40. Fibers from Lacewing Silk

Lacewings are insects that lay their eggs on the tips of silken threads called egg stalks as seen from Fig. 40.1 [09Wei]. Unlike most silk-producing insects, green lacewing (Mallada signata, Neuroptera) produces two distinct types of silks depending on the life cycle of the insect [08Wei]. Silks produced by lacewing in the larval stage and during the final instar of cocoon production were found to be different. Primary structure of the lacewing silk is composed of motifs containing 16 amino acids with cysteine residues [12Bau]. The cocoon silk is composed of 49 kDa proteins, with >40 % alanine, and contains α-helical secondary structure, considerably smaller than the proteins (>200 kDa) seen in the classic β-sheet silks. In terms of secondary structure, lacewing silk was mainly composed of unique and distinct cross β-sheets that run perpendicular to the fiber axis unlike the silk produced by other insects. A model suggesting the arrangement of the cross β-sheets in lacewing silk is shown in Fig. 40.2 [13Lin]. Atomic force measurements and calculations have shown that the lacewing silk has a bending modulus three times higher than that of silkworm fibers [09Wei]. Tensile properties of the silk were found to be highly dependent on the water content (relative humidity) with modulus decreasing from 50 g/den to 11 g/den when the relative humidity was increased from 30 to 100 % and the corresponding change in breaking stress was from 2.0 to 0.6 g/den. This substantial change in properties due to change in humidity was supposed to be due to the transition of the cross β-sheets to parallel β-sheets caused by the weakening of the hydrogen bonds at high humidity [12Bau]. At low RH, the total strength of the hydrogen bonds in one layer of the stalk is higher than that of the disulfide bonds causing the fibers to absorb low energies. When the RH is high, the hydrogen bonds are weakened, and the disulfide bonds are now stronger than the sum of the hydrogen bonds in one layer causing the hydrogen bonds to break. Such breakage of the hydrogen bonds allows the rearrangement of the β-sheets [12Bau]. SEM image (Fig. 40.3) showed thinning of the fibers after stretching which was not reversible, again indicating the transformation of the β-sheets. The simple process by which lacewing secretes silks is considered to be more suitable for producing recombinant proteins [12Bau].