Base pairing equilibria between polynucleotides and complementary monomers

Equilibrium dialysis measurements and optical melting curve data have been used to study the formation and stability of a number of complexes between polynucleotides and complementary monomers. The cooperativity parameter, (dθ/d ln c)θ = 0.5, where θ is the fraction of U or C residues complexed, and c is the concentration of free monomer has been measured as 1.4 for the 2:1 poly U:d‐adenosine‐complex, and 2.05 for the 2:1 poly C:d‐guanosiue complex at pH 7. The variation of Tm with c for several complexes has been used to calculate their partial molar enthalpies of formation at the midpoint of the transition: in 1.0 MNa + at pH 7, for the 2:1 complex of poly‐U with 2‐amino‐adenine, this is − 18.7 kcal/mole of 2‐amino‐adenine, for poly‐U with adenosine it is − 18.7 kcal/ mole; for poly‐C with dG, it is − 16.8 kcal/mole. These results do not agree very well with calorimetric integral heats of reaction reported in the literature.33 Complexes with random copolymers were also studied. The random copolymer, poly‐UC, can form a mixed complex with dG and either dA or 2‐amino‐adenosine; the binding of dG is enhanced by an adenine derivative and vice versa.Similarly, poly AC can form a mixed complex with dG and 3‐methyl‐xanthine. In each case, it appears that the ideal composition is a 2:1 hydrogen‐bonded complex, but the actual stoichiometry is such that each base on the random polynucleotide binds less than one‐half of a molecule of its complementary monomer. Poly UG can bind dG and dA, but in a less cooperative and specific way.