Orientation and Conformation of Hydrophobin at the Oil–Water Interface: Insights from Molecular Dynamics Simulations

Abstract

Hydrophobins, a new class of potential protein emulsifiers, have been extensively employed in the food, pharmaceutical, and chemical industries. However, the knowledge of the underlying molecular mechanism of protein adsorption at the oil−water interface remains elusive. In this study, all-atom molecular dynamics simulations were performed to probe the adsorption orientation and conformation change of class II hydrophobin HFBI at the cyclohexane−water interface. It was proposed that a hydrophobic dipole of the protein could be used to quantitatively predict the orientation of the adsorbed HFBI. Simulation results revealed that HFBI adsorbed at the interface with the patch-up orientation toward the oil phase, regardless of its initial orientations. HFBI's secondary structure was maintained to be intact in the course of simulations despite relatively significant variations in the tertiary structure observed, which could well preserve the bioactivity of HFBI. From the energy analysis, the driving force for interface adsorption was primarily determined by van der Waals interactions between HFBI and cyclohexane. Further analysis indicated that the adsorption orientation and conformation of HFBI at the oil−water interface were typically regulated by the hydrophobic patch and some key residues. This study provides some insights into the orientation, conformation, and adsorption mechanism of proteins at the oil−water interface and theoretical guidelines for the design and development of novel biological emulsifiers involved in the food, pharmaceutical, and chemical industries.

Effect of Oil Polarity on the Protein Adsorption at Oil–Water Interfaces

“…The adsorption orientation at the oil–water interface is a main contribution factor to the interfacial activity of adsorbed proteins . An orientation angle θ was used to monitor the evolution of protein adsorption orientation with simulation time, which is defined as the angle between hydrophobic dipole moment and the normal vector of the oil–water interface . The orientations of proteins adsorbed at different oil–water interfaces are shown in Figure .…”

“…Generally, the oil–water interface (magenta dot-dashed lines) is defined as the position where the density of water is equal to that of oil . Therefore, it can be found from Figure that the established oil–water interfaces are located between 7.0 and 8.0 nm.…”

“…Furthermore, the oil–water bilayers were built and relaxed through a series of energy minimization and equilibration steps. The same procedure has been elaborated in our previous work …”

Aspergillus Hydrophobins: Physicochemical Properties, Biochemical Properties, and Functions in Solid Polymer Degradation

“…The Cys3–Cys4 and Cys7–Cys8 loops of class II hydrophobins have random structures; because these loops are short and the proportion of random structures is low, class II hydrophobins do not form rodlets but associate with each other without a large change in their three-dimensional structures to form a regularly aligned self-assembled monolayer [ 76 , 78 , 85 ]. Molecular dynamics simulation suggests that HFBI adsorbs at the water–oil interface with its hydrophobic patch facing the oil phase without a change in the secondary structure, regardless of the initial orientation [ 92 ].…”

The Unexpected Role of Cations in the Self-Assembly of Positively Charged Amphiphiles at Liquid/Liquid Interfaces

“…Of relevance to the current work is the liquid/liquid (L/L) interface between the oil and aqueous phases, which host a range of unique phenomena that are essential to both synthetic and natural systemse.g., unique solvation environments and chemical asymmetry that define the surface. The partitioning of amphiphiles, such as lipids, surfactants, − proteins, − extraction reagents, − nanoparticles, − and polymers − to these L/L interfaces provide an opportunity to leverage synergistic interactions between organized molecules localized in between the two bulk phases to achieve function. These phenomena also represent key steps in chemical separations, − phase transfer catalysis, and even applications in soft matter electronics. , …”