| Summary: | Hydrogels are of interest to the biomedical field because the hydrated networks can provide a physiological environment where biological species can survive or grow. Genetic engineering of protein polymers—a synthetic technique which provides a superior level of synthetic control without compromising natural composition—was used to prepare materials of the general architecture, rod-coil-rod. A naturally occurring motif, the leucine zipper, describes the rod domain. The leucine zipper can self-assemble when two amphipathic helices come together and are stabilized by contact along their hydrophobic face. The acidic leucine zipper domain, denoted ‘A,’ contains mostly glutamic acid in residues which flank the hydrophobic interface. A polyelectrolyte protein, of the repetitive sequence [(AG)3PEG], defines the coil domain, denoted ‘C.’ AC10Acys displayed reversible gelation as a function of pH and temperature, thus three aspects of the viscoelastic behavior were investigated. The gelation properties were studied by single particle tracking, which monitors the Brownian motion of fluorescent particles imbedded in a protein hydrogel or suspended in a protein solution. First, the physical crosslinks in an AC10Acys hydrogel network were diplaced by the addition of a leucine zipper domain, Atrp. A 2.23 mM AC10Acys hydrogel behaved as an elastic gel at pH 8.5, but upon addition of 1.13 mM Atrp, a viscous solution was obtained. Second, the effect of charge of the leucine zipper domains were examined using, AC10Acys, and BC10Bcys, where ‘B’ denotes a basic leucine zipper domain. Both protiens form viscous solutions at 1.78 mM, pH 8.5 or pH 7.4, however, upon combination of AC10Acys and BC10Bcys, a stiff elastic gel is formed. Finally, a series of triblock proteins with increasing midblock length were genetically engineered to study the influence of midblock length on the gelation behavior of triblock proteins. The pH and concentration dependences of gelation of ACxAcys, where x = (20, 30, 40, 50), were examined by single particle tracking. Whereas AC10Acys was found to gel around 2.23 MM, pH 8.0, the concentration required for gelation decreases to 1.27 mM for the protein with the longest midblock length, AC50Acys.
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