Divalent Ion-Mediated DNA-DNA Interactions: A Comparative Study of Triplex and Duplex

Area of Science:

• Biophysics
• Computational Biology
• Molecular Biology

Background:

• Ion-mediated interactions are crucial for DNA condensation.
• Divalent cations like Mg2+ are not typically expected to aggregate double-stranded DNA (dsDNA).
• Recent experiments show Mg2+ can aggregate triple-stranded DNA (tsDNA), but the mechanism is unclear.

Purpose of the Study:

• To investigate the microscopic mechanisms behind Mg2+-induced tsDNA aggregation.
• To compare ion-mediated interactions between tsDNA and dsDNA in Mg2+ solutions.
• To elucidate the role of ion-bridging in DNA aggregation.

Main Methods:

• All-atom dynamic simulations were used to calculate potentials of mean force (PMFs).
• PMFs were computed for tsDNA-tsDNA, dsDNA-dsDNA, and tsDNA-dsDNA interactions in Mg2+ solutions.
• Analysis focused on Mg2+ binding sites and ion-bridging between DNA helices.

Main Results:

• Potentials of mean force (PMFs) revealed attractive interactions between tsDNAs, strengthening with higher Mg2+ concentrations.
• PMFs between dsDNAs remained non-attractive even at high Mg2+ concentrations.
• Mg2+ binds more strongly externally to tsDNA than dsDNA, facilitating significant ion-bridging.

Conclusions:

• Stronger external Mg2+ binding to tsDNA promotes ion-bridging, leading to apparent attraction between tsDNAs.
• Ion-bridging is a key factor in the Mg2+-induced aggregation of tsDNA.
• The findings provide a microscopic explanation for the observed aggregation of tsDNA by Mg2+.