title: Role of loop residues and cations on the formation and stability of dimeric DNA G-quadruplexes.
abstract: Formation of guanine-quadruplexes by four DNA oligonucleotides with common sequence dG4-loop-dG4 has been studied by a combination of NMR and UV spectroscopy. The loops consisted of 1',2'-dideoxyribose, propanediol, hexaethylene glycol, and thymine residues. The comparison of data on modified and parent oligonucleotides gave insight into the role of loop residues on formation and stability of dimeric G-quadruplexes. All modified oligonucleotides fold into dimeric fold-back G-quadruplexes in the presence of sodium ions. Multiple structures form in the presence of potassium and ammonium ions, which is in contrast to the parent oligonucleotide with dT4 loop. 15N-filtered 1H NMR spectra demonstrate that all studied G-quadruplexes exhibit three 15NH4(+) ion binding sites. Topology of intermolecular G-quadruplexes was evaluated by NMR measurements and diffusion experiments. The spherical, prolate-ellipsoid and symmetric cylinder models were used to interpret experimental translational diffusion constants in terms of diameters and lengths of unfolded oligonucleotides and their respective G-quadruplexes. UV melting and annealing curves show that oligonucleotides with non-nucleosidic loop residues fold faster, exhibit no hysteresis, and are less stable than dimeric d(G4T4G4)2 which can be attributed to the absence of H-bonds, stacking between loop residues and the outer G-quartets as well as cation-pi interactions. Oligonucleotide consisting of hexaethylene glycol linkage with only two phosphate groups in the loop exhibits higher melting temperature and more negative deltaH(o) and deltaG(o) values than oligonucleotides with four 1',2'-dideoxyribose or propanediol residues.
authors.affiliation: Slovenian NMR Center, National Institute of Chemistry, Hajdrihova 19, PO Box 660, SI-1001 Ljubljana, Slovenia.
authors.forename: Mirko
authors.initials: M
authors.lastname: Cevec
authors.forename: Janez
authors.initials: J
authors.lastname: Plavec
mesh_terms: D004247:DNA / Q000737:chemistry*; D004058:Diffusion; D054856:G-Quadruplexes; D006147:Guanine / Q000737:chemistry*; D019906:Nuclear Magnetic Resonance, Biomolecular; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D009838:Oligodeoxyribonucleotides / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D014783:Viscosity
chemical_list: D009838:Oligodeoxyribonucleotides; D006147:Guanine; D004247:DNA