Length-dependent force characteristics of coiled coils

Coiled-coil domains within and between proteins play important structural roles in biology. They consist of two or more alpha helices that form a superhelical structure due to packing of the hydrophobic residues that pattern each helix. A recent continuum model showed that the correspondence between the chirality of the pack to that of the underlying hydrophobic pattern comes about because of the internal deformation energy associated with each helix in forming the superhelix. We have developed a coarse-grained atomistic model for coiled coils that includes the competition between the hydrophobic energy that drives folding and the cost due to deforming each helix. The model exhibits a structural transition from a non-coiled-coil to coiled-coil state as the contribution from the deformation energy changes. Our model is able to reproduce naturally occurring coiled coils and essential features seen in unzipping experiments. We explore the force-extension properties of these model coiled coils as a function helix length and find that shorter coils unfold at lower force than longer ones with the required unfolding force eventually becoming length independent.