Differential Scanning Calorimetry of Native Silk Feedstock

Abstract

Native silk proteins, extracted directly from the silk gland prior to spinning, offer access to a naturally hydrated protein that has undergone little to no processing. Combined with differential scanning calorimetry (DSC), it is possible to probe the thermal stability and hydration status of silk and thus investigate its denaturation and solidification, echoing that of the natural spinning process. It is found that native silk is stable between −10 °C and 55 °C, and both the high‐temperature enthalpy of denaturation (measured via modulated temperature DSC) and a newly reported low‐temperature ice‐melting transition may serve as useful quality indicators in the future for artificial silks. Finally, compared to albumin, silk's denaturation enthalpy is much lower than expected, which is interpreted within a recently proposed entropic desolvation framework which can serve to unveil the low‐energy aquamelt processing pathway.

Artificial spinning of natural silk threads

“…Thus, the DSC data presents a clear folding/unfolding transition, in which the heat input leads to a conformational change of the protein structure. These results agree well with previous DSC work on NSF, which presented similar transition peaks at T m = 67.8–68.6 °C and denaturation enthalpies of ∆H = 0.25–0.42 J/g 16–18 . Notably, DSC curves of RSF feedstock lacked any thermal response during both successive thermal runs.…”

“…Briefly, differential scanning calorimetry (DSC) thermograms of NSB featured endothermic transition peaks at T m = 72.2 ± 0.2 °C, which were characterized with denaturation enthalpies of ∆H = 1.49 ± 0.39 J/g (1 st heat run). No transition peaks were identifiable upon reheating (2 nd heat run), demonstrating irreversible denaturation characteristics 16–18 . Thus, the DSC data presents a clear folding/unfolding transition, in which the heat input leads to a conformational change of the protein structure.…”

Spinning Beta Silks Requires Both pH Activation and Extensional Stress

“…Upon calculating the work required for feedstock solidification, we found it centered around (7.48 ± 6.57 J g −1 ), which is similar to recent DSC (1.78 ± 0.25 J g −1 ) [45] and shear rheology (2.75 ± 1.37 J g −1 ) [44] studies. [44,45] Moving beyond an average and to verify pH as a source of fiber variation, we compared the work input to form a fiber, as defined by the integral of feedstock stretching stress (linked to acidification, Figure 1a), to its breaking stress (Figure 2d) and found a clear relationship between the two. The correlation between breaking stress against work input for each individual fiber indicates that the degree of gelation prior to processing affects fiber performance and by adjusting the feedstock pH as well as the spinning speed, fiber properties can be tuned to be very strong and stiff (high work input) or weaker but more extensible (low work input) (Figure 2d).…”

Mesoscale structure development reveals when a silkworm silk is spun

“…Ideally, this would be probed in vivo, and while this is becoming a reality for nanoscale structures via spectroscopy 6 , 24 , at the mesoscale there are currently significant practical and technological challenges that make this an impossibility. Hence previously this has been approached bottom-up, by looking at the structures formed by an aqueous silk feedstock as it subjected to a range of thermal, chemical and mechanical stress fields which serve to initiate protein self-assembly and denaturation 25 , 26 . An alternative approach is top-down, reverse engineering features present in the spun fibre, usually as a result of different spinning conditions 27 .…”