Eur. Phys. J. E 10, 249-263 (2003)
DOI: 10.1140/epje/i2002-10113-2

Force-extension behavior of folding polymers

S. Cocco¹, J.F. Marko², R. Monasson³, A. Sarkar² and J. Yan<sup>2

1</sup>  LDFC CNRS-ULP, Institut de Physique, 3 rue de l'Université, 67084 Strasbourg, France
²  Department of Physics, University of Ilinois at Chicago, 845 West Taylor Street, Chicago IL 60607-7059, USA
³  CNRS-Laboratoire de Physique Theorique de l'ENS, 24 rue Lhomond, 75231 Paris 05, France

jmarko@uic.edu

(Received 7 August 2002 and Received in final form 7 March 2003 / Published online: 15 April 2003)

Abstract
The elastic response of flexible polymers made of elements which can be either folded or unfolded, having different lengths in these two states, is discussed. These situations are common for biopolymers as a result of folding interactions intrinsic to the monomers, or as a result of binding of other smaller molecules along the polymer length. Using simple flexible-chain models, we show that even when the energy associated with maintaining the folded state is comparable to , the elastic response of such a chain can mimic usual polymer linear elasticity, but with a force scale enhanced above that expected from the flexibility of the chain backbone. We discuss recent experiments on single-stranded DNA, chromatin fiber and double-stranded DNA with proteins weakly absorbed along its length which show this effect. Effects of polymer semiflexiblity and torsional stiffness relevant to experiments on proteins binding to dsDNA are analyzed. We finally discuss the competition between electrostatic self-repulsion and folding interactions responsible for the complex elastic response of single-stranded DNA.

PACS
87.14.Gg - DNA, RNA.
87.37.Rs - Single molecule manipulation of proteins and other biological molecules.
81.16.Fg - Supramolecular and biochemical assembly.
36.20.Ey - Conformation (statistics and dynamics).

© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2003