Chiral, Biological Nanostructures: Conformational Changes and Elasticity

In a classical thermoplastic material, the phase transition of a structural units within the polymer matrix enables the reshaping of the material into an alternative form by melting and reprocessing. The lack of chemical cross-linking within the materials means that the two forms, before and after deformation, have no significant difference in entropy. Here melting is associated whit a complete loss of structural information and shape. Shape reconfiguration is achieved by the break and forming of bonds between adjacent polymers chains and provides mechanical integrity to the two formed shapes. Here we study the elastic properties of D,L-alternating peptides able to form double stranded ?-helix. Furthermore, we make a comparative study with the mechanism of elasticity proposed for different elastic proteins. Elastic proteins are characterized by being able to undergo significant deformation, without rupture, before returning to their original state when the stress is removed. The sequence of elastic proteins contains elastomeric domains, which comprise repeated sequences, which in many cases appear to form ?-turns. In addition, the majority also contains domains that form intermolecular cross-link, which may be covalent or non-covalent. The mechanism of elasticity varies between the different proteins and appears to be related to the biological role of the proteins. D,L-Alternating peptides fold into secondary structures stabilized by multiple interactions that require the participation of both backbones and side chains, with the overwhelming majority being ?-helical conformations possessing a hollow core (? between 1 and 9 ? depending of the helix type). In this work we focus on the slow interconversion equilibrium of two right-handed ???5,6-helices in order to understand the mechanism of the conversion. A comparison with elastic mechanism of proteins permit us to design heterochiral building blocks with elastic function.