Isoelectric Point of Proteins at Hydrophobic Interfaces

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.

Composition, Structure, and Techno-Functional Characteristics of the Flour, Protein Concentrate, and Protein Isolate from Purslane (Portulaca oleracea L.) Seeds

“…In fact, because the majority of dietary proteins are acidic in nature, their solubility is lowest around pH 4–5, increasing as the pH value rises [ 18 ]. Furthermore, the protein is less soluble at the isoelectric point (pI) due to peptides' net charge and the surface hydrophobicity [ 19 ]. Protein solubility is complex and could be affected by different factors, such as electrostatic/hydrophobic interactions and hydrogen bonding.…”

Effect of Surfactants on the Molecular Structure of the Buried Oil/Water Interface

“…[14] Accordingly, SFG spectroscopy has been utilized to investigate the acid–base equilibrium of a cationic surfactant [15] or to determine the isoelectric point of protein solutions at the water/air interface [16] and at hydrophobic interfaces. [17] …”

Effects of Relative Molecular Weight Distribution and Isoelectric Point on the Swelling Behavior of Gelatin Films

“…pI is the pH at which there is no charge on the surface of a molecule ( Lautenbach et al, 2021 ). Due to the presence of different amino acid residues, proteins have different isoelectric points ( Hitchcock 1929 ).…”