Chemical sequence control of crystallization in periodic polypeptides of the sequence poly(AG)XEG/

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)\sb3GluGly (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)\sb3EG 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)\sb3EG. Solid state structural analysis on poly(AG)\sb4EG-I, poly(AG)\sb5EG-I, poly(AG)\sb6EG-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)\sb3EG-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)\sb3EG-II (type II crystalline modification prepared by dialysis of aqueous LiBr solutions) and poly(AG)\sb3EG-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)\sb3EG-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.

Synthesis and Characterization of Periodic Polypeptides Containing Repeating —(AlaGly)_xGluGly— Sequences

We have expressed in E. coli a series of periodic polypeptides represented by sequence 1. Our objective has been an understanding of the role of chemical sequence in determining the chain folding behavior of periodic macromolecules. Molecular organization has been examined by infrared spectroscopy and ^1H and ^(13)C NMR methods and a preliminary model of the folded structure has been developed

Genetic Synthesis of Periodic Protein Materials

Genetic engineering offers a novel approach to the development of advanced polymeric materials, in particular protein-based materials. Biological synthesis provides levels of control of polymer chain architecture that cannot yet be attained by current methods of chemical synthesis. In addition to employing naturally occurring genetic templates artificial genes can be designed to encode completely new materials with customized properties. In the present paper we: 1) review the concepts and technology of creating protein-based materials by genetic engineering, 2) discuss the merits of producing crystalline lamellar proteins by this approach, and 3) review progress made by our group in generating such materials by genetic strategies. Full descriptions appear elsewhere about the parameters to be considered in designing artificial protein genes of this type, the effectiveness of different gene construction and expression strategies utilized by us thus far and, the specific properties of the various materials derived from these efforts (1,2). Progress made by other groups involved in developing periodic proteins by molecular biological strategies are described in refs. 3-8. The latter studies include genetic engineering of artificial silk-like proteins (3,4), poly-aspartylphenylalanine (5), an α/β barrel domain (octarellin; 6), the collagen tripeptide GlyProPro (7) and human tropoelastin (8). Advances with the silk-like proteins (SLP) have been particularly impressive. In addition to producing multi-gram quantities of pure SLP homopolymers, this group has successfully generated block copolymers of SLP interspersed with core peptides of mammalian elastin and the human fibronectin cell attachment element. While publications are still lacking it appears that a numiber of groups are striving to create genetically engineered variants of the repetitive bioadhesive proteins produced by mussels and barnacles (9)

Biocatalytic Synthesis of Polymers of Precisely Defined Structures

The fabrication of functional nanoscale devices requires the construction of complex architectures at length scales characteristic of atoms and molecules. Currently microlithography and micro-machining of macroscopic objects are the preferred methods for construction of small devices, but these methods are limited to the micron scale. An intriguing approach to nanoscale fabrication involves the association of individual molecular components into the desired architectures by supramolecular assembly. This process requires the precise specification of intermolecular interactions, which in turn requires precise control of molecular structure

Controlled assembly of macromolecular beta-sheet fibrils

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EFFECT OF WATER ON THE STRUCTURE OF A MODEL POLYPEPTIDE

Vibrational spectroscopy in conjunction with X-ray and gravimetric methods has been used to study the structural stability of a hydrated polypeptide prepared in a genetically engineered strain of Escherichia coli. The sample adopts a β-sheet conformation in the solid state with a well-defined crystalline stem length and fold surfaces believed to be decorated with carboxylic acid groups. Ionization and subsequent hydration of these acid groups are found to have a major effect on the crystal packing and chain conformation. We have also established that the structural changes accompanying hydration of this model polypeptide occur in a stepwise fashion. First, because of their high accessibility, the carboxylic acid or carboxylate groups on the lamellar surfaces can readily interact with water molecules. In the second step, water penetrates into the regions between the hydrogen-bonded sheets; however, the resulting expansion in the intersheet distance can occur without altering the chain conformation. Lastly, when the water content is high, the hydrogen-bonded sheets are disrupted, leading to a change in chain conformation from ,&strands to helical or disordered chains

Neurosarcoidosis resembling meningioma: MRI characteristics and pathologic correlation.

A 37-year-old woman had visual changes. Magnetic resonance imaging showed an extraaxial mass in the anterior clinoid region that was presumed to be meningioma. There was no evidence of systemic or leptomeningeal disease. Pathologic findings were consistent with sarcoidosis. Isolated mass-like neurosarcoidosis, without systemic or leptomeningeal disease is difficult to diagnose preoperatively