So far numerous experimental and computational studies have been performed on model proteins such as serum albumin, beta-lactoglobulin, or lysozyme to unravel their specific interactions with different surfaces. Hence, molecular understanding of the protein behavior at different surfaces and interfaces is of great importance in many fields including biopharmaceutical development, drug targeting, development of implant materials, or biomembrane processing. Similarly, 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). For instance, 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). These changes in protein structure not only affect protein functionality but also may trigger their abnormal folding. Moreover, 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). For instance, alteration in physiological factors such as temperature, ion concentration, or pH often results in changes in the secondary or tertiary structure of proteins, their solubility, or colloidal stability ( Sarkar et al., 2009 Maldonado-Valderrama et al., 2010). Proteins are flexible macromolecules that react sensitively to environmental conditions and external stimulators. The pH dependence behavior of proteins was correlated to their amino acid composition and degree of hydrophobicity. 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. We used polystyrene (PS) as a model hydrophobic surface and determined the isoelectric point (IEP) of four structurally different model proteins. 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. 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.
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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. Due to their flexibility, proteins tend to respond to their environmental conditions and can undergo structural and conformational changes. Structural and colloidal stability of proteins at different surfaces and interfaces is of great importance in many fields including medical, pharmaceutical, or material science. Institute of Particle Technology (LFG), Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), Erlangen, Germany.Vanessa Lautenbach †, Saman Hosseinpour* † and Wolfgang Peukert*