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ABSTRACT
The quasi-equilibrium evolution of the helical fraction occurring in a biopolymer network (gelatin gel) under an applied stress has been investigated by observing modulation in its optical activity. Its variation with the imposed chain extension is distinctly nonmonotonic and corresponds to the transition of initially coiled strands to induced left-handed helices. The experimental results are in qualitative agreement with theoretical predictions of helices induced on chain extension. This new effect of mechanically stimulated helix-coil transition has been studied further as a function of the elastic properties of the polymer network: crosslink density and network aging.
INTRODUCTION
Biopolymer networks have been widely studied over the recent years because of their many practical applications in fields such as food, biomedical, and even photographic industries. For example, they have been extensively used for capsules, adsorbent padding, tissue regeneration and also as neutral density filters for optics. More fundamentally, biological networks (e.g., cytoskeletons with actin and microtubules) play an important role in cells by providing a structural framework which is responsible for the mechanical stability and the locomotion of the whole cell, as well as intracellular transport processes (1). Collagen networks form the core structure of bone, ligaments, and other living tissues. They have suscitated a high interest for a growing community of researchers at the interface of physics and biology (2,3).
Biopolymer networks also offer new alternatives to explore the relationship between the structure and responses at molecular length-scales. Certain homopolypeptides form regular α-helices under appropriate conditions. In this case, the molecular configurations are well understood according to the Zimm-Bragg model (4) (and its many subsequent modifications (5)). This model assumes that each segment along the polymer chain has access to only two conformational states, a random-coil state and a helical state, where the particular residue forms a hydrogen bond with other specific residues at a certain distance along the backbone. By modifying the end-to-end distance R of individual chains, the equilibrium state between the helical and the random-coil segments can evolve and a coil-to-helix transition can be induced. To determine the relationship between the helical state and the chain end-to-end distance, one needs to control the distance R and simultaneously measure the helical content. With the recent development of single-molecule force spectroscopy (SMFS), it has...