Topography of nucleic acid helices in solutions. IV. Effect of polyamines on RNase‐catalyzed hydrolysis of polyadenylic acid

The effect of polyamines on the ribonuclease‐catalyzed hydrolysis of polyriboadenylic acid, cytidine‐2′,3′‐cyclic phosphate, and cytidylyl‐3′,5′‐adenosine phosphate (CpA) is reported. It is found that the salts, (I) \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm H}_3 \mathop {\rm N}\limits^ + \left( {{\rm CH}_2 } \right)_n \mathop {\rm N}\limits^ + {\rm H}_3 \cdot 2{\rm Br}^ - $\end{document} inhibit the rate of hydrolysis of polyadenylic acid, whereas the hydrolysis of cytidine‐2′,3′‐cyclic phosphate, and CpA is accelerated. It is concluded that polyamines inhibit the RNase‐catalyzed hydrolysis of polyadenylic acid by binding to adjacent phosphate anions of the polynucleotide. The effect of other polyamines, i.e., the substituted salts (II) \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm RMe}_2 \mathop {\rm N}\limits^ + \left( {{\rm CH}_2 } \right)_n \mathop {\rm N}\limits^ + {\rm Me}_2 {\rm R} \cdot 2{\rm Br}^ - $\end{document} (where R = Me, Et, Pr, Bu, Pen, and —CH2CH2OH), (III) \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm Et}_3 \mathop {\rm N}\limits^ + \left( {{\rm CH}_2 } \right)_n \mathop {\rm N}\limits^ + {\rm Et}_3 \cdot 2{\rm Br}^ - $\end{document}, spermidine, and spermine, on the RNase‐catalyzed hydrolysis of polyadenylic acid was studied at 37 and 50°C. At 37°C., maximum inhibition of the hydrolytic reaction occurs at n = 4 for the salts I and II. At 50°C., maximum inhibition of the RNase‐catalyzed hydrolysis of polyadenylic acid occurs at n = 3 for the unsubstituted salts I and the salts II where R = Et, and shifts to n = 2 for the salts II where R = Pr. Increasing the size of the substituent R of the salts, II, enhances the degree of inhibition. Substituting —CH2CH2OH for Et or Pr of the salts, II, decreases the extent of inhibition. Spermine and spermidine interact at very low concentrations with polyadenylic acid, causing significant inhibition of the RNase reaction. The result suggests that they interact with four and three adjacent negative charges of the polynucleotide, respectively. It is concluded that the salts, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R}_1 {\rm R}_{\rm 2} {\rm R}_{\rm 3} \mathop {\rm N}\limits^ + \left( {{\rm CH}_2 } \right)_n {\rm NR}_{\rm 1} {\rm R}_{\rm 2} {\rm R}_{\rm 3} \cdot 2{\rm Br}^ - $\end{document} interact with polyadenylic acid by electrostatic, hydrogen bonding, and hydro‐phobic forces. The electrostatic and hydrogen‐bonding interactions are important for binding the salts where R1R2 and R3 are small, i.e., H. Hydrophobic interactions are very important in binding the highly substitutde salts (II and III). The above results are compared with those obtained from the melting of rA–rU and rA–rU2 in the presence of the salts I and II. The following conclusions may be made: (1) the surface of a multistranded helix is more polar than that of the single strand; (2) the Tm of a helix–coil transition in the presence of the diammonium salts depends mainly on the interaction of the latter with the helix; (3) the overall structure of polyadenylic acid at pH 6.20 does not appear to change significantly upon forming the multistranded helices, rA–rU and rA–rU2.