Changes in Silk Feedstock Rheology during Cocoon Construction: The Role of Calcium and Potassium Ions.

Peter R Laity,Elizabeth Baldwin,Chris Holland

doi:10.1002/mabi.201800188

Abstract

Variation in silk feedstocks is a barrier both to our understanding of natural spinning and biomimetic endeavors. To address this, compositional changes are investigated in feedstock specimens from the domesticated silkworm (Bombyx mori). It is found that the feedstock viscosity decreased systematically by over two orders of magnitude during cocoon construction. Potential factors such as protein concentration, molecular weight, pH, or the presence of trehalose are excluded, whereas a clear correlation appear between viscosity and the relative concentrations of Ca2+ and K+ ions. It is expected that Ca2+ ions would favor "salt bridges" between acidic (Asp and Glu) amino acids, leading to an increased viscosity, whereas K+ ions would compete for these sites, thereby reducing viscosity. Thus, these findings suggest a simple, systematic yet sophisticated control of feedstock viscosity in the silkworm, which in turn can be applied to future industrial silk production.

Highlights

50 to over 6000 Pa s.[18,19] While a small part of this rheological variation may be related to observed changes in the solidsVariation in silk is often perceived to be a barrier both to our content of the feedstock, a definitive understanding of natural spinning and successful biomimetic explanation for the largest part remains elusive.implementation of similar methods
Silk fibers are produced by a diverse range of arthropods orders of magnitude during the course of cocoon construction, including, most notably, true spiders and caterpillars.[1,2,3,4,5,6,7,8]
Several compositional changes were identified, which may be related to metabolic changes prior to or during cocoon construction, but were unlikely to be the main cause of changes to the feedstock rheology

Summary

Introduction

50 to over 6000 Pa s.[18,19] While a small part of this rheological variation may be related to observed changes in the solidsVariation in silk is often perceived to be a barrier both to our content of the feedstock (from ≈18 to 30 wt%), a definitive understanding of natural spinning and successful biomimetic explanation for the largest part remains elusive.implementation of similar methods. Silkworm body mass, and analyses of properties are largely understood, variation in the unspun feedstock composition using a range of techniques are reported protein feedstock is not. This is addressed in the present study. A more detailed discussion of the rheological modulus, strength, and toughness, rivalling high performance effects will be presented in the subsequent paper by Schaefer man-made fibers.[6,7,8,9,10,11] they are produced under physi- et al.[20] From a biological perspective, these changes may be ological conditions, from an aqueous protein solution (feedstock) associated with switching the feedstock from fibroin producthat is prepared and stored inside the animal at ambient tempera- tion and storage to fiber spinning. Understanding tures, without large energy inputs, or use of harmful chemicals.[12] the interactions between dissolved ions and fibroin may have

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Discussion

Conclusion