β‑Sheet Nanocrystalline Domains Formed from Phosphorylated Serine-Rich Motifs in Caddisfly Larval Silk: A Solid State NMR and XRD Study
2013-04-08T00:00:00Z (GMT) by
Adhesive silks spun by aquatic caddisfly (order Trichoptera) larvae are used to build both intricate protective shelters and food harvesting nets underwater. In this study, we use <sup>13</sup>C and <sup>31</sup>P solid-state NMR and wide angle X-ray diffraction (WAXD) as tools to elucidate molecular protein structure of caddisfly larval silk from the species Hesperophylax consimilis. Caddisfly larval silk is a fibroin protein based biopolymer containing mostly repetitive amino acid motifs. NMR and X-ray results provide strong supporting evidence for a structural model in which phosphorylated serine repeats (pSX)<sub>4</sub> complex with divalent cations Ca<sup>2+</sup> and Mg<sup>2+</sup> to form rigid nanocrystalline β-sheet structures in caddisfly silk. <sup>13</sup>C NMR data suggests that both phosphorylated serine and neighboring valine residues exist in a β-sheet conformation while glycine and leucine residues common in GGX repeats likely reside in random coil conformations. Additionally, <sup>31</sup>P chemical shift anisotropy (CSA) analysis indicates that the phosphates on phosphoserine residues are doubly ionized, and are charge-stabilized by divalent cations. Positively charged arginine side chains also likely play a role in charge stabilization. Finally, WAXD results finds that the silk is at least 7–8% crystalline, with β-sheet interplane spacings of 3.7 and 4.5 Å.