Self‐Assembly of N 2 ‐Modified Guanosine Derivatives: Formation of Discrete G‐Octamers

In the presence of Na + ions, two N 2 ‐modified guanosine derivatives, N 2 ‐(4‐ n ‐butylphenyl)‐2′,3′,5′‐ O ‐triacetylguanosine ( G1 ) and N 2 ‐(4‐pyrenylphenyl)‐2′,3′,5′‐ O ‐triacetylguanosine ( G2 ), are found to self‐associate into discrete octamers that contain two G‐quartets and a central ion. In each octamer, all eight guanosine molecules are in a syn conformation and the two G‐quartets are stacked in a tail‐to‐tail fashion. On the basis of NMR spectroscopic evidence, we hypothesize that the π–π‐stacking interaction between the N 2 ‐side arms (phenyl in G1 and pyrenyl in G2 ) can considerably stabilize the octamer structure. For G1 , we have used NMR spectroscopic saturation‐transfer experiments to monitor the kinetic ligand exchange process between monomers and octamers in CD 3 CN. The results show that the activation energy ( E a ) of the ligand exchange process is 31 ±5 kJ mol −1 . An Eyring analysis of the saturation transfer data yields the enthalpy and entropy of activation for the transition state: Δ H ≠ =29 ±5 kJ mol −1 and Δ S ≠ =−151 ±10 J mol −1  K −1 . These results are consistent with an associative mechanism for ligand exchange.