title: Using diffusion NMR to characterize guanosine self-association: insights into structure and mechanism.
abstract: 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 [G 1](16)4 K(+)4 pic(-) in CD(3)CN. Furthermore, hexadecamer formation from 5'-TBDMS-2',3'-isopropylidene G 1 and K(+) picrate was shown to be a cooperative process in CD(3)CN. In the second study, diffusion NMR studies on 5'-(3,5-bis(methoxy)benzoyl)-2',3'-isopropylidene G 4 showed that hierarchical self-association of G(8)-octamers is controlled by the K(+) cation. Evidence for formation of both discrete G(8)-octamers and G(16)-hexadecamers in CD(2)Cl(2) 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 G 7 and Na(+) picrate form a doubly charged octamer [G 7](8)2 Na(+)2 pic(-) 9 in CD(2)Cl(2). 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA.
authors.forename: Mark S
authors.initials: MS
authors.lastname: Kaucher
authors.forename: Yui-Fai
authors.initials: YF
authors.lastname: Lam
authors.forename: Silvia
authors.initials: S
authors.lastname: Pieraccini
authors.forename: Giovanni
authors.initials: G
authors.lastname: Gottarelli
authors.forename: Jeffery T
authors.initials: JT
authors.lastname: Davis
mesh_terms: D002942:Circular Dichroism; D006151:Guanosine / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008958:Models, Molecular; D008968:Molecular Conformation; D012996:Solutions; D012997:Solvents; D013816:Thermodynamics
chemical_list: D012996:Solutions; D012997:Solvents; D006151:Guanosine