Abstract
Whilst flow is the basis for silk fibre formation, subtle changes in a silk feedstocks’ chemical environment may serve to increase both energetic efficiency and control hierarchical structure development during spinning. Despite the role of pH being largely understood, the influence of metal ions is not, only being inferred by correlative work and observations. Through a combination of rheology and microscopy, we provide a causative study of how the most abundant metal ions in the silk feedstock, Ca2+ and K+, affect its flow properties and structure. Our results show that Ca2+ ions increase viscosity and prevent molecular alignment and aggregation, providing ideal storage conditions for unspun silk. In contrast, the addition of K+ ions promotes molecular alignment and aggregation and therefore seems to transfer the silk feedstock into a spinning state which confirms recent ‘sticky reptation’ modelling hypotheses. Additionally, we characterised the influence of the ubiquitous kosmotropic agent Li+, used to prepare regenerated silk solutions, and find that it promotes molecular alignment and prevents aggregation which may permit a range of interesting artificial silk processing techniques to be developed. In summary, our results provide a clearer picture of how metal ions co-ordinate, control and thus contribute towards silk protein self-assembly which in turn can inspire structuring approaches in other biopolymer systems.
Highlights
Silk spinning in Nature is a sophisticated process occurring in a benign environment, and with minimal energy input yields strong and tough fibres that can compete against synthetic alternatives [1,2,3,4]
We show that reconcentration does not change the flow properties of native silk and we were able to investigate the effects of several metal ions on native B. mori silk feedstock
Native silk proteins were obtained from the middle-posterior gland section of fifth instar B. mori silkworms during ‘early’ or ‘late’ stages of cocoon construction, based on the amounts of fibre spun
Summary
Silk spinning in Nature is a sophisticated process occurring in a benign environment, and with minimal energy input yields strong and tough fibres that can compete against synthetic alternatives [1,2,3,4]. The ‘secret’ behind such remarkable properties lies in the self-assembly of proteins into hierarchical structures under carefully controlled conditions It has been known for over a century [5] that the solidification of silk proteins during natural spinning is initiated by mechanical energy input via flow [4,6,7,8,9]]; more recent work has suggested that a change of pH and ions in the silk glands of spiders [10,11,12,13,14,15] and silkworms [16,17,18,19,20] may be important. The role of metal ions clearly has an impact on both the spinning process and fibre properties, and is an important factor to understand and control during any attempt at artificial silk spinning [33]
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