Complexes of poly(adenylic acid) with complementary monomers

Abstract

The interaction of a number of potentially complementary monomers with poly(A) has been investigated by equilibrium dialysis, optical rotatory dispersion and ultraviolet absorption measurements. Experiments were conducted at pH 7.0, 0.15 M Na+, where poly(A) exists as a random coil with some degree of base-stacking, and at pH 6.0, 0.15 M Na+, where poly(A) adopts the protonated double-helical acid form structure below 15 degrees C. Binding isotherms show that, at 3.5 degrees C, poly(A) forms a 1 : 1 complex with xanthine at pH 6, and a 2:1 complex at pH 7, while oxoformycin forms a 1:1 complex with poly(A) at both pH 6 and pH 7. Poly(A) forms a complex, tentatively assigned 1:1 stoichiometry, with 8-azaxanthine at pH 6, but no complexing occurs at pH 7. The complexes have been characterized by their optical rotatory dispersion and ultraviolet spectra, their thermal stabilities, and their rates of formation at low temperature. All the complexes are laevorotatory at long wavelengths (greater than 300 nm) and unplex formation at low temperature is a slow process requiring many hours for completion. The complexes of poly(A) with 3-methylxanthine have been reinvestigated and shown to undergo normal helix-coil transitions; the anomalous melting behaviour noted previously [Biopolymers, 10, 21 -- 33 (1971)] has been explained. From a comparison of optical rotatory dispersion spectra, it is concluded that the poly(A) with 3-methylxanthine have similar structures, which are quite different from the structures of the corresponding complexes with 7-methylxanthine. The structures and properties of the poly(A) - monomer complexes are discussed, and compared with those of other polynucleotide - monomer complexes. No significant interaction was observed between poly(A) and hypoxanthine, allopurinol, 6,8-dihydroxypurine, 1-methylxanthine, 9-methylxanthine, theophylline, theobromine or 3,9-dimethylxanthine.