Mechanical unfolding of a homopolymer globule studied by self-consistent field modelling

TL;DR

This study uses self-consistent field modeling to analyze the mechanical unfolding of a polymer globule, revealing three deformation regimes and a first-order transition with hysteresis, which could be observed experimentally.

Contribution

It provides a detailed numerical analysis of globule unfolding, identifying distinct deformation regimes and the nature of the transition, extending previous theoretical and simulation work.

Findings

Identified three deformation regimes: linear, tadpole, and stretched chain.

Discovered a first-order transition with hysteresis between tadpole and stretched states.

Predicted force-deformation patterns observable in single-molecule experiments.

Abstract

We present results of numerical self-consistent field (SCF) calculations for the equilibrium mechanical unfolding of a globule formed by a single flexible polymer chain collapsed in a poor solvent. In accordance with earlier scaling theory and stochastic dynamics simulations findings we have identified three regimes of extensional deformation: (i) a linear response regime characterized by a weakly elongated (ellipsoidal) shape of the globule at small deformations, (ii) a tadpole structure with a globular "head" coexisting with a stretched "tail" at intermediate ranges of deformations, and (iii) an uniformly stretched chain at strong extensions. The conformational transition from the tadpole to the stretched chain is accompanied by an abrupt unfolding of the depleted globular head and a corresponding jump-wise drop in the intrachain tension. The unfolding-refolding cycle demonstrates a hysteresis loop in the vicinity of the transition point. These three regimes of deformation, as well as the first-order like transition between the tadpole and the stretched chain conformations, can be experimentally observable provided that the number of monomer units in the chain is large and/or the solvent quality is sufficiently poor. For short chains, on the other hand, under moderately poor solvent strength conditions, the unfolding transition is continuous. Upon an increase in the imposed end-to-end distance the extended globule retains a longitudinally uniform shape at any degree of deformation. In all cases the system exhibits a negative extensional modulus in the intermediate range of deformations. We anticipate that predicted patterns in force-deformation curves for polymer molecules in poor solvent can be observed in single molecule atomic spectroscopy experiments.