Study of salt effects on adenosine-polyuridylic acid interaction

Complex formation between poly(U) and adenosine in solutions of salts that stabilize (Na2SO4), destabilize (NaClO4), or have little effect on the water structure (NaCl), as well as the poly(U)·poly(A) interaction in NaClO4, was studied by equilibrium dialysis and uv spectroscopy. At 3°C and neutral pH, Ado·2 poly(U) is formed in 1M NaCl and 0.33M Na2SO4. In NaClO4 solutions under the same conditions, an Ado·poly(U) was found over the whole range of salt concentration investigated (10 mM−1M), which has not been previously observed under any conditions. The Ado‐poly(U) was also found in a NaCl/NaClO4 mixture, the transition from the triple‐ to the double‐helical complex occurring within a narrow range of concentration of added NaClO4. In the presence of 1M NaCl this transition is observed on adding as little as 10 mM NaClO4, i.e., at a [ClO 4−]/[Cl−] ratio of about 1:100. However, when NaClO4 is added to a 1M solution of the stabilizing salt Na2SO4, no transition occurs even at a [ClO 4−]/[SO 42−] ratio of 1:4. Investigation of melting curves and uv spectra has shown that in an equimolar mixture of the polynucleotides, only a double‐helical poly(U)·poly(A) exists in 1M NaClO4 at low temperatures; this also holds for 1M NaCl. This changes to a triple‐helical 2 poly(U)·poly(A) and then dissociates as the temperature increases. At low temperatures and the poly(U)/poly(A) concentration ratio of 2:1, a mixture of 2 poly(U)·poly(A) and poly(U)·poly(A) was observed in 1M NaClO4, in contrast to the case of 1M NaCl. Thus, sodium perchlorate, a strong destabilizer of water structure, promotes formation of double‐helical complexes both in the polynucleotide–monomer and the polynucleotide–polynucleotide systems. Beginning with a sufficiently high ionic strength (μ ≃ 0.9), a further increase in the salt molarity results in an increase of the poly(U)·adenosine melting temperature in both stabilizing and neutral salts and a decrease in the destabilizing salt. In Na2SO4 concentrations higher than 1.2M Ado·2 poly(U) precipitates at room temperature. Analysis of the binding isotherms and melting profiles of the complexes between poly(U) and adenosine according to Hill's model shows that the cooperativity of binding, due to adenosine stacking on poly(U), increases in the order NaClO4 < NaCl < Na2SO4. The free energy of adenosine stacking on the template is similar to that of hydrogen bonding between adenosine and poly(U) and ranges from −1 to −2 kcal/mol. The values of ΔHt [th