Abstract
The Zimm‐Bragg theory is extended to treat the melting of the triple helix poly (A + 2U) for a solution with a 1 : 2 mole ratio of poly A to poly U. Only the case for long chains is considered. For a given set of parameters the theory predicts the fraction of segments in the triple helix, double helix, and random coil states as a function of temperature.
Four nucleation parameters are introduced to describe the two order–disorder transitions (poly (A + 2U) ⇄ poly A + 2 poly U and poly (A + U) ⇄ poly A + poly U) and the single order–order transition (poly (A + 2U) ⇄ poly (A + U) + poly U). A relation between the nucleation parameters is obtained which reduces the number of independent parameters to three. A method for determining these parameters from experiment is presented. From the previously published data of Blake, Massoulié and Fresco^8^ for [Na^+^] = 0.04, we find σ~T~ = 6.0 × 10^−4^, σ~D~ = 1.0 × 10^−3^, and σσ* = 1.5 × 10^−3^. σ~T~ and σ~D~ are the nucleation parameters for nucleating a triple helix and double helix, respectively, from a random coil region. σσ* is the nucleation parameter for nucleating a triple helix from a double helix and a single strand.
Melting curves are generated from the theory and compared with the experimental melting curves.