Synopsis
Complex formation between poly(U) and adenosine in solutions of salts that stabilize (NazSOd), destabilize (NaC104), or have little effect on the water structure (NaCl), as well as the poly(U)-poly(A) interaction in NaC104, 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 NazS04. In NaC104 solutions under the same conditions, an Ado.poly(U) was found over the whole range of salt concentration investigated (10 mM-lM), which has not been previously observed under any conditions. The Ado.poly(U) was also found in a NaCl/NaC104 mixture, the transition from the triple-to the double-helical complex occurring within a narrow range of concentration of added NaC104. In the presence of 1M NaCl this transition is observed on adding as little as 10 mM NaC104, i.e., at a [ClO;]/[Cl-] ratio of about 1:lOO. However, when NaC104 is added to a 1 M solution of the stabilizing salt NazS04, no transition occurs even a t a [ClO;]/[SOZ-] 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 NaC104 a t low temperatures; this also holds for 1 M 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 NaC104, in contrast to the case of 1M NaCI. Thus, sodium perchlorate, a strong destabilizer of water structure, promotes formation of double-helical complexes both in the polynucleotide-monomer and the polynucleotidepolynucleotide systems.
Beginning with a sufficiently high ionic strength ( p N 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 NazS04 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 NaC104 < NaCl < NazS04. 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 AHt [the effective enthalpy of adenosine binding to poly(U) next to an occupied site, obtained from the relationship between complex melting temperature and free monomer concentration at the midpoint of the transition] are -14.2, -18.3, and -16.8 kcal/mol for 1 M solutions of NaC104, NaCI, and NazS04, respectively. The results indicate that the effects of anions of the salts studied are related to water structure alterations rather than to their direct interaction with the complexes between poly(U) and adenosine.