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Chemical sequence control of crystallization in periodic polypeptides of the sequence poly(AG)(X)(EG)
Abstract
We are interested in exploiting the known sequence-dependent secondary structures observed in polypeptides to construct chain folded lamellar crystals of specified thickness and surface functionality. To utilize this technique for the production of polymeric materials, artificial oligonucleotide (DNA) monomers encoding two repeats of the oligopeptide sequence (AlaGly)$\sb3$GluGly (1.4) were synthesized chemically. Polymerization of the DNA, and cloning and expression in a bacterial host, resulted in a polypeptide with the expected composition. Structural characterization of poly(AG)$\sb3$EG shows features characteristic of antiparallel $\beta$-sheets (ap$\beta$-sheets). Wide angle x-ray diffraction experiments demonstrate the crystalline nature of this material, and indicate that the crystals are oriented with cylindrical symmetry about the a axis. The results are commensurate with a model constructed from stacked, regularly chain-folded crystalline lamellae composed of polypeptide chains that reverse polarity in register with the sequence periodicity of poly(AG)$\sb3$EG. Solid state structural analysis on poly(AG)$\sb4$EG-I, poly(AG)$\sb5$EG-I, poly(AG)$\sb6$EG-I shows similar crystalline architectures and supports a model in which chain folding is directed by the chemical sequence periodicity. Although experimental evidence collected on poly(AG)$\sb3$EG-I and other members in the series does not provide detailed information regarding the turn geometry, evidence collected on similar systems favors a model that exploits $\beta$-turns to reverse chain polarity. Comparison of the infrared and Raman spectra of poly(AG)$\sb3$EG-II (type II crystalline modification prepared by dialysis of aqueous LiBr solutions) and poly(AG)$\sb3$EG-I shows that substantial differences exist in the frequencies and intensities of the observed bands, suggesting that these materials have different crystal structures. The experimental evidence collected on poly(AG)$\sb3$EG-II supports a crystalline structure composed of chain folded lamellae constructed from the lateral stacking of sheets, in which the peptide backbone is highly contracted in comparison with the ap$\beta$-sheet structure. In this model, the proposed contraction results from the glycine residues adopting a left handed $\alpha$-helical conformation similar to that proposed by Lotz and coworkers for the structure of PLAG-II. (Abstract shortened by UMI.)
Subject Area
Polymer chemistry|Genetics
Recommended Citation
Krejchi, Mark Thomas, "Chemical sequence control of crystallization in periodic polypeptides of the sequence poly(AG)(X)(EG)" (1993). Doctoral Dissertations Available from Proquest. AAI9408296.
https://scholarworks.umass.edu/dissertations/AAI9408296