Do Anions Exert any Structural Changes on Dimeric d(G4T4G4)2 Quadruplex in Aqueous Solution

It is well known that the presence of cations is necessary for G-quartet formation due to their role in reducing repulsions amongst guanine carbonyl oxygen atoms and additionally enhancing base-base stacking interactions. However, not much is known about the influence of anions on G-quadruplex formation by G-rich oligonucleotides in aqueous solution. In the presence of sodium halides, acetate, nitrate and BPh4- folding was almost complete, while only partial folding has been observed with NaBF4 and NaPF6. Detailed evaluation of 1H NMR spectra of d(G4T4G4)2 in the presence of nine different sodium salts indicates minor differences in loop regions as well as in structure of G-quadruplex core. Comparison of inter-proton distances obtained from NOESY spectra showed slightly different orientations of thymine residues in loop regions and changes in stacking interactions between G-quartets, which are most pronounced in the case of G-quadruplex folded in the presence of NaPF6