Inducing β-Sheets Formation in Synthetic Spider Silk Fibers by Aqueous Post-Spin Stretching

Abstract

As a promising biomaterial with numerous potential applications, various types of synthetic spider silk fibers have been produced and studied in an effort to produce manmade fibers with mechanical and physical properties comparable to those of native spider silk. In this study, two recombinant proteins based on Nephila clavipes Major ampullate Spidroin 1 (MaSp1) consensus repeat sequence were expressed and spun into fibers. Mechanical test results showed that fiber spun from the higher molecular weight protein had better overall mechanical properties (70 KD versus 46 KD), whereas postspin stretch treatment in water helped increase fiber tensile strength significantly. Carbon-13 solid-state NMR studies of those fibers further revealed that the postspin stretch in water promoted protein molecule rearrangement and the formation of β-sheets in the polyalanine region of the silk. The rearrangement correlated with improved fiber mechanical properties and indicated that postspin stretch is key to helping the spider silk proteins in the fiber form correct secondary structures, leading to better quality fibers.

Secondary Structure Transition and Critical Stress for a Model of Spider Silk Assembly

“…22,23 Up to a limit of 50mm/s, XRD results show that the higher reeling speed also helps the orientation and formation of the silk network, while no influence on the secondary structure composition could be determined. [24][25][26] Premature invitro self-assembly poses one of the issues involved with the synthetic production of silk fibers.…”

Identification of Wet-Spinning and Post-Spin Stretching Methods Amenable to Recombinant Spider Aciniform Silk

“…In an attempt to promote favorable structural changes that have been shown to improve mechanical properties for other silks 9,12,43,[51][52][53] , post-spin stretching of AS W 3 fibers was carried out. Notably, organic solvent is typically employed in the post-spin stretching step because many silk-based AS fibers cannot withstand water penetration without being dissolved 9,11,12,15,42,44,45,59 .…”

“…Most studies characterizing post-spin stretching of silk-based fibers have reported carrying out this step manually with the use of forceps or tweezers 9,11,12,42 , with some reports of using controlled post-spin stretching apparatuses (e.g. 16,44 ).…”

“…Additionally, the tensile properties of these fibers fall within the range of previously reported wet-spun fibers from recombinant spidroin solubilized in HFIP. For other types of spidroin, strength and extensibility have ranged between 7-36 MPa and 1-5% for AS fibers 9,11,12,42,44,59 , and, 14-508 MPa and 3-307% for PS (3-5x) fibers 9-12, 42-44, 59 . They are also consistent with reports of post-spin stretching non-silk-based protein fibers in water, specifically native 65 and regenerated 66 hagfish slime threads as well as recombinant vimentin fibers 67 .…”

“…Although post-spin stretching of MA silk may increase β-sheet content 12 , the general behavior of MA silk discussed above implies that post-spin stretching results in association of amorphous β-sheets to form oriented β-sheet microcrystallites. With W 3 , post-spin stretching resulted in enrichment of β-sheet character and a corresponding depletion (but, corresponding to native aciniform silk, not a complete loss) of α-helical character.…”