Influence of CO2 on the micro-structural properties of spider dragline silk: X-ray microdiffraction results

The mechanical properties of spider dragline fibres are altered by CO2 exposure under anaesthetizing conditions during the spinning process. In order to relate these macroscopic changes to a microscopic model, the extrusion of dragline silk was studied by synchrotron radiation microdiffraction. A brief exposure of a female Nephila senigalensis spider to CO2 results after an incubation time of less than 7 min in the extrusion of a thread (two fibres) swollen with water. The data are interpreted for a model of crystalline β-sheet domains containing nanofibrils, which reinforce a network of protein chains. The protein network absorbs water, leaving the nanofibrils unaffected. A continuous flow of CO2 results in a co-extrusion of a dragline thread and an isotropic silk fraction, which probably has a glycine-rich sequence. Long CO2 exposure reduces the axial alignment of nanofibrils, presumably due to a partial destruction of the amorphous network.