(336a) Elasticity of ?-Helices, in Solutions and in Gels

A key structural motif in proteins is the a-helix whose rigidity supports the protein structure even as the protein opens or closes. It is predicted that in the human genome, 0.2% of all proteins contain an isolated stable α-helices and up to 11% of the proteins contain some sort of coiled coil. Using light scattering techniques, we have measured the persistence length of model poly-l-lysine (PLL) α-helices polypeptides across the coil-helix transition. As the backbone folds into a helix, the persistence length, a measure of the mechanical strength of the backbone, changes from 1- 2 nm to ~ 20 nm. Upon chemical crosslinking in solution, the helices remain semiflexible chains between crosslinks when the chains are coils and helices. We find that the equilibrium modulus does not change, but as the materials are stretched, the response changes from strain softening to strain hardening as the chains fold into a helix. Overall, the random coil–helix transition has a significant effect on the nonlinear mechanical properties of PLL hydrogels.