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NMR studies of mechanical mechanisms in whelk egg capsule Han, Shu


Whelks (sea snails) deposit their eggs into tough protein capsules that protect the eggs from the harsh marine environment. The mechanical properties of the protein capsule are fascinating. At low strain the capsule material is stiff but elastic, while at higher strain the modulus decreases and the material stretches dramatically. X-ray diffraction measurements and Raman spectroscopy suggest that the capsules’ high extensibility is due to a reversible phase transition of component protein building blocks from a compact ɑ-helical conformation to a more extended softer conformation called β*. In this work, a variety of Nuclear Magnetic Resonance (NMR) spectroscopy experiments were performed to study the microscopic basis of the egg capsules’ structure and mechanical properties under extension. We have used deuterated probe molecules to study the molecular alignment of whelk egg capsule (WEC). ²H residual quadrupolar splitting measurements are consistent with a two component model for the molecular alignment of WEC, in agreement with optical measurements showing a cross-plied fibre structure. Measurements of ¹H dipolar linewidth and T₁ time as functions of strain are consistent with a previous model that proposed a gradual transition from ɑ-helical coiled-coils to poorly structured worm-like chains with strain. Residual quadrupolar couplings of absorbed water are not strongly affected by strain. Overall, results obtained from these experiments provide useful information to understand the transition mechanism, and may contribute to the development of WEC-like materials for the applications such as new bioencapsulants for delicate tissue implants.

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