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. Copyright © 1968 John Wiley & Sons, Inc.