Induced helicity in biopolymer networks under stress

Abstract

By combining dynamic mechanical and optical measurements in probing the internal structure of a biopolymer network (gelatin gel), we studied the quasi-equilibrium evolution of helical content as a function of the applied stress. Assuming that the net optical activity is proportional to the concentration of secondary helices of collagen chains, and assuming that affine mechanical deformation, we find a nonmonotonic relationship between the helical domains and an imposed deformation. The results are in qualitative agreement with theoretical predictions of ␣-helices induced by chain end-to-end stretching, and give a consistent picture of mechanically stimulated helix-coil transition in networks of denatured polypeptides.collagen ͉ helix ͉ optical rotation ͉ gelatin ͉ chirality I n recent years, a new field has emerged at the interface of physics and biology, aiming to explore structure and responses at molecular-length scales. Many single-molecule experiments have been performed to measure forces generated by biopolymers and their response to applied extension forces. The now classical work on DNA stretching (1) is just one of a number of significant recent advances in this field. By monitoring the response of a single molecule to pulling and twisting its ends [using atomic force microscopy (AFM), and magnetic trap and optical tweezer methods (2, 3)], one can probe the question of how chiral biopolymers are held in their native state, and their pathways of folding and unfolding. However, although the current techniques used in single-molecule force experiments reveal spectacular force-extension curves, they give little direct information about the structural transitions that occur on extension of these chains. Theoretical modeling of the behavior of single biopolymer molecules on extension has been hampered by this lack of information concerning structural changes. Using a new macroscopic approach of combining mechanical and optical methods in probing the internal structure of the biopolymer network, we found a direct relationship between helical domains and an externally imposed end-to-end distance of chains.Certain homopolypeptides form regular ␣-helices under appropriate conditions. In this case, the molecular configurations are well understood and are described according to the ZimmBragg model (4) [and its many modifications (5)], which assumes that each chain segment has access to only two conformational states, the random-coil state and the helical state. The average helical fraction of the chain can then be calculated for any number of model interactions between these two states. Although the two-state model of polypeptides has a lot of support, especially revealed in the Ramachandran plots, showing that peptide backbone has two well separated states corresponding to the ␣-helix and the ␤-sheet (the high-temperature denatured coil being the random mixing between the two), from the fieldtheoretical point of view, it is desirable to have a continuum model of chain conformation, as a function of interaction pot...

Competition for hydrogen-bond formation in the helix-coil transition and protein folding

“…Tamashiro and Pincus are in good agreement with some experimental studies [63], however there was no intention to study the effect of stretching onto reentrant helix-coil transition, and no solvent effects were included into the consideration.…”

Conformational Preferences of the Full Chicken Prion Protein in Solution and Its Differences with Respect to Mammals

“…This clarifies the nature of the higher end-to-end distance identified, indicating in fact that even if the end-to-end distances can be helpful in detecting the presence of an equilibrium value, this could be not sufficient to detect a correct equilibration of the system, in terms of secondary structure, as also pointed out by Courty et al for the induced helicity in biopolymer networks. [46] To use an analogy, upon pulling a helical spring, its end-to-end distance changes but chirality does not or does so to a lesser extent.…”

Solvent control of crack dynamics in a reversible hydrogel

“…One estimates average mesh sizes ξ ∼ (kT /µ) 1/3 of order 10 nm, i.e. coil segments involving a few 100 units (residues) 6 . Moreover, in the presence of pressure gradients, the solvent diffuses through the network.…”