Material Properties of Honeybee Silk

Abstract

Colourless honeybee silk, ~3 μm diameter, is produced through a spinneret at the tip of the labium-hypopharynx. Successive generations of brood apply silk to the cell walls, making the cells smaller, as silk is deposited in the old brood combs. X-ray diffraction data show that honeybee silk contains ά-helical proteins ordered into coiled-coil structures, with an axial periodicity of about 28 nm, and form a four-stranded array parallel to the fibre axis. Honeybee fibroin is crystalline, but, when hydrated, is only half as stiff as when dry, although they are equal in strength. The fibroin is hygroscopic and highly distensible when solvated because of its molecular conformation. The mechanical properties of silk are independent of temperature. Lithium thiocyanate and urea virtually eliminate the yield point of honeybee silk tested both dry and in distilled water, and values for stress in the slope of the solvent-related curves is reduced. The solvents act directly on hydrogen bonds and then the silks behave as unconnected bends during tensile deformation. The components, hierarchical structure and the conditions of their production all affect the mechanical properties of natural silks. The amino acid sequence in honeybee silk protein provides an explanation of why the coiled-coil packing is atypically tight, and the most abundant core residue is the small amino acid, alanine. An atomistic simulation for the unfolding behaviour of ά-helical protein shows that two discrete transition states correspond to two fracture mechanisms. Six honeybee silk genes have now been identified, using a combination of genomic and proteomic techniques.