Chain Sliding versus beta-Sheet Formation upon Shearing Single alpha-Helical Coiled Coils

Coiled coils (CCs) are key building blocks of biogenic materials and determine their mechanical response to large deformations. Of particular interest is the observation that CC-based materials display a force-induced transition from alpha-helices to mechanically stronger beta-sheets (alpha beta T). Steered molecular dynamics simulations predict that this alpha beta T requires a minimum, pulling speed-dependent CC length. Here, de novo designed CCs with a length between four to seven heptads are utilized to probe if the transition found in natural CCs can be mimicked with synthetic sequences. Using single-molecule force spectroscopy and molecular dynamics simulations, these CCs are mechanically loaded in shear geometry and their rupture forces and structural responses to the applied load are determined. Simulations at the highest pulling speed (0.01 nm ns(-1)) show the appearance of beta-sheet structures for the five- and six-heptad CCs and a concomitant increase in mechanical strength. The alpha beta T is less probable at a lower pulling speed of 0.001 nm ns(-1) and is not observed in force spectroscopy experiments. For CCs loaded in shear geometry, the formation of beta-sheets competes with interchain sliding. beta-sheet formation is only possible in higher-order CC assemblies or in tensile-loading geometries where chain sliding and dissociation are prohibited.