Formylation of Recombinant Spider Silk in Formic Acid and Wet Spinning Studied Using Nuclear Magnetic Resonance and Infrared Spectroscopies.

Abstract

We reported wet spinning of recombinant spider silk protein (RSSP) and formylation of RSSP in formic acid (FA). First, FA was selected as the spinning solvent and the detailed spinning condition was determined. Next, the mechanical property was compared between the RSSP fiber spun after allowing the spinning solution dissolved in FA to stand for 2 days and the fiber spun immediately after being dissolved in FA for 4 h. The tensile strength of the former fiber was lower than the strength of the latter fiber. This difference can be explained by the difference in the degree of formylation as follows. FA is a known formylating agent, although most researchers who prepared silk fiber by wet spinning with FA have not pointed out about formylation. The formylation of the Ser OH group was confirmed by 13C solution nuclear magnetic resonance (NMR), and the time course of formylation of the RSSP film prepared from the FA solution was tracked by Fourier transform infrared spectroscopy. The 13C solid-state NMR spectra were also compared between two kinds of the formylated RSSP fibers and indicated that the packing state was tighter for the latter fiber than the former one, which could explain higher tensile strength of the latter fiber in the dry state. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis indicated that the RSSP sample decomposed gradually with storage time in FA and the decomposition has begun partly even at 2 h after dissolution in FA. The decomposition by formylation seems to have no significant effect on the backbone structure of the RSSP fiber, although the packing of the fiber becomes loose as a whole. Finally, preliminary trial of deformylation of the formylated RSSP fiber was performed.