Physical properties of honeybee silk: a review
Honeybee silk is released from secretory cells and polymerises as birefringent tactoids in the lumen while silk is spun by a spinneret at the tip of the labium–hypopharynx and contains ά-helical proteins arranged in a four-strand coiled-coil structure. Wet fibres are only half as stiff as dried ones, but are equal in strength. The fibroin is hygroscopic and lithium thiocyanate and urea eliminate the yield point tested on both dry and wet fibres. The slopes of the solvent-related curves are reduced compared to those tested in water. Silk sheets are independent of temperature when deformed in tension. This fibre is rather crystalline and its hydration sensitivity, expressed as the ratio of the elastic modulus of wet to that of dry fibre, is 0.53. The ά-helical fibroins are predicted to have an antiparallel tetrameric configuration that is shown as a possible structural model. The molecular structure of ά-helical proteins maximizes their robustness with minimal use of building materials. In conclusion, it appears that the composition, molecular topology and amino acid content and sequence are a highly conserved feature in the evolution of silk in Apis species.