Atomic force microscopy to assess the mechanical properties of individual casein micelles

Nanoscale Investigation of Elasticity Changes and Augmented Rigidity of Block Copolymer Micelles Induced by Reversible Core-Cross-Linking

Drug-delivery systems have attracted considerable attention due to their potential to increase the bioavailability of certain drugs and mitigate side effects by enabling targeted drug release. Reversibly core-cross-linked block copolymer micelles providing a hydrophilic and potentially nonimmunogenic shell and a hydrophobic core suitable for the uptake of hydrophobic drugs are frequently considered because of their high stability against environmental changes and dilution. Ultimately, triggering core-de-cross-linking enables the implementation of strategies for targeted drug release, which requests insights into the impact of varying nanomechanical properties on the stability of individual micelles. Here, atomic force microscopy nanoindentation in aqueous media is applied to intact α-allyl-PEG80-b-P(tBGE52-co-FGE12) micelles to quantify changes in their nanomechanical properties induced by dithiobismaleimidoethane (DTME)-mediated Diels-Alder cross-linking of furfuryl moieties and sequential de-cross-linking by reduction of its disulfide bond by tris(2-carboxyethyl)phosphine. As a result of crosslinking by DTME, the apparent Young's modulus of the micelles roughly doubles to 1.18 GPa. Changes to the Young's modulus can be largely reversed by de-cross-linking. Cross-linked and de-cross-linked micelles maintain their structural integrity even in diluted aqueous media below the critical micelle concentration, in contrast to the micelles prior to crosslinking. Understanding the structure-property relationships associated with the observed augmented mechanical stability in native environments is crucial for improving the efficiency of drug encapsulation and introducing refined temporal and spatially controlled drug-release mechanisms.

Blending pectin and κ-carrageenan converted the liquid yogurt induced by pectin into the solid yogurt

Effects of 0.11 %-0.17 % pectin and the mixture of 0.03 % κ-carrageenan and 0.11 %-0.17 % pectin on texture and microstructure of yogurt were investigated in this work. Rheology analysis demonstrated that adding 0.11 %-0.17 % pectin before fermentation inhibited gelation of yogurt and liquid yogurt was formed. However, when the above κ-carrageenan/pectin mixture was added, yogurt was gelled and solid-like. It was demonstrated by CLSM that milk protein aggregated into separated particles in the liquid yogurt induced by pectin, while milk protein aggregated into a continuous network in the solid yogurt induced by the mixture. Adding 0.11 %-0.17 % pectin into the casein micelle suspension induced aggregation of casein micelles into separated particles, which was the same in the corresponding liquid yogurt samples. Moreover, casein micelles precipitated in the pectin/casein micelle mixtures after storage for 3 h. However, when the above mixture was added into the casein micelle suspension, tightly-connected casein micelle aggregates appeared and the resultant κ-carrageenan/pectin/casein micelle mixtures were stable after storage for 3 h. These results indicated that the pectin-casein micelle interaction played an essential role in formation of the liquid yogurt and κ-carrageenan altered this interaction. Thus, blending κ-carrageenan and pectin converted the liquid yogurt induced by pectin into the solid yogurt.

Exploration of dynamic interaction between β-lactoglobulin and casein micelles during UHT milk process

The protein aggregation induced by UHT treatment shortens the shelf life of UHT milk. However, the mechanism of β-Lg induced casein micelle aggregation remains unclear. Herein, the dynamic interaction between β-Lg and casein micelles during UHT processing was investigated by experimental techniques and molecular dynamics simulations. Results showed that β-Lg decreased the stability of casein micelles, increased their size and zeta potential. Raman and FTIR spectra analysis suggested that hydrogen and disulfide bonds facilitated their interaction. Cryo-TEM showed that the formation of the casein micelle/β-Lg complex involved rigid binding, flexible linking, and severe cross-linking aggregation during UHT processing. SAXS and MST demonstrated β-Lg bound to κ-casein on micelle surfaces with a dissociation constant (Kd) of 3.84 ± 1.14 μm. Molecular docking and dynamic simulations identified the interacting amino acid residues and clarified that electrostatic and van der Waals forces drove the interaction. UHT treatment increased hydrogen bonds and decreased total binding energy. The non-covalent binding promoted the formation of disulfide bonds between β-Lg and casein micelles under heat treatment. Ultimately, it was concluded that non-covalent interaction and disulfide bonding resulted in casein micelle/β-Lg aggregates. These findings provided scientific insights into protein aggregation in UHT milk.

Preparation of liquid yogurt in the presence of pectin and its formation mechanism

We had previously observed that adding pectin into milk before fermentation inhibited gelation of yogurt but did not affect the pH. Thus, this work aimed to prepare such liquid yogurt and clarify its formation mechanism. It was found that liquid yogurt was obtained in the presence of 0.10%-0.20% pectin. However, at lower or higher pectin concentrations, yogurt was gelled. Confocal laser scanning microscopy analysis demonstrated that 0.10%-0.20% pectin induced milk protein aggregating into separated particles rather than a continuous network, which explained why liquid yogurt was formed. Moreover, adding 0.10%-0.20% pectin into the casein micelle suspension induced aggregation of casein micelles at pH 6.8. After pH decreased to 4.3, casein micelles showed more aggregation but they were still separated particles, which was the same in the corresponding yogurt samples. These results suggested that pectin changed the aggregation mode of casein micelles and induced formation of liquid yogurt.

Application of small angle scattering (SAS) in structural characterisation of casein and casein-based products during digestion

In recent years, small and ultra-small angle scattering techniques, collectively known as small angle scattering (SAS) have been used to study various food structures during the digestion process. These techniques play an important role in structural characterisation due to the non-destructive nature (especially when using neutrons), various in situ capabilities and a large length scale (of 1 nm to ∼20 μm) they cover. The application of these techniques in the structural characterisation of dairy products has expanded significantly in recent years. Casein, a major dairy protein, forms the basis of a wide range of gel structures at different length scales. These gel structures have been extensively researched utilising scattering techniques to obtain structural information at the nano and micron scale that complements electron and confocal microscopy. Especially, neutrons have provided opportunity to study these gels in their natural environment by using various in situ options. One such example is understanding changes in casein gel structures during digestion in the gastrointestinal tract, which is essential for designing personalised food structures for a wide range of food-related diseases and improve health outcomes. In this review, we present an overview of casein gels investigated using small angle and ultra-small angle scattering techniques. We also reviewed their digestion using newly built setups recently employed in various research. To gain a greater understanding of micro and nano-scale structural changes during digestion, such as the effect of digestive juices and mechanical breakdown on structure, new setups for semi-solid food materials are needed to be optimised.

Effect of sodium alginate on the yogurt stability was dependent on the thickening effect and interaction between casein micelles and sodium alginate

The effect of sodium alginate (SA) on the yogurt stability and the related mechanisms were investigated. It was found that low-concentration SA (≤0.2 %) increased the yogurt stability, while high-concentration SA (≥0.3 %) decreased the yogurt stability. Sodium alginate increased the viscosity and viscoelasticity of yogurt and this effect was positively correlated with its concentration, suggesting that SA worked as the thickening agent in yogurt. However, addition of ≥0.3 % SA damaged the yogurt gel. These results suggested that interaction between milk protein and SA might play an important role in the yogurt stability besides the thickening effect. Addition of ≤0.2 % SA did not change the particle size of casein micelles. However, addition of ≥0.3 % SA induced aggregation of casein micelles and increased the size. And the aggregated casein micelles precipitated after 3 h storage. Isothermal titration calorimetry analysis showed that casein micelles and SA were thermodynamically incompatible. These results suggested that the interaction between casein micelles and SA induced aggregation and precipitation of casein micelles, which was critical in the destabilization of yogurt. In conclusion, the effect of SA on the yogurt stability was dependent on the thickening effect and the interaction between casein micelles and SA.

Exploring the impact of PEGylation on the cell-nanomicelle interactions by AFM-based single-molecule force spectroscopy and force tracing

PEGylation has been considered the gold standard method for the modification of various drug delivery systems since the last century. However, the impact of PEGylation on the dynamic interaction between drug carriers and cell membranes has not been quantitatively clarified. Herein, the cellular binding and receptor-mediated endocytosis of a model PEGylated polypeptide nanomicelle were systematically investigated at the single-particle level using AFM-based single-molecule force spectroscopy (SMFS) and force tracing. A self-assembled elastin-like polypeptide (ELP) nanomicelle, which is capable of cross-linking, gastrin-releasing peptide (GRP) modification, and PEGylation was prepared. The cross-linked ELP-based nanomicelles exhibited outstanding stability in a broad temperature range of 4-40 °C, which facilitate the drug loading, as well as our cell-nanomicelle study at the single particle level. The unbinding force between the cross-linked ELP-based nanomicelles and the GRP receptor (GRPR)-containing cell (PC-3) membranes was quantitatively measured by AFM-SMFS. It is found that the PEGylated GRP-displaying nanomicelles exhibit the highest unbinding force, indicating the enhanced specific binding effect of PEGylation. Furthermore, the receptor-mediated endocytosis of the cross-linked ELP-based nanomicelles was monitored with the help of force tracing based on AFM-SMFS. Our results show that PEGylation decreases the endocytic force, duration, and engulfment depth of the PEGylated GRP-displaying nanomicelles, but increases their endocytic velocity, which results from the elimination of non-specific interactions during endocytosis. These observations demonstrate the diverse and complex roles of PEGylation on the interaction of polypeptide nanomicelles to cell membranes and may shed light on the rational design of organic polymer-based drug delivery systems aiming for active and passive targeting strategies. STATEMENT OF SIGNIFICANCE: A self-assembled elastin-like polypeptide (ELP) nanomicelle, which can be easily cross-linked, gastrin-releasing peptide (GRP) modified, and PEGylated, is designed. The AFM-SMFS experiment shows that PEGylation can enhance specific binding of the nanomicelles to the receptors on cell membranes. The force tracing experiment indicates that PEGylation decreases the endocytic force as well as engulfment depth of the nanomicelles through the elimination of non-specific interactions. PEGylation can benefit the drug delivery systems aiming at active targeting, while might not be an ideal modification for drug carriers designed for passive targeting, whose cellular uptake mainly depends on non-specific interactions.