Unfolding of globular polymers by external force.

Area of Science:

• Polymer Physics
• Biophysics
• Thermodynamics

Background:

• Polymer chains in poor solvents form compact globules.
• Force-clamp atomic force microscopy (AFM) is crucial in molecular biophysics.
• Understanding polymer behavior under tensile force is essential.

Purpose of the Study:

• To analyze the thermodynamic ensemble of a polymer chain under constant tensile force.
• To investigate the phase transition from a compact globule to an extended chain.
• To compare force-clamp conditions with imposed extension regimes.

Main Methods:

• Utilizing Flory mean-field theory to model surface interactions.
• Applying Gibbs thermodynamic ensemble framework.
• Developing analytical expressions for energy barriers.

Main Results:

• A first-order phase transition (all-or-nothing unfolding) occurs under increasing tensile force.
• This unfolding behavior contrasts with the partial globule/extended chain coexistence under imposed extension.
• Transition forces correlate with solvent quality and polymer degree of polymerization.

Conclusions:

• The study provides a theoretical framework for interpreting force-clamp AFM experiments.
• Analytical expressions for energy barriers aid in understanding unfolding kinetics.
• The force-ramp experiment analysis shows rupture force depends on loading rate.