Modifications of spider silk sequences in an attempt to control the mechanical properties of the synthetic fibers

Bacteria were genetically engineered to produce two spider silk protein variants composed of basic repeat units combining a flagelliform elastic motif ([GPGGX](4)) and a major ampullate silk strength motif ([linker/poly-alanine]. The secondary structures of the pure recombinant proteins in solution were determined by circular dichroism. The data presented suggest that the nature of the 5th and 10th amino acid (X) in the [GPGGX](2) elastic motif and temperature have an impact on the amount of beta-sheet structures present in the proteins. More specifically, increasing temperatures seem to be positively correlated with beta-sheet formation for both proteins and this state is irreversible or reversible when both X (5th and 10th) in the elastic motif are hydrophilic or hydrophobic respectively. Moreover, each pure silk-like protein was able to spontaneously self-assemble into films from aqueous solutions. Two kinds of synthetic fibers were made by pulling fibers from these preassembled films as well as spinning fibers from each protein resolubilized in HFIP. The mechanical data show that the pulled fibers are far tougher than the spun fibers suggesting a better fiber organization.