Thesis (PH.D.) -- Syracuse University, 2007. / "Publication number AAT 3281759"

Genetic engineering of protein-based polymers offers distinct advantages over conventional synthesis of polymers. Microorganisms can synthesize high molecular weight materials, in relatively large quantities, that are inherently stereoregular, monodisperse, and of controlled sequence. In addition, specific secondary and higher order structures are determined by this protein sequence. As a result, scientists can design polymers to have unique structural features found in natural protein materials and functional properties that are inherent in certain peptide sequences. For this reason, genetic engineering principles were used to create a set of artificial genes that encode twelve macromolecules having both $\alpha$-helical and disordered coil protein sequences with the last amino acid being cysteine (cys) or tryptophan (trp). Triblock copolymer sequences having coiled-coil protein ends, A or B, where A and B represent $\alpha$-helical acidic and basic leucine zipper proteins, separated by a water soluble flexible spacer coil protein, C, where C represents ((AG)$\sb3$PEG) $\sb{\rm n}$ (n = 10 or 28), showed reversible physical gelation behavior. This behavior is believed to result from the aggregation of two or more helices that form physical crosslinks with the disordered coil domain retaining solvent and preventing precipitation of the chain. Diffising wave spectroscopy was used to investigate the gelation behavior of AC$\sb{10}$Acys in buffer when environmental conditions such as pH, temperature, and concentration were varied. The dynamic intensity autocorrelation function recorded over time for 5% (w/v) AC$\sb{10}$Acys showed that the protein behaved as a gel at pH 6.7-8.0 and that the melting point was between 40$\sp\circ$C and 48$\sp\circ$C. In addition to the triblock results, the incorporation of 5$\sp\prime$,5$\sp\prime$,5$\sp\prime$-trifluoroleucine (Tfl) in place of leucine (Leu) in the A and B blocks was accomplished by synthesizing proteins in bacterial hosts auxotrophic for Leu. The substitution of Tfl for Leu in A and B was confirmed by electrospray mass spectrometry. Amino acid analyses performed on purified Tfl A and Tfl B populations suggested 66% and 38% levels of Tfl substitution, respectively. Thermal denaturation temperatures measured by circular dichroism of the Tfl containing helices were higher than those of the corresponding Leu containing helices by 8$\sp\circ$C and 13$\sp\circ$C for A and B respectively.

Principles of genetic engineering and recombinant DNA technology were applied to the modification and microbial synthesis of artificial extracellular matrix proteins, which were designed for eventual application in vascular prostheses. Proteins were synthesized, in which the elastin-like pentapeptide (VPGIG) was repeated alone or in a blocky structure with a cell binding domain derived from the CS5 region of fibronectin. To control physical and mechanical properties, unique amine functionality was added to the termini of these proteins by the incorporation of lysine-containing fusions. Detailed purification protocols were developed for the isolation of three CS5-containing proteins having the same primary sequence, yet distinct lengths, and one CS5-free protein. The success of these protocols was verified by SDS-PAGE, Western analysis, amino acid analysis, and, in two cases, NMR spectroscopy. The integrity of the elastin-like sequence was further verified by the presence of a lower critical solution temperature (LCST). Reacting the engineered proteins with glutaraldehyde resulted in crosslinked biomaterials that exhibited improved stability, with respect to solubility, and controllable mechanical response. The elastic moduli of these biomaterials was found to be inversely related to the protein masses from which they were derived, and approached modulus values of native elastin. Approximately, three out of every 4 amines participated in the crosslinking reaction, which was observed to be complicated one. Measurement of biological properties was limited to the evaluation of substrate specificity, with respect to cellular adhesion and spreading. Human umbilical vein endothelial cells (HUVECs) were observed to adhere to crosslinked, CS5-containing substrates at levels similar to that of fibronectin (FN) and at levels significantly greater than to the CS5-free control. HUVEC adhesion was found to be independent of blocking conditions. The binding mechanism did not involve the elastin binding protein, yet was one in which the CS5 domain participated. Human umbilical artery smooth muscle cells (UASMCs) behaved in the same manner and with an unknown binding mechanism. Glutaraldehyde reaction products were implicated as participants in a nonspecific binding mechanism. Due to the ambiguities introduced by glutaraldehyde, a bis-N-hydroxysuccinimidyl ester of polyethylene glycol was employed as a crosslinking agent. The aqueous reaction conditions, although controllable to a certain extent, through pH and temperature, proved to be deleterious in achieving stable networks. HUVEC seeding indicated that this strategy, or other similar ones, may not alter binding characteristics in a significant manner.

Precise control over the responsive behavior of hydrogels is crucial for medical applications of these materials. To better understand how pH-triggered gelation and materials properties of the resultant gels can be controlled, a series of multiblock copolymers has been prepared by genetic engineering. The advantage of genetically engineered materials is that they are well-defined, monodisperse materials. In particular, the exact length and sequence of each block of a multiblock copolymer can be controlled. In this system triggerable gelation is based on the self-association of the naturally occurring Fos leucine zipper domain to form crosslinks in response to changes in pH. A polyelectrolyte domain (C) is interspersed with the leucine zipper domains (F) and renders the network water-soluble. Three block copolymers, consisting of three (L2FC3), five (L2FC5), and seven (L2FC7) blocks, respectively, were prepared via bacterial protein expression. The responsive behavior of these materials with respect to pH and temperature was characterized using circular dichroism and rolling ball viscometry. The different architectures of these block copolymers play a role in modulating the response to pH. L2FC5 and L2FC7 exhibit a stronger pH-response as compared to L2FC3, but no difference was observed between the two longer block copolymers.

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