Connecting caddisworm silk structure and mechanical properties: combined infrared spectroscopy and mechanical analysis

The underwater silk of an aquatic casemaking caddisfly larvae (Hesperophylax occidentalis) is viscoelastic, and displays distinct yield behaviour, large strain cycle hysteresis and near complete recovery of its initial strength and stiffness when unloaded. Yield followed by a stress plateau has been attributed to sequential rupture of serial Ca2+-cross-linked phosphoserine (pS) beta-domains. Spontaneous recovery has been attributed to refolding of the Ca2+/pS domains powered by an elastic network. In this study, native Ca2+ ions were exchanged with other metal ions, followed by combined mechanical and FTIR analysis to probe the contribution of pS/metal ion complexes to silk mechanical properties. After exchange of Ca2+ with Na+, the fibres are soft elastomers and the infrared spectra are consistent with C-v3 symmetry of the -OPO32- groups. Multivalent metal ions decreased the -OPO32- symmetry and the symmetric stretching modes (v(s)) split in a manner characteristic of ordered phosphate compounds, such as phosphate minerals and lamellar bilayers of phosphatidic acid lipids. Integrated intensities of the v(s) bands, indicative of the metal ion's effect on transition dipole moment of the P-O bonds, and thereby the strength of the phosphate metal complex, increased in the order: Na+ < Mg2+ < Sr2+ < Ba2+ < Ca2+ < Eu3+ < La3+ < Zn2+ < Fe2+. With a subset of the metal ion series, the initial stiffness and yield stress of metal ion-exchanged fibres increased in the same order: Na+ << Mg2+ < Ca2+ < La3+ < Zn2+ approximate to Fe2+, establishing the link between phosphate transition dipole moments and silk fibre strength.