| Summary: | 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 NH4+15 ions uncovered a mixed mono-K+-mono-NH4+15 form that represents intermediate in the conversion of di-NH4+15 into di-K+ form. Analogously, NH4+15 ions were found to replace Na+ ions inside d(G3T4G4)2 quadruplex. The preference of NH4+15 over Na+ ions for the two binding sites is considerably smaller than the preference of K+ over NH4+15 ions. The two cation binding sites within the G-quadruplex core differ to such a degree that NH4+15 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 NH4+15 ions by Gibbs free energies of approximately −24 and −18 kJ mol−1 which includes differences in cation dehydration energies, respectively.
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