The Effect Magnesium Cations to the Formation of G-Quadruplex Studied by Single-Molecule Spectroscopy

G-quadruplex is a non-canonical DNA structure with a single-stranded DNA composited by tandem repeats of Guanine-rich (G-rich) motifs. This G-rich motif folds into a quadruple-folded structure at the presence of sodium (Na+), potassium (K+) and other cations while it is held together by the Hoogsteen hydrogen bonding network.The stability of G-quadruplex is of great interest due to the stably folded G-quadruplex down regulates the amplification of oncogene promotor, therefore, efforts have been made to stabilize the G-quadruplex structures by binding to antibodies or ligands, which can be potential anti-cancer drugs. G-quadruplex is long believed to be intrinsically stabilized by some monovalent cations, especially the K+ cation, possibly due to the hydration effect that is sensitive to the ion radius. Physiologically abundant Mg2+ cations are usually considered unimportant or even carrying a negative effect on the stabilization of G-quadruplex based on previous studies carried out in bulk solutions. However, the high DNA concentration required in these measurements may also lead to the interference of the inter- and intra-molecular G-quadruplex, or aggregations.In this work, we utilized single-molecule FRET spectroscopy to study the isolated G-quadruplex molecules and found that the presence of relatively low concentrations of Mg2+ cations alone efficiently promotes the folded structure formation of the human telomere 21 (HT21) and B-cell lymphoma 2 (bcl2) G-quadruplex sequences, compared to the solely presented K+ cations. In addition, the Mg2+ assisted folding of HT21 shows a preference for the antiparallel configuration. The stabilities of the Mg2+ and K+ bound G-quadruplex are also addressed by the kinetic analysis of the interconversion between unfolded and various folded conformations.