A study of the interaction of oligonucleotides with polynucleotides by the method of equilibrium gel filtration. The effect of magnesium ions and the composition of the oligonucleotides on the stability of the complex.

The interaction of the oligonucleotides ApApA, ApApU and ApApC with polyribouridylic acid and of hexariboinosinic acid with and without phosphate near the 3'-end with polyribocytidylic acid was studied by the method of equilibrium gel filtration through Sephadexes. The free energy, the energy and entropy of the complexing were calculated from isotherms of the adsorption of the oligonucleotides on the polynucleotides in relation to the composition of the oligonucleotides and the concentration of magnesium ions in the buffer. It was shown that in the case of the interaction of ApApA and ApApC with poly(U) a perfect triple complex is formed, while in the case of the interaction of ApApU with poly(U) the noncomplementary base is partially displaced from the complex. The free energy of the interaction of neighboring oligonucleotides in a complex with polynucleotides is from --1000 to --3000 cal/mole depending on the type of the complex. It was shown that after the interaction of hexainosinic acid with poly(C) a double complex is formed. The free energy of the interaction of neighboring oligonucleotides in this complex is formed. The free energy of the interaction of neighboring oligonucleotides in this complex is about --1200 cal/mole. Magnesium ions have a different effect on the formation of triple and double complexes in the interaction of oligonucleotides with polynucleotides.