The influence of metal ions on native silk rheology

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.