破译pH值和金属离子编程的家蚕纺丝机制

Decoding silkworm spinning programmed by pH and metal ions

Silk is one of the toughest fibrous materials known despite spun at ambient temperature and pressure with water as a solvent.It is a great challenge to reproduce high-performance artificial fibers comparable to natural silk by bionic for the incomplete understanding of silkworm spinning in vivo.Here,we found that amphipol and digitonin stabilized the structure of natural silk fibroin(NSF)by a large-scale screening in vitro,and then studied the close-to-native ultrastructure and hierarchical assembly of NSF in the silk gland lumen.Our study showed that NSF formed reversible flexible nanofibrils mainly composed of random coils with a sedimentation coefficient of 5.8 S and a diameter of about 4 nm,rather than a micellar or rod-like structure assembled by the aggregation of globular NSF molecules.Metal ions were required for NSF nanofibril formation.The successive p H decrease from posterior silk gland(PSG)to anterior silk gland(ASG)resulted in a gradual increase in NSF hydrophobicity,thus inducing the sol-gelation transition of NSF nanofibrils.NSF nanofibrils were randomly dispersed from PSG to ASG-1,and self-assembled into anisotropic herringbone patterns at ASG-2 near the spinneret ready for silkworm spinning.Our findings reveal the controlled self-assembly mechanism of the multi-scale hierarchical architecture of NSF from nanofibrils to herringbone patterns programmed by metal ions and p H gradient,which provides novel insights into the spinning mechanism of silk-secreting animals and bioinspired design of high-performance fibers.