DNA on a tube: electrostatic contribution to stiffness

Area of Science:

• Biophysics
• Molecular Biology
• Computational Chemistry

Background:

• DNA stiffness is crucial for its biological functions.
• Electrostatic interactions between charged phosphate groups are a key factor influencing DNA structure and mechanics.
• Understanding these contributions is essential for modeling DNA behavior.

Purpose of the Study:

• To estimate the electrostatic contributions to the stiffness of short DNA fragments.
• To quantify the electrostatic persistence length of B-DNA under varying monovalent salt concentrations.
• To compare electrostatic effects with non-electrostatic contributions to DNA stiffness.

Main Methods:

• Utilized two simple models to simulate DNA fragments.
• Model 1: DNA as two helical strands around a cylinder; employed Debye-Hückel potentials for electrostatic interactions.
• Model 2: DNA as two helical strands around a curved tube; evaluated electrostatic potential energy.

Main Results:

• Electrostatic persistence length for 105-130 bp B-DNA was 125.64 Å (37 bp) at 5 mM and 76.05 Å (23 bp) at 10 mM monovalent salt.
• At 10 mM, considering condensed fraction (θ=0.715) yielded 108.28 Å (32 bp); end effects reduced this to 72.87 Å (21 bp).
• At 5 mM, electrostatic effects contributed 21.8% to DNA stiffness for 105-130 bp fragments, with total persistence length ~575.64 Å (171 bp).

Conclusions:

• Electrostatic interactions are a significant determinant of DNA stiffness, particularly at low salt concentrations.
• The contribution of electrostatics to DNA persistence length diminishes substantially at physiological monovalent cation concentrations.
• Findings align with counterion condensation models and experimental observations, enhancing our understanding of DNA mechanics.