Sericin Promotes Fibroin Silk I Stabilization Across a Phase-Separation

Abstract

Natural silk spinning offers several advantages over the synthetic fiber spinning, although the underlying mechanisms of this process are yet to be fully elucidated. Silkworm silks, specifically B. mori, comprise two main proteins: fibroin, which forms the fiber, and sericin, a coextruded coating that acts as a matrix in the resulting nonwoven composite cocoon. To date, most studies have focused on fibroin's self-assembly and gelation, with the influence of sericin during spinning receiving little to no attention. This study investigates sericin's effects on the self-assembly of fibroin via their natural phase-separation. Through changes in sample opacity, FTIR, and XRD, we report that increasing sericin concentration retards the time to gelation and β-sheet formation of fibroin, causing it to adopt a Silk I conformation. Such findings have important implications for both the natural silk spinning process and any future industrial applications, suggesting that sericin may be able to induce long-range conformational and stability control in silk fibroin, while being in a separate phase, a factor that would facilitate long-term storage or silk feedstocks.

Differential Scanning Calorimetry of Native Silk Feedstock

“…This rise is most likely due to the dehydration of the proteins present, and the return to the baseline is due to either the structural reconfiguration of the proteins (e.g., gelation which agrees well with recent rheological studies) or could imply that proteins present are able to become partially rehydrated over the timescale of the experiment (discussed in more detail in Section S3, Supporting Information). This rehydration phenomena may be attributed to sericin itself, which is known to resolubilize more readily than fibroin (perfectly exemplified by degumming) or perhaps acting as a renaturing/protective agent for fibroin which is consistent with other recent observations, or finally, that certain portions of the fibroin protein are more hydrophilic than others (Section S3, Supporting Information).…”

The rheological properties of native sericin

“…Furthermore, we observe that sericin flows in a quasi-Newtonian manner throughout the fibroin shearing range (i.e. until UC,Fib), at which point its viscosity drops, which may be the result of selection for the need to maintain the consistent sericin-fibroin interface required for stable long-term fibroin storage [126], but later thin in order to ease the transport of an increasingly solid fibroin core through our recently proposed "solidity gradient" model [91]. These arguments are further supported by recent work into drying rates of newly spun fibres which found that the external (sericin) layers remain wet for several seconds [116], and that when spun under high humidity, flow can continue between cocoon layers [33], suggesting sericin does not solidify within the animal.…”

The Biomedical Use of Silk: Past, Present, Future

“…The Bombyx mori protein fiber is a composite material comprising a semi‐crystalline silk core (i.e., silk fibroin), which is mainly responsible for the load‐bearing capacity, and an outer layer of sericin, which functions as a gumming agent . However, emerging evidence suggests that sericin also inhibits the premature conversion of soluble silk (silk I) into β‐sheet‐rich silk …”