Abstract
Structural and colloidal stability of proteins at different surfaces and interfaces is of great importance in many fields including medical, pharmaceutical, or material science. Due to their flexibility, proteins tend to respond to their environmental conditions and can undergo structural and conformational changes. For instance, alterations in physiological factors such as temperature, ions concentration, or pH as well as the adsorption to an interface can initiate protein aggregation. Therefore, at different surfaces and interfaces the characterization of the structural and colloidal stability of proteins, which is mainly influenced by their electrostatic and hydrophobic interactions, is of fundamental importance. In this study, we utilized sum frequency generation (SFG) spectroscopy to assess the role of solution pH on the polarity and magnitude of the electric field within the hydration shell of selected model proteins adsorbed to a hydrophobic surface. We used polystyrene (PS) as a model hydrophobic surface and determined the isoelectric point (IEP) of four structurally different model proteins. Comparing the measured IEP of proteins at the PS/solution or air/solution interface with that determined in the bulk solution via zeta potential measurement, we found significant similarities between the IEP of surface adsorbed proteins and those in the bulk aqueous phase. The pH dependence behavior of proteins was correlated to their amino acid composition and degree of hydrophobicity.
Highlights
Proteins are flexible macromolecules that react sensitively to environmental conditions and external stimulators
The isoelectric point (IEP) of four proteins BSA, lysozyme, Antifreeze protein type III (AFP III), and hemoglobin were investigated in contact with a hydrophobic layer at the Deuterated polystyrene (dPS)-water interface, using sum frequency generation (SFG) spectroscopy
We utilized an inherently surface sensitive nonlinear spectroscopic tool, sum frequency generation (SFG) spectroscopy, to assess the impact of solution pH on selected model proteins adsorbed at a solid hydrophobic surface
Summary
Proteins are flexible macromolecules that react sensitively to environmental conditions and external stimulators. The adsorption of proteins to different surfaces, e.g., cell membranes and implants in biological systems or pipings during industrial processing, is often accompanied by proteins’ structural and conformational alterations (Yano, 2012; FaulónMarruecos et al, 2018; Mitra, 2020). These changes in protein structure affect protein functionality and may trigger their abnormal folding. The hydrophobicity of cell membranes can initiate protein aggregation, which is assumed to be the cause of severe neuronal diseases such as Alzheimer’s or Parkinson’s disease (Beyer, 2007; Gonzalez-Garcia et al, 2021; Saghir et al, 2021). In downstream processing and adsorption chromatography of proteins irreversible binding of proteins on commercially available hydrophobic adsorbents is accompanied by structural changes in proteins, a process that is influenced by solution pH (Millitzer et al, 2005)
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