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-tri acetylguanosine (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 pi-pi-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 CD3CN. The results show that the activation energy (E.) 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: Delta H-+/- = 29 +/- 5 kJ mol(-1) and Delta S-+/- = 151 +/- 10 kJ mol(-1) K-1. These results are consistent with an associative mechanism for ligand exchange.