Functionalizing maize zein as viscoelastic polymers through β-sheet-rich protein networks
Abstract
As a major byproduct of the corn wet-milling industry, considerable attention has been directed toward identifying high value-added applications for the zein protein fraction—a complex mixture of aqueous alcohol-soluble proteins present in the endosperm of the corn kernel. Viscoelastic properties have been observed in zein above its glass transition temperature, and it is believed that this functionalization of the protein could have great potential for enhancing zein's applicability in food and biomaterial systems. In viscoelastic zein systems, the formation of intermolecular β-sheet structures has been shown to greatly influence mechanical strength and elasticity; however, the molecular interactions responsible for driving the formation of these assemblies have not been identified. This thesis presents a model systems approach for developing key structure-function relationships to understand how viscoelastic properties are generated in zein systems and to identify strategies for how these properties can be further manipulated for optimal performance in various applications. Zein resins were formed via precipitation from aqueous ethanolic environments, and the effects of zein/plasticizer and zein/co-protein interactions were evaluated as a possible strategy to control zein's state transition and mechanical properties. Plasticizing interactions with oleic acid reduced zein's water absorption, decreased the protein's glass transition temperature, and decreased low frequency β-sheet secondary structures, resulting in low elasticity/high extensibility resins. Co-protein interactions with casein increased moisture absorption, increased zein's glass transition temperature, and increased low frequency β-sheet secondary structures to form highly elastic resin systems with improved mechanical strength. Changes in zein's conformation were also investigated during the protein's self-assembly in increasingly hydrophilic solvent systems, using different ethanol-water ratios to manipulate solvent polarity. Circular dichroism spectra showed increases in water content decreased α-helix structures in favor of random coil and β-sheets, similar to observations of zein in viscoelastic dough systems. Evidence of conformational changes in zein being mediated by solvent polarity was further supported by changes in Thioflavin T fluorescence emission spectra and intrinsic viscosity measurements. These findings emphasize the importance of structural changes during zein's aggregation and provide further insight into the mechanisms by which the protein is functionalized. All-atom molecular dynamics simulations were performed to identify the key interactions that stabilize the β-sheet secondary structures connected with zein's elastic properties. Twelve model peptides were selected from three common α-zein variants and demonstrated a range of affinities for forming β-sheet structures. This behavior indicates that discrete regions of α-zein's primary structure have differing capacities for influencing system functionality, a property that appears to be contingent upon peptides forming a dense network of backbone hydrogen bonds. In the search to identify strategies to improve zein's versatility in various applications, these interaction types are a likely target for promoting or disrupting β-sheet associations and thus modify the protein's mechanical properties. Through this work, viscoelasticity and its structural basis in zein systems have been evaluated across a variety of scales (macro, micro, molecular). Understanding these fundamental mechanisms that drive this unique functionality will be valuable insight for enhancing the overall applicability of this underutilized biopolymer.
Degree
Ph.D.
Advisors
Campanella, Purdue University.
Subject Area
Food Science
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