Molecular dynamics study on membrane fouling by oppositely charged proteins

Abstract

Membrane fouling continues to hamper the performance of membrane‐filtration processes. A challenge with macromolecular foulants like proteins is that macroscopic characterizations, like net electrical charge, may be poorly correlated with membrane fouling. This necessitates a molecular‐scale analysis of the local interactions. In this study, molecular dynamics simulations have been performed to understand the interactions between two similar‐sized proteins with opposite overall charges (namely, lysozyme and α‐lactalbumin) and a negative‐charged membrane. Surprisingly, the protein–membrane distances and adsorption probabilities of both proteins are similar. Compared with the positive‐charged lysozyme, the negative‐charged α‐lactalbumin exhibits (a) greater protein–membrane attractive interaction energy due to synergy among adsorption sites; (b) lower root‐mean‐squared deviations (RMSD); and (c) greater number of residues that show low root‐mean‐squared fluctuations (RMSF). These results indicate that local interactions are critical and thus highlight the pitfall of using the overall protein characteristics as predictors of membrane fouling.

Investigation of membrane fouling using molecular dynamics simulation

“…Simulation models have to be validated against experiments to ensure reliability and accuracy. Composition of feed/membrane [92,93] Conductivity [36,94] Cross-linking degree [95,96] Contact angle [97,98] Density of grafting sites on the membrane surface [26,29] DOTM A [55] Density of membrane [30,40,99,100] Flux decline profile [30,33,37,101] Spectroscopy (e.g., XPS B , FTIR C ) [35,102,103] Isotherm of static adsorption (e.g., by QCM-D D ) [104][105][106] Zeta potential [107] Rejection of foulant [35,102] SEM E…”

“…It should be noted that there is no universal threshold value for separation distance that can be used to distinguish whether a foulant is adsorbed or unabsorbed, so variour studies have sought different means to determine adsorption or lack thereof. We have used the average separation distance (𝑑 ̅ ) and fraction of time whereby the separation distance is less than 4 Å (𝑓 4 ) to classify MD trajectories into categories of stable adsorption, transient adsorption, and no adsorption [33,107]. Another quantity commonly used to compare adsorption is the number of contact atoms that are within a specified distance from the membrane surface.…”

“…For a macromolecule like protein, it may have various adsorption or anchoring points onto the membrane, which results in different interaction behaviors. To identify dominant amino acid residues responsible for the anchoring of foulants onto the membrane surface, some studies assessed the separation distance of each individual residue and the membrane surface [33,116,118,119]. As a representative example, Fig.…”

“…As mentioned above, interaction energy also depends on the exact anchoring point of the foulant onto the membrane surface, which means the adsorption orientation is worth investigating. A few studies [33,56,115] have analyzed the interaction energy between specific regions of the foulant and the membrane, and indicated distinctly different energies particularly for macromolecules with multiple domains of different properties (e.g., a protein is made up of amino acid residues with diverse properties).…”

“…( 5) is applied for the foulant, it must be assured that within the time frame studied, the foulant still exhibits a normal diffusion behavior (i.e., an approximate linear relation between MSD and 𝑡) [61,104,109]. Generally, RMSD, especially the z-component that is perpendicular to the membrane surface [44,109], is used to characterize foulant mobility, with a lower value indicating a higher tendency to remain deposited on the membrane surface and thus cause more severe fouling [33,58].…”