Self-assembly of N2-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 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 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: DeltaH(not =)=29 +/-5 kJ mol(-1) and DeltaS(not =)=-151 +/-10 J mol(-1) K(-1). These results are consistent with an associative mechanism for ligand exchange.
Department of Chemistry, Queen's University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada.
Sanela
S
Martić
Xiangyang
X
Liu
Suning
S
Wang
Gang
G
Wu
D018360:Crystallography, X-Ray; D006151:Guanosine / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy
D008024:Ligands; C520048:N2-(4-n-butylphenyl)-2',3',5'-O-triacetylguanosine; D006151:Guanosine