Influence of Protein Self-Association on Complex Coacervation with Polysaccharide: A Monte Carlo Study

Coarse-grained Monte Carlo simulations have been applied to study complex coacervation of pectin with bovine serum albumin (BSA) and two isomers of beta-lactoglobulin (BLGA and BLGB). The influence from the specific distribution of charge and hydrophobic patches in protein surfaces on the self-association of proteins and their complex coacervation were investigated. A simple and direct method to quantify the contribution of hydrophobic interaction on protein complex formation was introduced. Highly accordant pH dependence of charges in proteins and phase boundaries for the complex coacervation was observed. Comparing to BSA, beta-lactoglobulin had a higher probability and a broader pH window to form complex coacervate. The major cause is the higher self-association proneness of beta-lactoglobulin, as evidenced by the more negative second virial coefficients. The double-point mutations of G64D/V118A from BLGB to BLGA caused the latter one to have a stronger self-association proneness. It was revealed that the larger negative charge patch in BLGA synergistically enhanced the attraction of the strongest binding site, a positive charge patch, when pH was close to or above the isoelectric point of the protein. These findings suggest that the coarse grained simulation is competent to explore the delicate influences from different proteins in protein–polysaccharide complex coacervates

Replica Exchange Monte Carlo Simulation of Human Serum Albumin–Catechin Complexes

Replica exchange Monte Carlo simulation equipped with an orientation-enhanced hydrophobic interaction was utilized to study the impacts of molar ratio and ionic strength on the complex formation of human serum albumin (HSA) and catechin. Only a small amount of catechins was found to act as bridges in the formation of HSA–catechin complexes. Selective binding behavior was observed at low catechin to HSA molar ratio (<i>R</i>). Increase of catechin amount can suppress HSA self-aggregation and diminish the selectivity of protein binding sites. Strong saturation binding with short-range interactions was found to level off at around 4.6 catechins per HSA on average, while this number slowly increased with <i>R</i> when long-range interactions were taken into account. Meanwhile, among the three rings of catechin, the 3,4-dihydroxyphenyl (B-ring) shows the strongest preference to bind HSA. Neither the aggregation nor the binding sites of the HSA–catechin complex was sensitive to ionic strength, suggesting that the electrostatic interaction is not a dominant force in such complexes. These results provide a further molecular level understanding of protein–polyphenol binding, and the strategy employed in this work shows a way to bridge phase behaviors at macroscale and the distribution of binding sites at residue level

Monte Carlo Simulation on Complex Formation of Proteins and Polysaccharides

In protein–polysaccharide complex systems, how nonspecific interactions such as electrostatic and van der Waals interactions affect complex formation has not been clearly understood. On the basis of a coarse-grained model with the specificity of a target system, we have applied Monte Carlo (MC) simulation to illustrate the process of complex coacervate formation from the association of proteins and polysaccharides. The coarse-grained model is based on serum albumin and a polycation system, and the MC simulation of pH impact on complex coacervation has been carried out. We found that complex coacervates could form three ways, but the conventional association through electrostatic attraction between the protein and polysaccharide still dominated the complex coacervation in such systems. We also observed that the depletion potential always participated in protein crowding and was weakened in the presence of strong electrostatic interactions. Furthermore, we observed that the sizes of polysaccharide chains nonmonotonically increased with the number of bound proteins. Our approach provides a new way to understand the details during protein–polysaccharide complex coacervation at multiple length scales, from interaction and conformation to aggregation

Medium-Chain Sugar Amphiphiles: A New Family of Healthy Vegetable Oil Structuring Agents

Vegetable oils are frequently structured to enhance their organoleptic and mechanical properties. This is usually achieved by increasing the net amount of saturated and/or trans fatty acids in the oil. With the risk of coronary heart diseases associated with these fatty acids, the food industry is looking for better alternatives. In this context, the medium-chain dialkanoates of low-calorie sugars (sugar alcohol dioctanoates) are investigated as a healthy alternative structuring agent. Precursors of sugar amphiphiles, being FDA-approved GRAS materials, exhibited high cell viability at a concentration ∼50 μg/mL. They readily formed nanoscale multilayered structures in an oil matrix to form a coherent network at low concentrations (1–3 wt %/v), which immobilized a wide range of oils (canola, soybean, and grapeseed oils). The structuring efficiency of sugar amphiphiles was computed in terms of mechanical, thermal, and structural properties and found to be a function of its type and concentration

Effect of a Labile Methyl Donor on the Transformation of 5‑Demethyltangeretin and the Related Implication on Bioactivity

Polymethoxyflavones (PMFs) belong to a subgroup of flavonoids that particularly exist in the peels of citrus fruits. Despite their many health-beneficial biofunctionalities, the lipophilic nature of PMFs limits their water solubility and oral bioavailability. To investigate the effect of the delivery system on the improvement of PMF bioavailibility, a lecithin-based emulsion was formulated for the delivery of two PMF compounds, tangeretin and 5-demethyltangeretin. While the emulsion system improved the digestion kinetics and the total solubilized PMF concentrations in in vitro lipolysis studies, the concentration of 5-demethyltangeretin decreased due to chemical transformation to its permethoxylated counterpart, tangeretin. The emulsifier lecithin used in this emulsion formulation contained a choline headgroup as a labile methyl group donor. The presence of a methyl donor potentially caused the transformation of 5-demethyltangeretin and reduced its anti-cancer-cell-proliferation activities. Moreover, this is the first report in the literature of the transformation from 5-demethyltangeretin to tangeretin in a lecithin-based emulsion during lipolysis, and the mechanism underlying this phenomenon has also been proposed for the first time

Structure, Morphology, and Assembly Behavior of Kafirin

Prolamins from grains have attracted intensive attention in recent years due to their potential in satisfying the demand for environmentally friendly (biodegradable), abundantly available (sustainable), and cost-effective biomaterials. However, for kafirin, the prolamin from sorghum, its composition, structure, morphology, and self-assembly behaviors have not been fully characterized. In this paper, kafirin was extracted from the whole sorghum grain and found to contain 68, 14, 6, and 12% of α-, β-, and γ-fractions and cross-linked kafirin, respectively. Freeze-dried kafirin contained ∼49% α-helix in the solid state. When dissolved in 65% (v/v) isopropanol, 60% (v/v) <i>tert</i>-butanol, and 85% (v/v) ethanol aqueous solvents, the relative α-helix content in kafirin increased with the decrease of solvent polarity. Structural analysis using small-angle X-ray scattering (SAXS) indicated that kafirin (2 mg/mL) took stretched and extended conformations with dimensions of 118 × 15 × 15 and 100 × 11 × 11 Å in 60% <i>tert</i>-butanol and 65% isopropanol, respectively. More elongated conformation of individual kafirin with high-order assembly was observed in 85% ethanol. Protein aggregation occurred as protein concentration increased in its good solvent. The morphology of kafirin assemblies captured by atomic force microscopy (AFM) revealed that kafirin protein took uniform particle morphology at low concentration, and disk-like or rod-like structures resulting from solvent evaporation induced particle interactions emerged at high concentrations. These results suggest that both protein concentration and solvent polarity can effectively regulate kafirin assemblies from thick rod-like to slim rod-like structures, a convenient way to tune the fibrillation of prolamin-based biomaterials

Understanding the Dissolution of α‑Zein in Aqueous Ethanol and Acetic Acid Solutions

Zein is a corn prolamin that has broad industrial applications because of its unique physical properties. Currently, the high cost of extraction and purification, which is directly related to the dispersion of zein in different solvents, is the major bottleneck of the zein industry. Solution behaviors of zein have been studied for a long time. However, the physical nature of zein in different solvents remains unclear. In this study, small-angle X-ray scattering (SAXS), static light scattering (SLS), and rheology were combined to study the structure and protein–solvent interaction of α-zein in both acetic acid and aqueous ethanol solutions. We found that the like-dissolve-like rule, the partial unfolding, and the protonation of zein are all critical to understanding the solution behaviors. Zein holds an elongated conformation (i.e., prolate ellipsoid) in all solutions, as revealed from SAXS data. There is an “aging effect” for zein in aqueous ethanol solutions, as evidenced by the transition of Newtonian rheological profiles for fresh zein solutions to the non-Newtonian shear thinning behavior for zein solutions after storage at room temperature for 24 h. Such shear thinning behavior becomes more pronounced for zein solutions at higher concentrations. The SLS results clearly show that acetic acid is a better solvent to dissolve zein than aqueous ethanol solution, as supported by a more negative second virial coefficient. This is majorly caused by the protonation of the protein, which was further verified by the dissolution of zein in water (a nonsolvent for zein) with the addition of acids

Monte Carlo Study of Polyelectrolyte Adsorption on Mixed Lipid Membrane

Monte Carlo simulations are employed to investigate the adsorption of a flexible linear cationic polyelectrolyte onto a fluid mixed membrane containing neutral (phosphatidyl-choline, PC), multivalent (phosphatidylinositol, PIP₂), and monovalent (phosphatidylserine, PS) anionic lipids. We systematically study the effect of chain length and charge density of the polyelectrolyte, the solution ionic strength, as well as the membrane compositions on the conformational and interfacial properties of the model system. In particular, we explore (i) the adsorption/desorption limit, (ii) the interfacial structure variations of the adsorbing polyelectrolyte and the lipid membrane, and (iii) the overcharging of the membrane. Polyelectrolyte adsorbs on the membrane when anionic lipid demixing entropy loss and polyelectrolyte flexibility loss due to adsorption are dominated by electrostatic attraction between polyelectrolyte and anionic lipids on the membrane. Polyelectrolytes with longer chain length and higher charge density can adsorb on the membrane with increased anionic lipid density under a higher critical ionic concentration. Below the critical ionic concentration, the adsorption extent increases with the charge density and chain length of the polyelectrolyte and decreases with the ionic strength of the solution. The diffusing anionic lipids prohibit the polyelectrolyte chain from forming too long tails. The adsorbing polyelectrolyte with long chain length and high charge density can overcharge a membrane with low charge density, and conversely, the membrane charge inversion forces the polyelectrolyte chain to form extended loops and tails in the solution

Physical and Antimicrobial Properties of Peppermint Oil Nanoemulsions

The mixture of peppermint oil (PO) with medium-chain triacylglycerol was emulsified in water and stabilized with a food-grade biopolymer, modified starch, to form PO nanoemulsions. The effects of emulsifying conditions including homogenization pressure, the number of processing cycles, and oil loading on the mean diameters and viscosities of nanoemulsions were characterized by dynamic light scattering, optical microscopy, and rheological measurements. The formulated PO nanoemulsions with mean diameters normally <200 nm showed high stability over at least 30 days of storage time. Their antimicrobial properties related to those of PO have also been evaluated by two assays, the minimum inhibitory concentration (MIC) and time-kill dynamic processes, against two Gram-positive bacterial strains of Listeria monocytogenes Scott A and Staphylococcus aureus ATCC 25923. Compared with bulk PO, the PO nanoemulsions showed prolonged antibacterial activities. The results suggest that the nanoemulsion technology can provide novel applications of essential oils in extending the shelf life of aqueous food products