Topography of nucleic acid helices in solutions. 3. 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, R¹, R², and R³ lowers the degree of stabilization of the helical structure. The disproportionation reaction, 2rA–rU→rA–rU₂ + rA occurs readily with salts II and III, especially when the substituents, R₁, R₂, and R₃ are small, i.e., H or Me. Spermine is found to stabilize the rA–rU² and rI–rC helices to approximately the same extent; however, large differences between the degree of stabilization of rA–rU₂ and rI-rC helices are observed when the substituents R₁, R₂, and R₃ 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–rU₂ and rI–rC helices suggest that the latter helix is denser.