Encapsulation is a widespread biological process particularly in the formation of protective egg cases of oviparous animals. The egg capsule wall of the channelled whelk Busycon canaliculum is an effective shock absorber with high reversible extensibility and a stiffness that changes significantly during extension. Here we show that post-stretch recovery in egg capsules is not driven by entropic forces as it is in rubber. Indeed, at fixed strain, force decreases linearly with increasing temperature, whereas in rubber elasticity the force increases. Instead, capsule wall recovery is associated with the internal energy arising from the facile and reversible structural alpha-helix <--> beta-sheet transition of egg capsule proteins during extension. This behaviour is extraordinary in the magnitude of energy dissipated and speed of recovery and is reminiscent of strain-induced crystallization in some polymeric fibres and of superelastic deformations associated with diffusionless phase transitions in shape-memory alloys.

The egg capsules of marine prosobranch gastropods, commonly know as whelks, function as a protective encapsulant for whelk embryos in wave-swept marine environments. The proteinaceous sheets comprising the wall of whelk egg capsules (WEC) exhibit long-range reversible extensibility with a hysteresis of up to 50 per cent, previously suggested to result from reversible changes in the structure of the constituent protein building blocks. Here, we further investigate the structural changes of the WEC biopolymer at various hierarchical levels using several different time-resolved in situ approaches. We find strong evidence in these biological polymers for a strain-induced reversible transition from an ordered conformational phase to a largely disordered one that leads to the characteristic reversible hysteretic behaviour, which is reminiscent of the pseudoelastic behaviour in some metallic alloys. On the basis of these results, we generate a simple numerical model incorporating a worm-like chain equation to explain the phase transition behaviour of the WEC at the molecular level.

The robust, proteinaceous egg capsules of marine prosobranch gastropods (genus Busycotypus) exhibit unique biomechanical properties such as high elastic strain recovery and elastic energy dissipation capability. Capsule material possesses long-range extensibility that is fully recoverable and is the result of a secondary structure phase transition from α-helix to extended β-sheet rather than of entropic (rubber) elasticity. We report here the characterization of the precursor proteins that make up this material. Three different proteins have been purified and analyzed, and complete protein sequences deduced from messenger ribonucleic acid (mRNA) transcripts. Circular dichroism (CD) and Fourier transform infrared (FTIR) spectra indicate that the proteins are strongly α-helical in solution and primary sequence analysis suggests that these proteins have a propensity to form coiled-coils. This is in agreement with previous wide-angle x-ray scattering (WAXS) and solid-state Raman spectroscopic analysis of mature egg capsules.