Stretching multistate flexible chains and loops.

Area of Science:

• Polymer physics
• Biophysics
• Statistical mechanics

Background:

• Polymer loop structures are fundamental in biological processes like DNA looping and denaturation.
• Loop formation alters a polymer chain's elastic properties due to entropic changes.
• Reversible loop formation introduces complex multistate conformational behavior.

Purpose of the Study:

• To model and analyze the mechanical and thermodynamic properties of multistate reversible polymer loops.
• To investigate the force-extension relationships in looping and zipping Gaussian chain models.
• To explore the phase behavior of a Gaussian necklace composed of alternating loop and zipped segments.

Main Methods:

• Development of theoretical models for looping and zipping Gaussian chains.
• Calculation of force-extension relations in fixed-extension (Helmholtz) and fixed-force (Gibbs) ensembles.
• Construction of a force-temperature phase diagram for a Gaussian necklace model.

Main Results:

• Multistate systems exhibit distinct tensile elasticity and ensemble inequivalence compared to single chains.
• The models reveal unique behaviors arising from reversible loop and zip formation.
• A force-induced phase transition from a looped to a mixed state was identified in the Gaussian necklace.

Conclusions:

• Reversible multistate polymer loops display complex elasticity and ensemble-dependent properties.
• Theoretical models provide insights into the behavior of biological polymers under force.
• The findings suggest potential for force-controlled conformational changes in polymer systems.