The influence of surface functional groups on β-lactoglobulin adsorption equilibrium Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/kp78gm56s

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  • Interactions between proteins and contact surfaces can have important implications in the food industry. Such interactions contribute to the course of fouling of membrane surfaces and they appear to mediate bacterial and spore adhesion to some degree as well. In addition to protein and solution properties, interfacial behavior is strongly influenced by contact surface properties. Among these, hydrophobicity and the potential to take part in acid-base interaction have received considerable attention, but in a quantitative sense we know very little about their respective influences on protein adsorption. It was the purpose of this research to quantify the equilibrium adsorptive behavior of the milk protein β-lactoglobulin as it is influenced by the presence of different contact surface functional groups. Monocrystalline and polished silicon surfaces were modified to be hydrophilic by oxidation and hydrophobic by silanization with dichlorodiethylsilane (DDES), dichlorodimethylsilane (DDMS), and dichlorodiphenylsilane (DDPS), each used at concentrations of 0.82, 3.3, and 82 mM. Surface hydrophobicities were evaluated with contact angle methods. Adsorption isotherms were constructed after allowing each modified silicon surface to independently contact β-lactoglobulin (0.01 M phosphate buffer, pH 7.0) at concentrations ranging between 200 and 2000 mg/L for eight h at room temperature. Surfaces were then rinsed and dried. Optical properties of the bare- and film-covered surfaces, necessary for calculation of adsorbed mass, were obtained by ellipsometry. Plots of adsorbed mass as a function of protein concentration exhibited attainment of plateau values beyond a protein concentration of about 200 mg/L. At high silane concentration, the plateau values associated with surfaces exhibiting ethyl groups were observed to be greatest followed by those exhibiting phenyl, methyl, then hydrophilic (OH) groups. At the low DDMS and DDES concentrations (0.82 and 3.3 mM), adsorbed mass did not increase beyond that value recorded for the hydrophilic surface. This is likely due to some critical spacing of methyl and ethyl groups being required to produce a favorable hydrophobic effect on adsorption. For surfaces treated with dichlorodiphenylsilane, adsorbed mass increased with silane concentration. Apparently, a favorable acid-base interaction effected by the hydrophilic surface is inhibited by the presence of small amounts of methyl and ethyl groups, but somewhat less inhibited by the presence of phenyl groups because the latter have the ability to undergo acid-base interaction.
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