Cooperative nonenzymic base recognition. Thermodynamics of the helix—coil transition of a monomer—polymer double helix

The double‐helical complex formed from m 6 m 9 A and poly U has been characterized by circular dichroism and u.v. spectrophotometry. The circular dichroism of the complex is similar to that of the double‐helical poly A poly U complex both in shape and in magnitude and thus indicates a quite similar structure. The double Helix–coil transition has been studied at various nucleotide concentrations and at three different ionic strengths. As expected for the binding of a base to a polymer, the Helix–coil transition is shifted to higher temperatures by increasing nucleotide concentrations, but is not affected by changes of the ionic strength. The melting curves are analyzed according to a linear Ising model taking the stacking of the monomers into account. At 0°C the equilibrium constant for nucleation is found to be 2–5 M −1 and that for chain growth is 500 M −1 . The enthalpy change associated with chain growth is −11.2 ± 1 kcal M −1 .