Two distinct overstretched DNA structures revealed by single-molecule thermodynamics measurements

Area of Science:

• Biophysics
• Molecular Biology
• Thermodynamics

Background:

• Double-stranded DNA (dsDNA) exhibits structural plasticity under varying conditions.
• The precise structural states of highly stretched DNA remain incompletely understood.
• DNA overstretching involves a transition around 65 pN, causing ~1.7-fold contour length elongation.

Purpose of the Study:

• To simultaneously determine the entropy (ΔS) and enthalpy (ΔH) changes for the two distinct transitions during DNA overstretching.
• To elucidate the thermodynamic properties and structural implications of DNA overstretching.

Main Methods:

• Simultaneous determination of entropy and enthalpy changes for DNA overstretching transitions.
• Analysis of transition force response to salt concentration to infer strand proximity.
• Development of a multidimensional phase diagram to illustrate transition selection based on DNA base-pair stability.

Main Results:

• The hysteretic peeling transition exhibited ΔS ≈ 20 cal/(K·mol) and ΔH ≈ 7 kcal/mol.
• The nonhysteretic transition showed ΔS ≈ -3 cal/(K·mol) and ΔH ≈ 1 kcal/mol.
• Salt concentration response indicated strand separation after peeling and proximity after the nonhysteretic transition.

Conclusions:

• The study provides the first simultaneous thermodynamic characterization of DNA overstretching transitions.
• Results suggest distinct structural outcomes (strand separation vs. proximity) depending on the transition.
• DNA base-pair stability dictates the transition pathway, with implications for in vivo DNA mechanics.