title: Unfolding of globular polymers by external force.
abstract: We examine the problem of a polymer chain, folded into a globule in poor solvent, subjected to a constant tensile force. Such a situation represents a Gibbs thermodynamic ensemble and is useful for analysing force-clamp atomic force microscopy measurements, now very common in molecular biophysics. Using a basic Flory mean-field theory, we account for surface interactions of monomers with solvent. Under an increasing tensile force, a first-order phase transition occurs from a compact globule to a fully extended chain, in an "all-or-nothing" unfolding event. This contrasts with the regime of imposed extension, first studied by Halperin and Zhulina [Europhys. Lett. 15, 417 (1991)], where there is a regime of coexistence of a partial globule with an extended chain segment. We relate the transition forces in this problem to the solvent quality and degree of polymerisation, and also find analytical expressions for the energy barriers present in the problem. Using these expressions, we analyse the kinetic problem of a force-ramp experiment and show that the force at which a globule ruptures depends on the rate of loading.
authors.affiliation: Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom.
authors.forename: Samuel
authors.initials: S
authors.lastname: Bell
authors.affiliation: Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom.
authors.forename: Eugene M
authors.initials: EM
authors.lastname: Terentjev
mesh_terms: D007700:Kinetics; D018625:Microscopy, Atomic Force; D056004:Molecular Dynamics Simulation; D011108:Polymers / Q000737:chemistry*; D058767:Protein Unfolding; D012997:Solvents / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D011108:Polymers; D012997:Solvents