Morphology and primary crystal structure of a silk-like protein polymer synthesized by genetically engineered Escherichia coli bacteria.

The morphology and primary crystal structure of SLPF, a protein polymer produced by genetically engineered Escherichia coli bacteria, were characterized. SLPF is a segmented copolymer consisting of amino acid sequence blocks modeled on the crystalline segments of silk fibroin and the cell attachment domain of human fibronectin. Wide angle x-ray scattering (WAXS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and molecular simulations were used to analyze the primary crystal structure of SLPF. TEM experiments conducted on SLPF droplets cast from formic acid on amorphous carbon film demonstrated that these protein films have a microstructure formed of woven sheaves. The sheaves are composed of well-defined whisker crystallites. The width of the whiskers, 11.8 +/- 2.2 nm, may be correlated to the length of the silk-like segment in SLPF as predicted by molecular simulations. WAXS data, TEM images, SAED, patterns, molecular simulations, and theoretical diffraction patterns all were consistent with the crankshaft model proposed for Silk I by Lotz and Keith.