Water and humidity severely affect the material properties of spider major ampullate silk, causing the fiber to become plasticized, contract, swell and undergo torsion. Several amino acid residue types have been proposed to be involved in this process, but the complex composition of the native fiber complicates detailed investigations. Here, we observe supercontraction in biomimetically produced artificial spider silk fibers composed of defined proteins. We found experimental evidence that proline is not the sole residue responsible for supercontraction and that tyrosine residues in the amorphous regions of the silk fiber play an important role. Furthermore, we show that the response of artificial silk fibers to humidity can be tuned, which is important for the development of materials for applications in wet environments, eg producing water resistant fibers with maximal strain at break and toughness modulus.

Efficient production of artificial spider silk fibers with properties that match its natural counterpart has still not been achieved. Recently, a biomimetic process for spinning recombinant spider silk proteins (spidroins) was presented, in which important molecular mechanisms involved in native spider silk spinning were recapitulated. However, drawbacks of these fibers included inferior mechanical properties and problems with low resistance to aqueous environments. In this work, we show that ≥5 h incubation of the fibers, in a collection bath of 500 mM NaAc and 200 mM NaCl, at pH 5 results in fibers that do not dissolve in water or phosphate buffered saline, which implies that the fibers can be used for applications that involve wet/humid conditions. Furthermore, incubation in the collection bath improved the strain at break and was associated with increased β-sheet content, but did not affect the fiber morphology. In summary, we present a simple way to improve artificial spider silk fiber strain at break and resistance to aqueous solvents.

Silk fibres derive their exceptional properties from hierarchical protein organisation, yet the molecular pathways that guide this structural transformation remain poorly resolved. During regenerated silk fibroin gelation under biomimetic gradual acidification, we identify a stepwise assembly pathway comprising nanoscale clustering, domain growth within clusters, and mesoscale network formation. Time-resolved small-angle neutron scattering performed simultaneously with turbidity and fluorescence emission (NUrF) identifies unique intermediates and a regulated onset of β -contacts and β -sheets assembly, indicating that fibril formation requires prior compaction and network connectivity. By contrast, methanol-induced gelation bypasses these intermediates, driving rapid aggregation. These findings define the sequence and timing of events that construct silk’s hierarchical architecture without accidental aggregation, showing how pathway selection governs material outcomes. This multiscale resolution achieved by NUrF provides a broadly applicable strategy for probing hierarchical assembly in silk and other protein materials.

Silk fibres derive their exceptional properties from hierarchical protein organisation, yet the molecular pathways guiding this evolution remain poorly resolved. During regenerated silk fibroin gelation under biomimetic gradual acidification, we resolve a stepwise assembly pathway comprising nanoscale clustering, growth of domains within clusters, and mesoscale network formation. Time-resolved small-angle neutron scattering performed simultaneously with UV and fluorescence emission (NUrF) identifies unique intermediates and a regulated onset of β-sheet assembly, indicating that fibril formation requires prior compaction and network connectivity. By contrast, methanol-induced gelation bypasses these intermediates, driving rapid aggregation. These findings define the sequence and timing of events that construct silk’s hierarchical architecture without accidental aggregation, showing how pathway selection governs material outcomes. This multiscale resolution achieved by NurF provides a broadly applicable strategy for probing hierarchical assembly in silk and other protein materials.