Identification of mixed di-cation forms of G-quadruplex in solution.
Multinuclear NMR study has demonstrated that G-quadruplex adopted by d(G3T4G4) exhibits two cation binding sites between three of its G-quartets. Titration of tighter binding K+ ions into the solution of d(G3T4G4)2 folded in the presence of 15NH4+ ions uncovered a mixed mono-K+-mono-15NH4+ form that represents intermediate in the conversion of di-15NH4+ into di-K+ form. Analogously, 15NH4+ ions were found to replace Na+ ions inside d(G3T4G4)2 quadruplex. The preference of 15NH4+ over Na+ ions for the two binding sites is considerably smaller than the preference of K+ over 15NH4+ ions. The two cation binding sites within the G-quadruplex core differ to such a degree that 15NH4+ ions bound to the site, which is closer to the edge-type loop, are always replaced first during titration by K+ ions. The second binding site is not taken up by K+ ion until K+ ion already resides at the first binding site. Quantitative analysis of concentrations of the three di-cation forms, which are in slow exchange on the NMR time scale, at 12 K+ ion concentrations afforded equilibrium binding constants. K+ ion binding to sites U and L within d(G3T4G4)2 is more favorable with respect to 15NH4+ ions by Gibbs free energies of approximately -24 and -18 kJ mol(-1) which includes differences in cation dehydration energies, respectively.
Slovenian NMR Center, National Institute of Chemistry, Hajdrihova 19, PO Box 660, SI-1001 Ljubljana, Slovenia.
Primoz
P
Sket
Martin
M
Crnugelj
Janez
J
Plavec
D001665:Binding Sites; D002412:Cations / Q000737:chemistry; D004247:DNA / Q000737:chemistry*; D054856:G-Quadruplexes; D006147:Guanine / Q000737:chemistry*; D019906:Nuclear Magnetic Resonance, Biomolecular; D009841:Oligonucleotides / Q000737:chemistry*; D011188:Potassium / Q000737:chemistry; D000644:Quaternary Ammonium Compounds / Q000737:chemistry
D002412:Cations; D009841:Oligonucleotides; D000644:Quaternary Ammonium Compounds; C501842:d(G3T4G4)2; D006147:Guanine; D004247:DNA; D011188:Potassium