Crystal and liquid crystal structure of natural silk fibroin and synthetic polypeptides
Abstract (Summary)This research involves in vitro crystallization and the characterization of crystal structure of silk I. This polymorph, silk I, was discovered in the contents of the air-dried silk gland half a century ago. However, the metastable nature and the lack of orientation in the silk I samples have thwarted the understanding of this structure. We have successfully reproduced a silk I structure in vitro by foaming and enzymatic hydrolysis of silk fibroin. The processing parameters, the solution concentration and the pH, as well as the amorphous portion and the amino acid composition in the crystalline sequence of silk fibroin, were examined via studies of crystallization, the crystal structure of silk I foams, the Cp fraction, and a silk-like sequential polypeptide, Poly (L-Ala-Gly). The characterization techniques employed include electron diffraction coupled with TEM imaging, X-ray diffraction, infrared spectroscopy, and molecular simulations and simulated diffraction. Re-examination of the ' crankshaft model ' indicated that the crankshaft conformation proposed for silk I is incorrect and the hydrophilic serine residues are not negligible for the packing of the silk I structure. We proposed a tentative silk I model based on an almost fully extended chain conformation with a slight twist. Six staggered protein chains make a crystallographic repeat for an orthorhombic unit cell of 22.66 Ã? Ã?-- 5.7 Ã? Ã?-- 20.82 Ã? with a rise per residue of 3.47 Ã?. The solid state analog of the liquid crystalline structure of monodisperse poly (Ã?Â³-benzyl Ã?Â±,L-glutamate) (PBLG) was studied using atomic force microscopy and electron diffraction coupled with TEM imaging. These PBLG molecules were produced by recombinant DNA technology, and display a smectic ordering resultant from rigid Ã?Â±-helical rods of uniform length. TEM and AFM have revealed a banded morphology, which indicates a twist of the director field as in a cholesteric or twisted smectic. Detailed examination of the relative orientation of the banding and the diffraction pattern leads to the conclusion that the structure observed is a twist-grain-boundary (TGB)-like phase. This is consistent with the superposition of a smectic-A layering resulting from the uniform rod length on the supramolecular twist (the cholesteric) present in the cholesteric.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/2000