Using diffusion NMR to characterize guanosine self-association: Insights into structure and mechanism

This paper presents results from a series of pulsed field gradient (PFG) NMR studies on lipophilic guanosine nucleosides that undergo cation-templated assembly in organic solvents. The use of PFG-NMR to measure diffusion coefficients for the different aggregates allowed us to observe the influences of cation, solvent and anion on the self-assembly process. Three case studies are presented. In the first study, diffusion NMR confirmed formation of a hexadecameric G-quadruplex [G1](16).4K(+).4pic(-) in CD3CN. Furthermore, hexadecamer formation from 5'-TBDMS-2',3'-isopropylidene G1 and K+ picrate was shown to be a cooperative process in CD3CN. In the second study, diffusion NMR studies on 5'-(3,5-bis(methoxy)benzoyl)-2',3'-isopropylidene G4 showed that hierarchical self-association of G8-octamers is controlled by the K+ cation. Evidence for formation of both discrete G8-octamers and G(16)-hexadecamers in CD2Cl2 was obtained. The position of this octamer-hexadecamer equilibrium was shown to depend on the K+ concentration. In the third case, diffusion NMR was used to determine the size of a guanosine self-assembly where NMR signal integration was ambiguous. Thus, both diffusion NMR and ESI-MS show that 5'-O-acetyl-2',3'-O-isopropylidene G7 and Na+ picrate form a doubly charged octamer [G7](8)-2Na(+).2pic(-) 9 in CD2Cl2. The anion's role in stabilizing this particular complex is discussed. In all three cases the information gained from the diffusion NMR technique enabled us to better understand the self-assembly processes, especially regarding the roles of cation, anion and solvent.