Differential scanning fluorimetry illuminates silk feedstock stability and processability

Abstract

The ability to design and implement silk feedstock formulations for tailored spinning has so far eluded the bioengineers. Recently, the high throughput screening technique of differential scanning fluorimetry (DSF) demonstrated the link between the instability transition temperature (T_i) and the processability of the silk feedstock. Using DSF we screened a large set of chemicals known to affect solvent quality. A multivariate analysis of the results shows that, regardless of the diversity of chemicals, three groupings are significantly distinguishable: G1 = similar to native silk; G2 = largely dominated by electrostatic interactions; and G3 = dominated by chelating interactions. We propose a thermodynamic analysis based on a pre- and post-transition fit to estimate the van't Hoff enthalpies (ΔH_v) and the instability temperature (T_i). Our analysis shows that the ΔT_i and ΔH_v values were distinct: G1 (ΔT_i = 0.23 ± 0.2; ΔH_v = −159.1 ± 5.6 kcal mol⁻¹), G2 (ΔT_i = −7.3 ± 0.7; ΔH_v = −191.4 ± 5.5 kcal mol⁻¹), and G3 (ΔT_i = −19.9 ± 3.3; ΔH_v = −68.8 ± 6.0 kcal mol⁻¹). Our analysis further combined the ΔT_i value and the ΔH_v value using stability ΔΔG to find that G1 only marginally stabilizes native silks (ΔΔG = −0.15 ± 0.04 kcal mol⁻¹), whereas G2 and G3 destabilize native silk (ΔΔG = 3.8 ± 0.11 and ΔΔG = 3.8 ± 0.3 kcal mol⁻¹, respectively). Here our analysis shows that native silk has a complex multistep transition that is possibly non-cooperative. However, all three groupings also show a direct and cooperative transition with varied stabilization effects. This analysis suggests that native silks are able to sample multiple substates prior to undergoing (or to delay) the final transition. We conclude by hypothesizing that the observed energetic plasticity may be mediated by a fragile packaging of the silk tertiary structure that is readily lost when the solvent quality changes.