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Potassium ions stabilize parallel G-quadruplex DNA structures, while other cations destabilize them by favoring a coil conformation. This differential cation effect is likely common in similar DNA structures.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Structural Biology

Background:

  • G-quadruplexes are nucleic acid structures with potential roles in gene regulation.
  • The human VEGF gene promoter is known to form G-quadruplex structures.
  • Understanding the factors influencing G-quadruplex stability is crucial for their therapeutic targeting.

Purpose of the Study:

  • To investigate the effect of different cations on the thermal stability of a specific parallel G-quadruplex DNA sequence (Pu22-T12T13).
  • To elucidate the mechanisms by which stabilizing and destabilizing cations influence G-quadruplex conformation.
  • To determine if the observed cation effects are generalizable to other parallel intramolecular G-quadruplexes.

Main Methods:

  • Synthesis of a modified 22-meric DNA sequence (Pu22-T12T13) from the human VEGF gene promoter region.
  • Measurement of thermal stability (Tm) of the G-quadruplex in the presence of varying concentrations of potassium (K+), cesium (Cs+), and tetramethylammonium (TMA+) ions.
  • Analysis of cation-induced changes in G-quadruplex stability based on thermodynamic principles.

Main Results:

  • The Pu22-T12T13 DNA sequence formed a stable parallel-stranded G-quadruplex in a potassium solution.
  • Thermal stability (Tm) increased with potassium ion concentration, indicating stabilization.
  • Non-stabilizing cations (Cs+ and TMA+) significantly decreased Tm, favoring a coil conformation.
  • Counterion condensation around the coil state was estimated to involve approximately one more ion than the G-quadruplex state.

Conclusions:

  • The stability of the Pu22-T12T13 G-quadruplex is modulated by the differential binding of cations.
  • Internally bound potassium ions stabilize the G-quadruplex, while externally associated counterions destabilize it.
  • The observed phenomenon suggests a general principle for cation interactions with parallel intramolecular G-quadruplexes.