Investigation of the molecular level interactions between mucins and food proteins: Spectroscopic, tribological and rheological studies

The thesis investigated the structure and molecular-level interaction of β-lactoglobulin (BLG) and mucins, representing major components of the dairy products and saliva/digestion systems, respectively. Mucins are long glycoprotein molecules responsible for the gel nature of the mucous layer covers epithelial surfaces throughout the body. A literature review of the interactions of different mucin types and saliva mucins with several food proteins and food protein emulsions, as well as their functional properties related to the food oral processing is presented at the first chapter of the thesis (Paper V). Most of the studies suggest an electrostatic attraction between positively charged food proteins with negatively charged moieties of mucins (mainly on glycosylated region of mucins).
The structural changes occurring during the interaction between BLG, the major whey protein, and bovine submaxillary mucin (BSM), a major salivary protein, were studied using high and low field Nuclear Magnetic Resonance (NMR), Dynamic Light Scattering (DLS), and Circular Dichroism (CD) spectroscopy. The zeta potentials of the proteins were also measured to provide information on the role of electrostatic forces in the interaction. These spectroscopic results suggested that the interaction between BSM and BLG led to a compact aggregation. The interaction between the two proteins was concluded to be mostly of hydrophilic origin (Paper I). The interaction characteristics between mucins and BLG under tribological stress were investigated by comparing the lubricity of mixed solutions of mucin-BLG with that of neat protein solutions at compliant hydrophobic interfaces. BSM and porcine gastric mucin (PGM) showed distinctly higher adsorbed masses compared to BLG onto polydimethylsiloxane (PDMS) or polystyrene (PS) surfaces. The adsorbed masses of the mixed protein solutions, namely BLG-BSM and BLG-PGM, reduced significantly. The dominant lubrication mechanism of the protein solutions was boundary lubrication. The pH
2
dependent lubricating properties of BLG-BSM mixed solutions appeared to be determined by competitive adsorption of the two proteins onto the substrates, which suggests that they do not form as strong aggregates as BLG-saliva, especially under tribological stress (Paper II). Moreover, the interfacial rheological properties of solutions of BLG mixed with BSM have been investigated. BLG-BSM protein mixtures exhibited interfacial properties with lower elastic and viscous moduli than BLG, as a result of competitive displacement of BLG proteins with BSMs from the interface. It is suggested that hydrophobic patches of BSM can be imbedded into the BLG monolayer as driven by a strong hydrophobic interaction with air and disrupt the cohesive assembly of BLG, whereas the hydrophilic (negatively charged) parts of the BSM chain are protruding from the interface towards the bulk water (Paper III). To elucidate the interaction mechanisms of BLG and two types of mucins, BSM and PGM, specifically focusing on the role of hydrophobic residues of the proteins at different pH conditions, intrinsic fluorescence spectroscopy, the fluorescent dye ANS techniques and high field NMR spectroscopy were used. Results from intrinsic fluorescence spectroscopy indicated stronger hydrophobic interactions of BLG with PGM than with BSM, which was further supported by extrinsic fluorescence spectroscopy. Stronger interactions of BLG with PGM also suggest a more abundant presence of hydrophobic moieties in PGM than BSM. Furthermore, HF-NMR studies indicated that the hydrophilic interaction also contributed to the interactions with both mucins, especially at acidic conditions (Paper IV). In the final Chapter VI, the tribological and physicochemical properties (emulsion particle size, viscosity, contact angle measurements, microscopy) of a model mucus compound, namely highly concentrated BSM, with the negatively charged BLG-stabilized emulsion (at pH 6.8) were determined as an attempt to understand the physicochemical basis of BLG-stabilized emulsion in the oral environment.