SHADWICK, R.E.; RAPOPORT, H.S.; Univ. of California, San Diego: The development of mechanical properties in a natural protein elastomer from whelk egg capsules.

Marine gastropod whelks deposit their eggs in capsules that are highly resistant to degradation in the harsh marine environment. Characterization of egg capsules from Busycon and Kelletia revealed a fibrous protein polymer with unusual elastic properties. In particular, this material exhibits high stiffness and low hysteresis at strains below 5%, but at larger strains it appears to yield, resulting in a ten-fold reduction in stiffness and a doubling of hysteresis. This behaviour is entirely reversible and repeatable, endowing the capsule biopolymer with the properties of a stiff spring at low strains, changing to an energy absorbing elastomer at high strains. Production of the capsules begins internally by self assembly of capsule protein in the nidamental gland, but final processing occurs while the capsule is manipulated in the ventral pedal gland. Here covalent stabilization occurs and the capsule becomes insoluble and obtains the elastic properties described above. The mechanical properties develop in stages, with long range elasticity coming first, as putative cross-links set up between terminal domain ends of coiled-coils. As the material compacts with interchain end-domain linkages occurring, the high stiffness seen at initial extension develops. To study how assembly generates the high stiffness behavior we treated capsule samples with strong formic acid or heat. From these results we speculate on an H-bond model to explain the nuances of this material�s mechanical behavior. Understanding the molecular basis for the capsule mechanical properties may be helpful in development of biomimetic material designs.