Spider major ampullate silk (drag line): smart composite processing based on imperfect crystals

Abstract

SummaryA spider can spin major ampullate silk (drag line) at rates that differ by more than an order of magnitude, and yet obtain consistently reliable tensile properties. Silk samples collected at different rates and stained by infiltration with tin were studied by transmission electron microscopy. The stained microstructures indicate the presence of nonperiodic lattice crystals, the average size of which depends on the silking rate. A simple statistical point model demonstrates that the ordered regions in major ampullate silk can originate from statistical matches between adjacent chains. The minimum size of the ordered regions is dictated by the fixed monomer sequence in the chains and not by processing kinetics. Inductively coupled plasma-mass spectroscopy shows that the concentration of multivalent protein-binding cations in the unstained fibre is significantly reduced at the higher rates of silking, compared with their concentration at the lowest rates. Qualitatively, this result is consistent with the ions being used to maintain the shear sensitivity of silk secretion over the range of processing rates studied.

Nanostructural features of a spider dragline silk as revealed by electron and X-ray diffraction studies

“…The streaking in some single crystal diffraction patterns (Figs. 5, 9 and 10) suggested lattice strains, consistent with the NPL model [11][12][13] of larger crystals in spider dragline. These strains are likely to occur in the intersheet direction, where the distance is governed by van der Waals packing of side chain moieties, and where packing of the b-sheets will vary depending on the amino acid side groups present.…”

“…These differences in mechanical properties must arise from differences in microstructure, evidence of which has been reported for major ampullate silk reeled at different rates [13]. We chose to perform the present study on reeled silk so that samples would have a reproducible history, and in anticipation of the fact that biomimetic silk analogues will experience forced drawing as they are processed into fibers.…”

“…Larger crystals, ranging from 70-500 nm, have been detected in TEM studies of Nephila clavipes dragline silk; these have been named non-periodic lattice (NPL) crystals and can be thought of as regions of nearly (but not perfectly) repeated composition and order [11][12][13]. Their presence has also been inferred from an analysis of XRD patterns obtained from bundles of N. clavipes dragline [12].…”

Tensile properties of Attacus atlas silk submerged in liquid media

“…Beta-sheet microcrystallites embedded in an amorphous matrix occur in B. mori silk, 8 whereas major ampullate silk (the principal fiber in drag line) from N. clavipes spiders shows a more complex and hierarchical distribution of microcrystallites within the matrix. 9 The difficulty of correlating composition and microstructure has prompted some attempts to correlate composition with mechanical properties directly. The underlying assumption is that similar compositions should give rise to similar microstructures, with the further assumption that processing conditions are also similar.…”

Supramolecular organization of regenerated silkworm silk fibers

“…In this case the polyalanine runs, A n (n = 5-8), seem to be essential for the formation of ␤-nanocrystallites that are responsible for the structural integrity of the fiber. The other motifs found in spidroins, GA, GGX (X = Q, L or Y) and GPG have been related to the distinct features of spider silk not found in silkworm silk, such as supercontraction [8] and the formation of secondary structures like non-perfect crystals [9].…”