Protein adsorption on nanostructured silica surfaces
This Ph.D. thesis presents a study of the interaction of globular proteins with nanostructured silica surfaces. It aims to gain a better understanding of the consequences of a high surface curvature and confinement on the adsorption behavior and morphology of the adsorbed protein. The adsorption behavior of lysozyme and 𝛽-lactoglobulin on silica nanoparticles was studied from pH 2 to 11 and ionic strength until 100 mM. Within this pH range the surface charge distribution on proteins and silica changes drastically. Specifically it was possible to study the interactions between particles and 𝛽-lactoglobulin on both sides of the isoelectric point of the protein. Resulting data were evaluated with the Guggenheim – Anderson – de Boer adsorption isotherm equation which accounts for multilayer adsorption from liquid phases. The orientation of the globular protein cytochrome c adsorbed on silica nanoparticles was determined as a function of solution pH using small angle neutron scattering. The scattering data was evaluated with a form factor model accounting specifically for 'raspberry-shaped' structures. A pronounced shift in the adsorption orientation relative to the particle surface was detected between pH 3 and 4. This behavior was correlated with the dipole moment of the protein. Simulations of the dipole moment for different protonation states of cytochrome c show a distinctive change in the dipole moment orientation for the same pH region. In preceding studies it was found that protein adsorption can cause hetero-aggregation of the silica nanoparticles. Here, the large-scale structure of these aggregates was studied by confocal laser scanning microscopy with fluorescently labeled lysozyme. Stacks of 2D images were used to reconstruct the 3D aggregate structure and estimate the global structural properties in terms of their surface-area-to-volume ratio. Protein uptake in nanometer-sized pores depends on the accessible pore space and the interaction of the protein with the pore wall. The adsorption of lysozyme in mesoporous SBA-15 silica materials with native and chemically modified pore walls was studied over a wide pH range. To assess the observed differences in the protein uptake capacity of the materials, a geometrical pore filling model was developed that takes into account the different pore-size distribution of the materials. The melting point depression of eleven aqueous alkali halide systems confined in the mesopores of SBA-15 and MCM-41 silica was studied by DSC. It was found that the eutectic temperature in the pores is mainly dependent of the fraction of volume occupied by the salt crystallites, i.e. largest for salts forming oligohydrates in the pores. Salt-specific adsorption effects at the pore wall are only of minor importance.
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