Stretching of Bombyx mori Silk Protein in Flow

Abstract

The flow-induced self-assembly of entangled Bombyx mori silk proteins is hypothesised to be aided by the ‘registration’ of aligned protein chains using intermolecularly interacting ‘sticky’ patches. This suggests that upon chain alignment, a hierarchical network forms that collectively stretches and induces nucleation in a precisely controlled way. Through the lens of polymer physics, we argue that if all chains would stretch to a similar extent, a clear correlation length of the stickers in the direction of the flow emerges, which may indeed favour such a registration effect. Through simulations in both extensional flow and shear, we show that there is, on the other hand, a very broad distribution of protein–chain stretch, which suggests the registration of proteins is not directly coupled to the applied strain, but may be a slow statistical process. This qualitative prediction seems to be consistent with the large strains (i.e., at long time scales) required to induce gelation in our rheological measurements under constant shear. We discuss our perspective of how the flow-induced self-assembly of silk may be addressed by new experiments and model development.

Seeking Solvation: Exploring the Role of Protein Hydration in Silk Gelation

“…This implies an approximate equivalence between the strengths of monomer-monomer and monomersolvent interactions. Moreover, while it was not possible to observe the coil geometry by SAXS (or SANS [60]) at higher concentration (the interpretation of scattering data required non-overlapping coils), the configuration determined at low concentration was consistent with previous NMR results [46][47][48][49] and rheology [9,99,178], which demonstrated that the protein in NSF behaved as a typical polymer in solution, albeit slightly modified by its natural propensity to form transient 'sticky' ionic interactions [66,67]. (ii) Crucially, the combination of random coil geometry and uniform dynamics (from NMR studies [46][47][48][49]) implies similar levels of hydration across the entire fibroin coil, which precludes any explanation of gelation through interactions between hydrophobic regions.…”

“…These salts were selected because they lie towards the centre of the Hofmeister series [158][159][160][161][162] and are not expected to show any chemical affinity for the protein. In particular, monovalent cations avoid any possibility of bridging between carboxylate groups [65][66][67], which could affect aggregation and turbidity measurements. The turbidity is related to decreases in transmitted light, due to scattering from inhomogeneities in the dilute solution [98,100,[163][164][165][166][167].…”

“…As all the solutions studied by Greving et al were dilute (below 38 mg mL −1 , i.e., concentrations too low for chain overlap), we speculate that apparent variations in R G may have been due to the Ca 2+ concentrations in the solutions. It is known that Ca 2+ in the NSF can form ionic crosslinks (calcium bridges) between carboxylate-substituted amino acids, thereby raising the viscosity through 'sticky reptation' [65][66][67]. It is possible that a similar mechanism in dilute solutions could resist coil expansion (i.e., giving smaller R G values).…”

Structure of Silk I (Bombyx mori Silk Fibroin before Spinning) -Type II β-Turn, Not α-Helix-

“…In other words, the highly concentrated silk solution contained in the middle silk glands has residues in energetically favored conformations close to average random coil values [ 39 , 40 ], but forms a hydrogen-bonded network that keeps it in a repeated type II β-turn structure [ 21 , 22 ]. These data and insights became the starting point to understand the molecular mechanisms behind the flow behavior generating the high-performance silk fibers from viewpoints of rheology [ 83 , 84 , 85 ].…”

Theoretical rheo-physics of silk: Intermolecular associations reduce the critical specific work for flow-induced crystallisation

“…The manufacturing of both natural and artificial polymer-based fibres relies on flowinduced crystallisation in non-linear rheological conditions [1][2][3][4][5]. The energy input required by this process may be significantly reduced through tailored macromolecular interactions, as exemplified by natural silk [6][7][8][9]: This protein, of which the conformation closely resembles a random coil [10], self-assembles in flow in aqueous conditions under energy requirements orders of magnitude lower than its synthetic counterparts [6]. It has been hypothesised that flow-induced stretching of the chain disrupts a solvation layer and in turn enables crystallisation to commence.…”