase transitions in multistranded macromolecules.

Abstract

The theory of the helix–coil equilibrium in an n-stranded macromolecule is investigated under restrictions analogous to the matching case discussed in the preceding paper for two-stranded molecules. The problem is found to be formally identical to the two-strand case by showing that the probability of a looped state is proportional to k−a, where k is the number of links in each branch of the loop and a = 3(n − 1) / 2 for Gaussian chains. It follows that a first-order phase transition occurs at a critical value of the stability constant for n ≥ 3. Data bearing on the helix–coil transition in the three-stranded protein collagen are examined and found to suggest that a first-order phase transition would indeed occur under equilibrium conditions. The three-stranded polynucleotide complex formed from one stand of polyriboadenylic acid and two strands of polyribouridylic acid shows an apparent second-order transition. This is interpreted to mean that mismatching of strands occurs in this material.