Correlation between protein secondary structure and mechanical performance for the ultra-tough dragline silk of Darwin's bark spider

Abstract

The spider major ampullate (MA) silk exhibits high tensile strength and extensibility and is typically a blend of MaSp1 and MaSp2 proteins with the latter comprising glycine-proline-glycine-glycine-X repeating motifs that promote extensibility and supercontraction. The MA silk from Darwin's bark spider (<i>Caerostris darwini</i>) is estimated to be two to three times tougher than the MA silk from other spider species. Previous research suggests that a unique MaSp4 protein incorporates proline into a novel glycine-proline-glycine-proline motif and may explain <i>C. darwini</i> MA silk's extraordinary toughness. However, no direct correlation has been made between the silk's molecular structure and its mechanical properties for <i>C. darwini</i>. Here, we correlate the relative protein secondary structure composition of MA silk from <i>C. darwini</i> and four other spider species with mechanical properties before and after supercontraction to understand the effect of the additional MaSp4 protein. Our results demonstrate that <i>C. darwini</i> MA silk possesses a unique protein composition with a lower ratio of helices (31%) and β-sheets (20%) than other species. Before supercontraction, toughness, modulus and tensile strength correlate with percentages of β-sheets, unordered or random coiled regions and β-turns. However, after supercontraction, only modulus and strain at break correlate with percentages of β-sheets and β-turns. Our study highlights that additional information including crystal size and crystal and chain orientation is necessary to build a complete structure-property correlation model.