Browsing by Author "Zhang, Xiao"

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    Adsorption of Biomacromolecules onto Polysaccharide Surfaces
    Zhang, Xiao (Virginia Tech, 2014-10-02)
    Plant cell wall polysaccharides are abundant natural polymers making them potential sources for sustainable and biodegradable materials. Interfacial behavior, including adsorption and enzymatic degradation, of several plant cell wall polysaccharides and their derivatives were studied with a quartz crystal microbalance with dissipation monitoring (QCM-D), surface plasmon resonance (SPR) and atomic force microscopy (AFM). Xyloglucan adsorption isotherms were obtained to probe how cellulose-hemicellulose interactions were affected by the type of cellulose substrate and molar mass of xyloglucan. Xyloglucan as small as a heptasaccharide still adsorbed irreversibly onto cellulose. Carboxymethyl cellulose (CMC) adsorption onto cellulose and viscoelastic properties and water contents of the adsorbed CMC layers were obtained from a combination of QCM-D and SPR data. The CMC samples formed hydrated and viscoelastic layers compared to the relatively rigid xyloglucan layer. Pectin model surfaces were prepared by pectin adsorption from citric phosphate buffer onto gold substrates. These pectin model surfaces were used for subsequent interaction studies with xyloglucan and enzymatic degradation behavior. There is a strong correlation between the degree of esterification (DE) and film resistance to degradation with the high DE being the most susceptible to degradation. The adsorption of two mixed linkage glucans (MLG), barley and lichen MLG, onto regenerated cellulose (RC) surfaces in the absence and presence of other matrix polysaccharides was studied. Viscoelastic properties of the resulting layer were compared as a function of the proprotion of '-(1''3) linkages with lichen MLG forming softer gel-like layers on RC. The lichen MLG layers were further used for enzymatic degradation studies with respect to enzyme concentration, temperature, pH and ionic strength. These studies show that polymer adsorption is a promising strategy to modify material surfaces and provides fundamental understanding of interactions and biodegradation of cell wall polysaccharides at solid/liquid interfaces.
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    Binding Interactions of Keratin-Based Hair Fiber Extract to Gold, Keratin, and BMP-2
    de Guzman, Roche C.; Tsuda, Shanel M.; Ton, Minh-Thi N.; Zhang, Xiao; Esker, Alan R.; Van Dyke, Mark E. (PLOS, 2015-08-28)
    Hair-derived keratin biomaterials composed mostly of reduced keratin proteins (kerateines) have demonstrated their utility as carriers of biologics and drugs for tissue engineering. Electrostatic forces between negatively-charged keratins and biologic macromolecules allow for effective drug retention; attraction to positively-charged growth factors like bone morphogenetic protein 2 (BMP-2) has been used as a strategy for osteoinduction. In this study, the intermolecular surface and bulk interaction properties of kerateines were investigated. Thiol-rich kerateines were chemisorbed onto gold substrates to form an irreversible 2-nm rigid layer for surface plasmon resonance analysis. Kerateine-to-kerateine cohesion was observed in pH-neutral water with an equilibrium dissociation constant (KD) of 1.8 × 10⁻⁴ M, indicating that non-coulombic attractive forces (i.e. hydrophobic and van der Waals) were at work. The association of BMP-2 to kerateine was found to be greater (KD = 1.1 × 10−7 M), within the range of specific binding. Addition of salts (phosphate-buffered saline; PBS) shortened the Debye length or the electrostatic field influence which weakened the kerateine-BMP-2 binding (KD = 3.2 × 10⁻⁵ M). BMP-2 in bulk kerateine gels provided a limited release in PBS (~ 10% dissociation in 4 weeks), suggesting that electrostatic intermolecular attraction was significant to retain BMP-2 within the keratin matrix. Complete dissociation between kerateine and BMP-2 occurred when the PBS pH was lowered (to 4.5), below the keratin isoelectric point of 5.3. This phenomenon can be attributed to the protonation of keratin at a lower pH, leading to positive-positive repulsion. Therefore, the dynamics of kerateine-BMP-2 binding is highly dependent on pH and salt concentration, as well as on BMP-2 solubility at different pH and molarity. The study findings may contribute to our understanding of the release kinetics of drugs from keratin biomaterials and allow for the development of better, more clinically relevant BMP-2-conjugated systems for bone repair and regeneration..