Topography of nucleic acid helices in solutions. III. Interactions of spermine and spermidine derivatives with polyadenylic–polyuridylic and polyinosinic–polycytidylic acid helices

The synthesis of spermine derivatives (II), \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)_3 \mathop {\rm N}\limits^ + {\rm R}_{\rm 1} {\rm R}_{\rm 2} \left( {{\rm CH}_2 } \right)_2 ]_2 \cdot 4{\rm X}^ - $\end{document} , and spermidine derivatives (III), \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R}_1 {\rm R}_{\rm 2} {\rm R}_{\rm 3} \mathop {\rm N}\limits^ + \left( {{\rm CH}_2 } \right)_4 \mathop {\rm N}\limits^ + {\rm R}_{\rm 1} {\rm R}_{\rm 2} \left( {{\rm CH}_2 } \right)_3 \mathop {\rm N}\limits^ + {\rm R}_{\rm 1} {\rm R}_{\rm 2} {\rm R}_3 \cdot 3{\rm X}^ - $\end{document} , are reported. The effects of these salts on the helix–coil transition of rA–rU and rI–rC helices were examined. Increasing the size of the hydrophobic substituents, R 1 , R 2 , and R 3 lowers the degree of stabilization of the helical structure. The disproportionation reaction, 2rA–rU→rA–rU 2 + rA occurs readily with salts II and III, especially when the substituents, R 1 , R 2 , and R 3 are small, i.e., H or Me. Spermine is found to stabilize the rA–rU 2 and rI–rC helices to approximately the same extent; however, large differences between the degree of stabilization of rA–rU 2 and rI‐rC helices are observed when the substituents R 1 , R 2 , and R 3 are large hydrophobic groups. Similar results are also obtained for the spermidine series. Finally, differences in the interactions of the salts II and III with rA–rU 2 and rI–rC helices suggest that the latter helix is denser.