Retinol and retinoic acid bind to a surface cleft in bovine beta-lactoglobulin: a method of binding site determination using fluorescence resonance energy transfer

Towards understanding the interaction between ultrasound-pretreated β-lactoglobulin monomer with resveratrol

Increasingly, studies are using ultrasound to elevate the functional properties of proteins, so the interaction between phenolic compounds and proteins induced by ultrasound needs to be further understood. β-Lactoglobulin (β-LG) at pH 8.1, which exists mainly as monomers, was ultrasound treated at 20 kHz ultrasonic intensity and 30% amplitude for 0-5 min and subsequently interacted with resveratrol. Fluorescence data showed that ultrasound pretreatment improved binding constant (K_a ) from (1.62 ± 0.45) × 10⁵ to (9.43 ± 0.55) × 10⁵  M^-1 and binding number from 1.13 ± 0.09 to 1.28 ± 0.11 in a static quenching mode. Fluorescence resonance energy transfer (FRET) analysis indicated that resveratrol bound to the surface hydrophobic pocket of native and treated proteins with no obvious changes in energy transfer efficiency (E) and Föster's distance (r). Thermodynamic parameters indicated that ultrasonication shifted the main driving force from the hydrophobic force for native and 1-min treated β-LG to van der Waals forces and hydrogen bonding for both 3-min and 5-min treated proteins. Ultrasonication and resveratrol addition generated significant differences in surface hydrophobicity and the surface charge of the protein (P < 0.05), whereas they had little influence on the secondary structure of β-LG. Compared with the native β-LG/resveratrol complex, ultrasound-treated protein complexes showed significantly stronger 2,2-azinobis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) scavenging capacity (P < 0.05), and kept relatively stable after 180-min irradiation. Data provided by this study can lead to a better comprehension of the structure and molecular events occurring during the complexing process between an ultrasound-pretreated protein with polyphenol.

A Double Payload Complex between Hypericin and All-Trans Retinoic Acid in the β-Lactoglobulin Protein

Combined therapies are usually used to treat acne vulgaris since this approach can tackle various foci simultaneously. Using a combination of spectroscopic, computational, and microbiological techniques and methods, herein we report on the use of β-lactoglobulin as a double payload carrier of hypericin (an antimicrobial photodynamic agent) and all-trans retinoic acid (an anti-inflammatory drug) for S. aureus in vitro photodynamic inactivation. The addition of all-trans retinoic acid to hypericin-β-lactoglobulin complex renders a photochemically safe vehicle due to the photophysical quenching of hypericin, which recovers its photodynamic activity when in contact with bacteria. The ability of hypericin to photoinactivate S. aureus was not affected by retinoic acid. β-Lactoglobulin is a novel biocompatible and photochemically safe nanovehicle with strong potential for the treatment of acne.

PCSK9 as a Target for Development of a New Generation of Hypolipidemic Drugs

PCSK9 has now become an important target to create new classes of lipid-lowering drugs. The prevention of its interaction with LDL receptors allows an increase in the number of these receptors on the surface of the cell membrane of hepatocytes, which leads to an increase in the uptake of cholesterol-rich atherogenic LDL from the bloodstream. The PCSK9 antagonists described in this review belong to different classes of compounds, may have a low molecular weight or belong to macromolecular structures, and also demonstrate different mechanisms of action. The mechanisms of action include preventing the effective binding of PCSK9 to LDLR, stimulating the degradation of PCSK9, and even blocking its transcription or transport to the plasma membrane/cell surface. Although several types of antihyperlipidemic drugs have been introduced on the market and are actively used in clinical practice, they are not without disadvantages, such as well-known side effects (statins) or high costs (monoclonal antibodies). Thus, there is still a need for effective cholesterol-lowering drugs with minimal side effects, preferably orally bioavailable. Low-molecular-weight PCSK9 inhibitors could be a worthy alternative for this purpose.

Interaction characterization of zein with cyanidin-3-O-glucoside and its effect on the stability of mulberry anthocyanins and protein digestion

Ingredient interactions usually occur in food matrix, which may affect their functions and properties. This study aimed to investigate the interactive effects of mulberry and corn protein on pigment stability and zein digestibility. The interaction of main compounds in both ingredients, that is, cyanidin-3-O-glucoside (C3G) and zein, was characterized via their structural, morphological, thermal stability, and digestible properties using multi-spectroscopic techniques, scanning electron microscopy, high performance liquid chromatography, and in vitro digestion models. Results showed that zein exhibited a strong binding affinity for C3G via van der Waals forces and hydrogen bonds determined in fluorescence assays. The secondary structure of zein changed due to C3G binding, with a decrease in α-helix and an increase in β-sheet. The particle size of zein decreased after interacting with C3G. The zein complexation with mulberry anthocyanin-rich extracts in a simulative food system did not affect the digestibility of zein significantly but enhanced the thermal stability of pigments slightly. Specifically, anthocyanins did not change the susceptibility of zein to pepsin proteolysis, suggesting that binding sites of C3G might not be the cleavage sites of pepsins. These results provide important insight into the binding mechanism of zein and anthocyanins and might help guide the design of anthocyanin-based functional food. PRACTICAL APPLICATION: Zein, as a storage protein widely distributed in corn flour, was commonly co-existing with anthocyanins in starchy food. This study provides insights into the molecular interactions between zein and cyanidin-3-O-glucoside. However, the interaction might not impact the zein digestion but enhance anthocyanin thermal stability. The findings of this work could throw light on the selection of ingredients rich in zein and anthocyanins in the food industry.

pH-Dependent complexation between β-lactoglobulin and lycopene: Multi-spectroscopy, molecular docking and dynamic simulation study

This study aims to investigate the effect of pH levels (pH 7.0 and pH 8.1) on binding ability of β-lactoglobulin (β-LG) with lycopene (LYC) and elucidate interaction mechanisms using multi-spectroscopy and molecular docking study. β-LG at pH 8.1 showed a stronger binding affinity to lycopene than that at pH 7.0 according to binding constant, binding number, energy transfer efficiency, and surface hydrophobicity. Lycopene bound to protein mainly by van der Waals force in the form of static quenching mode and preferred to interact with β-LG at the top of barrel for both pH levels. Molecular dynamic simulation revealed that β-LG/LYC complex at pH 8.1 was more stable than at pH 7.0. β-LG/LYC complexes formed at pH 8.1 showed significantly higher ABTS radical scavenging activity than samples at pH 7.0 (p < 0.05). Data obtained may contribute valuable information for preparing a whey protein-based delivery system for lycopene.

β-Lactoglobulin and Glycodelin: Two Sides of the Same Coin?

The two lipocalins, β-lactoglobulin (βLg) and glycodelin (Gd), are possibly the most closely related members of the large and widely distributed lipocalin family, yet their functions appear to be substantially different. Indeed, the function of β-lactoglobulin, a major component of ruminant milk, is still unclear although neonatal nutrition is clearly important. On the other hand, glycodelin has several specific functions in reproduction conferred through distinct, tissue specific glycosylation of the polypeptide backbone. It is also associated with some cancer outcomes. The glycodelin gene, PAEP, reflecting one of its names, progestagen-associated endometrial protein, is expressed in many though not all primates, but the name has now also been adopted for the β-lactoglobulin gene (HGNC, www.genenames.org). After a general overview of the two proteins in the context of the lipocalin family, this review considers the properties of each in the light of their physiological functional significance, supplementing earlier reviews to include studies from the past decade. While the biological function of glycodelin is reasonably well defined, that of β-lactoglobulin remains elusive.

Partitioning Behavior and Interfacial Activity of Phenolic Acid Derivatives and their Impact on β-Lactoglobulin at the Oil-Water Interface

Proteins are able to stabilize dispersed food systems due to their amphiphilic nature, acting as emulsifiers. Their interfacial properties can be influenced by different methods, including the formation of protein-phenol nanocomplexes. In this study, the interfacial behavior of phenolic compounds and protein-phenol nanocomplexes was first characterized according to the oil-water partitioning behavior of phenolic acid derivatives according to their molecular structure and its impact on interfacial tension. The influence of the phenolic compounds on protein film formation and its properties by dilatational rheology was then evaluated. The most phenolic acid derivatives are predominantly present in the aqueous phase. Despite their hydrophobic benzene body, weak interfacial activity was observed depending on their chemical structure. This result supports possible protein-phenol nanocomplex formation in the aqueous phase and possible interactions at the oil-water interface. Protein-phenol nanocomplexes showed decreased interfacial adsorption properties and decreased viscoelastic interfacial behavior, depending on the expansion of the delocalized π-electrons in the phenol.

Characteristics of the interaction mechanisms of xylitol with β-lactoglobulin and β-casein: Amulti-spectral method and docking study

Proteins and functional polyols are essential food ingredients coexisting in the food matrix, and therefore, interactions between them inevitably occur. In this study, the interaction mechanisms of xylitol (XY) with bovine milk β-lactoglobulin (β-LG) and β-casein (β-CN) were studied using multispectral techniques and molecular docking. It was found that XY strongly quenched the intrinsic fluorescence of β-LG and β-CN by static quenching. The values of the binding constants were K_A(β-LG-XY) = 3.369 × 10⁴ L/mol and K_{A(β-CN- XY)} = 7.821 × 10⁴ L/mol, indicating that the binding affinity of XY to β-CN was higher than that for β-LG. Hydrogen bonding and van der Waals forces played a major role in the interactions of XY with β-LG and β-CN, and both interactions were exothermic. Simultaneous fluorescence, three-dimensional fluorescence, and circular dichroism spectroscopy showed that binding of XY did not change the secondary structure of β-LG. However, XY interaction with β-CN led to the conversion of α-helices to random coils and structural loosening. In addition, molecular docking predicted the most likely binding sites of XY in both proteins and the interaction forces involved in binding, confirming the spectroscopic results. This study improves the understanding of the interactions of XY with β-LG and β-CN in functional dairy products and provides a theoretical basis for the addition of XY in a functional milk base.

Locating the binding sites of citric acid and gallic acid on milk β-lactoglobulin

β-Lactoglobulin (β-LG) is a member of lipocalin superfamily of transporters for small hydrophobic and hydrophilic molecules. We located the binding sites of citric acid and gallic acid on β-lactoglobulin (β-LG) in aqueous solution at physiological conditions, using spectroscopic methods, thermodynamic analysis and molecular modeling. Thermodynamic parameters ΔH⁰ -9.5 to -6.9 (kJ mol^-1), ΔS⁰ 23.9 to 13.6 (J mol^-1K^-1) and ΔG⁰ -14.5 to -13.6 (kJ mol^-1) showed that acid binds protein via ionic contacts with gallic acid forming stronger protein conjugates consistent with theoretical modeling. Different amino acids are involved in gallic acid and citric acid complexation. Protein conformation was altered with reduction of β-sheet from 58% (free protein) to 46-43% and a major increase in α-helix from 11% (free protein) to 29-23% and random coil structure in the acid-protein, indicating a partial protein destabilization. Communicated by Ramaswamy H. Sarma.

Ferulic acid-ovalbumin protein nanoparticles: Structure and foaming behavior

Egg white was known for its excellent foaming properties, and some reports had studied the effect of polyphenol such as green tea on the foaming properties. However, ovalbumin, as the most abundant component of egg white protein, few literatures have reported the effects of polyphenols on its structure and foam property. In this study, ferulic acid (FA) was selected to explore the influence of polyphenol on the structure and foaming properties of ovalbumin (OVA). Results showed that hydrophobic interaction and hydrogen chemical bonds were the main driving force. FA could induce a significant decrease of free-SH content (12.76-3.72 μmol/g), a slight decline of surface hydrophobicity (716.39-577.65). Meanwhile, combined with the results of fluorescence spectroscopy and circular dichroism spectroscopy, we conclude that FA changed the structures and molecular flexibility of OVA. The increase of particle size and absolute zeta-potential showed there was a little aggregation between OVA molecules, proved FA could act as a cross-linker between OVA proteins. This behavior makes the adjacent films more firm and stable, therefore improved the foaming properties. This study suggested that FA could be a potential foaming agent to modify the foaming properties of OVA in the foam-related food industry.

Molecular interaction of tea catechin with bovine β-lactoglobulin: A spectroscopic and in silico studies

Polyphenols has attained pronounced attention due to their beneficial values of health and found to prevent several chronic diseases. Here, we elucidated binding mechanism between frequently consumed polyphenol “tea catechin” and milk protein bovine beta-lactoglobulin (β-Lg). We investigated the conformational changes of β-Lg due to interaction with catechin using spectroscopic and in silico studies. Fluorescence quenching data (Stern-Volmer quenching constant) revealed that β-Lg interacted with catechin via dynamic quenching. Thermodynamic data revealed that the interaction between β-Lg and catechin is endothermic and spontaneously interacted mainly through hydrophobic interactions. The UV-Vis absorption and far-UV circular dichroism (CD) spectroscopy exhibited that the tertiary as well as secondary structure of β-Lg distorted after interaction with catechin. Molecular docking and simulation studies also confirm that catechin binds at the central cavity of β-Lg with high affinity (~10⁵ M⁻¹) and hydrophobic interactions play significant role in the formation of a stable β-Lg-catechin complex.

The Effect of (-)-Epigallocatechin-3-Gallate Non-Covalent Interaction with the Glycosylated Protein on the Emulsion Property

The effect of (−)-epigallocatechin-3-gallate (EGCG) on protein structure and emulsion properties of glycosylated black bean protein isolate (BBPI-G) were studied and compared to native black bean protein isolate (BBPI). The binding affinity of BBPI and BBPI-G with EGCG belonged to non-covalent interaction, which was determined by fluorescence quenching. EGCG attachment caused more disordered protein conformation, leading to a higher emulsification property. Among the different EGCG concentrations (0.10, 0.25, 0.50 mg/mL), the result revealed that the highest level of the emulsification property was obtained with 0.25 mg/mL EGCG. Therefore, the BBPI-EGCG and BBPI-G-EGCG prepared by 0.25 mg/mL EGCG were selected to fabricate oil-in-water (O/W) emulsions. After the addition of EGCG, the mean particle size of emulsions decreased with the increasing absolute value of zeta-potential, and more compact interfacial film was formed due to the higher percentage of interfacial protein adsorption (AP%). Meanwhile, EGCG also significantly reduced the lipid oxidation of emulsions.

Fabrication and Characterization of Zein Composite Particles Coated by Caseinate-Pectin Electrostatic Complexes with Improved Structural Stability in Acidic Aqueous Environments

Zein composite particles coated with caseinate-pectin electrostatic complexes (zein-caseinate-pectin particles) were fabricated using an electrostatic deposition and liquid-liquid dispersion method without heating treatment. Compared to zein particles coated only with caseinate, the acidic stability of zein-caseinate-pectin particles was greatly improved, and the particle aggregation was suppressed at pH 3–6, especially at pH values near the isoelectric point of caseinate (pH 4–5). Besides, desirable long-term storage stability and re-dispersibility were observed. Under different zein to curcumin (Cur) feeding ratios (10:1, 20:1, 30:1 and 40:1, w/w), the Cur-loaded zein-caseinate-pectin particles had a spherical shape with an average diameter ranging from 358.37 to 369.20 nm, a narrow size distribution (polydispersity index < 0.2) and a negative surface charge ranging from −18.87 to −19.53 mV. The relatively high encapsulation efficiencies of Cur (81.27% to 94.00%) and desirable re-dispersibility were also achieved. Fluorescence spectroscopy indicated that the encapsulated Cur interacted with carrier materials mainly through hydrophobic interactions. The in-vitro release profile showed a sustained release of Cur from zein-caseinate-pectin particles in acidic aqueous environment (pH 4) up to 24 h, without any burst effect. In addition, the encapsulation retained more ABTS^•+ radical scavenging capacity of Cur during 4 weeks of storage. These results suggest that zein-caseinate-pectin particles may be used as a potential delivery system for lipophilic nutrients in acidic beverages.

Interactions between β-Lactoglobulin and 3,3'-Diindolylmethane in Model System

The compound 3,3′-diindolylmethane (DIM) has a broad spectrum of anticancer activities. However, low stability and bioavailability limit its application. Elucidating interactions between DIM and β-lactoglobulin (β-LG) may be useful for fabricating whey protein-based protecting systems. Interaction with DIM increased the diameter and absolute zeta potential value of β-LG. UV-absorption spectra suggested that there was a complex of DIM and β-LG. β-LG showed enhanced fluorescence intensity by complexing with DIM with a binding constant of 6.7 × 10⁵ M⁻¹. Upon interaction with DIM, β-LG was decreased in secondary structure content of helix and turn while increased in β-sheet and unordered. FT-IR spectra and molecular docking results indicated the roles of hydrophobic interaction and hydrogen bond for the formation of DIM and β-LG nanocomplexes. Data suggested that β-LG may be a good vehicle for making a protein-based DIM protection and delivery system due to the tight binding of DIM to β-LG.

Tracking of β-Lactoglobulin Binding Compounds with Biofingerprinting Chromatogram Analysis of Natural Products

The addition of milk to tea has been shown to prevent the vascular protective effects of the tea. Thus we had an interest in evaluating the possible binding of compounds in extracts of black tea, Camellia sinensis (L.) O. Kunze, and nettle, Urtica dioica L, to β-lactoglobulin (βLG), which is the main proteinaceous component of whey. Since our previously miniaturised 96-well plate method based on fluorescence quenching is suitable only for pure compounds, we have developed a method of ultrafiltration sampling combined with HPLC to study the βLG-binding property of the extracts. The method development was made with extract of Piper nigrum L. (black pepper), because its major component is piperine, which has been earlier shown to bind to βLG. The results showed that piperine from P. nigrum was highly and rutin from U. dioica slightly bound to βLG. No components from C. sinensis were bound to βLG. We showed for the first time, that the ultrafiltration/HPLC system can be used in βLG-binding studies with plant extracts. Furthermore, we demonstrated that βLG could bind compounds from extracts and in this way either enhance, or decrease, their health benefits, for example in food preparations.

Binding of Gallic Acid and Epigallocatechin Gallate to Heat-Unfolded Whey Proteins at Neutral pH Alters Radical Scavenging Activity of in Vitro Protein Digests

Preheated (80 °C for 9 min) whey protein isolate (HWPI) was reacted with 20, 120, and 240 μmol/g (protein basis) gallic acid (GA) or epigallocatechin gallate (EGCG) at neutral pH and 25 °C. Isothermal titration calorimetry and fluorometry showed a similar trend that GA binding to HWPI was moderate but weaker than EGCG binding. However, the shift of maximal fluorescence emission wavelength in opposite directions in response to GA (blue) and EGCG (red) suggests discrepant binding patterns. Electrophoresis results showed that EGCG induced formation of HWPI complexes while GA only had a marginal effect. Both free and phenolic-bound HWPI exhibited mild antiradical activity. However, when subjected to in vitro digestion, synergistic radical-scavenging activity was produced between the phenolics and peptides with the highest synergism being observed on 120 μmol/g phenolics.

Spectrofluorimetric and molecular docking studies on the interaction of cyanidin-3-O-glucoside with whey protein, β-lactoglobulin

The interaction of β-Lactoglobulin (β-Lg) with cyanidin-3-O-glucoside (C3G) was characterized using fluorescence, circular dichroism spectroscopy, and docking studies under physiological conditions. Fluorescence studies showed that β-Lg has a strong binding affinity for C3G via hydrophobic interaction with the binding constant, K_a, of 3.14×10⁴M^-1 at 298K. The secondary structure of β-Lg displayed an increase in the major structure of β-sheet upon binding with C3G, whereas a decrease in the minor structure of α-helix was also observed. In addition, evidenced by near UV-CD, the interaction also disrupted the environments of Trp residues. The molecular docking results illustrated that both hydrogen bonding and the hydrophobic interaction are involved as an acting force during the binding process. These results may contribute to a better understanding over the enhanced physicochemical proprieties of anthocyanins due to the complexation with milk proteins.

Multispectroscopic and molecular modeling studies on the interaction of two curcuminoids with β-lactoglobulin

This study demonstrates the binding properties of bisdemethoxycurcumin (BDMC) and diacetylbisdemethoxycurcumin (DABC) as bioactive curcuminoids with bovine β-lactoglobulin (BLG) variant B using fluorescence and circular dichroism (CD) spectroscopy; molecular docking, and molecular dynamics simulation methods. The estimated binding constants for BLG-BDMC and BLG-DABC complexes were (8.99±0.10)×10(4) M(-1) and (1.87±0.10)×10(2) M(-1), respectively. The distances between BLG and these curcuminoids were obtained based on the Förster's theory of non-radiative energy transfer. Molecular docking studies revealed the binding of BDMC and DABC to the protein surface cleft of protein by formation of four and one hydrogen bonds, respectively. Finally, molecular dynamics simulation results represent the conformational changes of BLG due to its interaction with BDMC. Also, the profiles of atomic fluctuations signified the rigidity of ligand binding site during the simulation.

Gum arabic and Fe²⁺ synergistically improve the heat and acid stability of norbixin at pH 3.0-5.0

Thermal and acid stabilities of norbixin are challenges for its application as a food colorant. In this work, gum arabic and Fe(2+) were studied for the possibility to improve the thermal and acid stabilities of norbixin. Norbixin was dissolved at 0.004% w/v in deionized water with and without 0.2% w/v gum arabic and/or 0.15 mM ferrous chloride, adjusted to pH 3.0-5.0, and heated at 90 or 126 °C for 30 min. Before heating, norbixin precipitated at pH 3.0-4.0, which was prevented by gum arabic. The thermal stability of norbixin was improved by the combination of gum arabic and Fe(2+). Fluorescence analyses indicated the complex formation between norbixin and gum arabic with and without Fe(2+). Particle size and atomic force microscopy results suggested Fe(2+) and gum arabic synergistically prevented the aggregation of norbixin at acidic pH and during heating. It was hypothesized that the core of gum arabic-norbixin complexes was strengthened by Fe(2+) to enable the synergy.

Native and thermally modified protein-polyphenol coassemblies: lactoferrin-based nanoparticles and submicrometer particles as protective vehicles for (-)-epigallocatechin-3-gallate

The interactions between native, thermally modified lactoferrin (LF) and (-)-epigallocatechin-3-gallate (EGCG) at pH 3.5, 5.0, and 6.5 were investigated. Turbidity, particle size, and charge of LF-EGCG complexes were mainly dominated by pH value and secondary structure of protein. At pH 3.5 and 5.0, LF-EGCG complexes were nanoparticles which had high ζ-potential, small size, and soluble state. At pH 6.5, they were submicrometer particles which exhibited low ζ-potential, large size, and insoluble state. The infrared spectra of freeze-dried LF-EGCG complexes showed that they were different from LF and EGCG alone. Far-UV CD results indicated that heat denaturation might irreversibly alter the secondary structure of LF and EGCG induced a progressive increase in the proportion of α-helix structure at the cost of β-sheet and unordered coil structure of LF at pH 3.5, 5.0, and 6.5. EGCG exhibited a strong affinity for native LF but a weak affinity for thermally modified LF at pH 5.0 and 6.5. An inverse result was observed at pH 3.5. These results could have potential for the development of food formulations based on LF as a carrier of bioactive compounds.

Probing of the interaction between β-lactoglobulin and the anticancer drug oxaliplatin

The potential carrier role of β-lactoglobulin (β-LG) and its interactions with oxaliplatin were studied using various spectroscopic techniques (fluorescence, UV-visible, and circular dichroism (CD)) in an aqueous medium at two temperatures of 25 and 37 °C in combination with a molecular docking study. Fluorescence measurements have shown that the observed quenching is a combination of static and dynamic quenching with a predominant contribution of static mode. The presence of a single binding site located in the internal cavity of the β-barrel of β-LG was confirmed by molecular docking calculations. Thermodynamic data as well as molecular docking indicated that the hydrophobic interactions dominate in the binding site. Results of fluorescence resonance energy transfer (FRET) measurements in combination with docking results imply that resonance energy transfer occurs between β-LG and its ligand oxaliplatin. Additionally, CD results revealed that oxaliplatin binding has no influence on the β-LG structure. The molecular docking results indicate that docking may be an appropriate method for the prediction and confirmation of experimental results. Complementary molecular docking results may be useful for the determination of the binding mechanism of proteins such as β-LG in pharmaceutical and biophysical studies providing new insight in the novel pharmacology and new solutions in the formulation of advanced oral drug delivery systems.

pH-driven encapsulation of curcumin in self-assembled casein nanoparticles for enhanced dispersibility and bioactivity

The poor water solubility and bioactivity of lipophilic phytochemicals can be potentially improved by delivery systems. In this study, a low-cost, low-energy, and organic solvent-free encapsulation technology was studied by utilizing the pH-dependent solubility properties of curcumin and self-assembly properties of sodium caseinate (NaCas). Curcumin was deprotonated and dissolved, while NaCas was dissociated at pH 12 and 21 °C for 30 min. The subsequent neutralization enabled the encapsulation of curcumin in self-assembled casein nanoparticles. The degradation of curcumin under encapsulation conditions was negligible based on visible light and nuclear magnetic resonance spectroscopy. The dissociation of NaCas at pH 12 and reassociation after neutralization were confirmed using dynamic light scattering and analytical ultracentrifugation. The curcumin encapsulated in casein nanoparticles showed significantly improved anti-proliferation activity against human colorectal and pancreatic cancer cells. The studied encapsulation method is promising to utilize lipophilic compounds in food or pharmaceutical industries.

Yeast mannoproteins improve thermal stability of anthocyanins at pH 7.0

Anthocyanins are food colourants with strong antioxidant activities, but poor thermal stability limits their application in neutral foods. In the present study, impacts of yeast mannoproteins on the thermal stability of anthocyanins were studied at pH 7.0. The degradation of anthocyanins at 80 and 126 °C followed first order kinetics, and the addition of mannoproteins reduced the degradation rate constant and increased the half-life by 4 to 5-fold. After heating at 80 and 126 °C for 30 min, mannoproteins improved the colour stability of anthocyanins by 4 to 5-fold and maintained the antioxidant capacity of anthocyanins. Visible light absorption, fluorescence spectroscopy, and zeta-potential results suggest that anthocyanins bound with the protein moiety of mannoproteins by hydrophobic interactions, and that the inclusion of anthocyanins in complexes effectively reduced the thermal degradation at pH 7.0. Therefore, mannoproteins may expand the application of anthocyanins as natural colours or functional ingredients.

Transglutaminase-induced crosslinking of gelatin-calcium carbonate composite films

The effects of transglutaminase (TGase) on the rheological profiles and interactions of gelatin-calcium carbonate solutions were studied. In addition, mechanical properties, water vapour permeability and microstructures of gelatin-calcium carbonate films were also investigated and compared. Fluorescence data suggested that the interaction of TGase and gelation-calcium carbonate belonged to a static quenching mechanism, and merely one binding site between TGase and gelatin-calcium carbonate was identified. Moreover, differential scanning calorimetry (DSC), the mechanical properties and the water vapour permeability studies revealed that TGase favoured the strong intramolecular polymerisation of the peptides in gelatin. The microstructures of the surfaces and cross sections in gelatin-calcium carbonate films were shown by scanning electron microscope (SEM) micrographs. The results of the fourier transform infrared spectroscopy (FTIR) indicated that TGase caused conformational changes in the proteins films. Therefore, TGase successfully facilitated the formation of gelatin-calcium carbonate composite films.

The differences in binding 12-carbon aliphatic ligands by bovine β-lactoglobulin isoform A and B studied by isothermal titration calorimetry and X-ray crystallography

Isoforms A (LGB-A) and B (LGB-B) of bovine lactoglobulin, the milk protein, differ in positions 64 (D↔G) and 118 (V↔A). Interactions of LGB-A and LGB-B with sodium dodecyl sulfate (SDS), dodecyltrimethylammonium chloride (DTAC) and lauric acid (LA), 12-carbon ligands possessing differently charged polar groups, were investigated using isothermal titration calorimetry and X-ray crystallography, to study the proton linkage phenomenon and to distinguish between effects related to different isoforms and different ligand properties. The determined values of ΔS and ΔH revealed that for all ligands, binding is entropically driven. The contribution from enthalpy change is lower and shows strong dependence on type of buffer that indicates proton release from the protein varying with protein isoform and ligand type and involvement of LA and Asp64 (in isoform A) in this process. The ligand affinities for both isoforms were arranged in the same order, DTAC < LA < SDS, and were systematically lower for variant B. The entropy change of the complexation process was always higher for isoform A, but these values were compensated by changes in enthalpy, resulting in almost identical ΔG for complexes of both isoforms. The determined crystal structures showed that substitution in positions 64 and 118 did not influence the overall structure of LGB complexes. The chemical character of the ligand polar group did not affect the position of its aliphatic chain in protein β-barrel, indicating a major role of hydrophobic interactions in ligand binding that prevailed even with the repulsion between positively charged DTAC and lysine residues located at binding site entrance.

Thymol nanoencapsulated by sodium caseinate: physical and antilisterial properties

In this work, thymol was encapsulated in sodium caseinate using high shear homogenization. The transparent dispersion at neutral pH was stable for 30 days at room temperature as determined by dynamic light scattering and atomic force microscopy, which agreed with high ζ potential of nanoparticles. The slightly decreased particle dimension during storage indicates the absence of Ostwald ripening. When molecular binding was studied by fluorescence spectroscopy, thymol was observed to bind with tyrosine and possibly other amino acid residues away from tryptophan of caseins. At pH 4.6 (isoelectric point of caseins), the stabilization of thymol nanoparticles against aggregation was enabled by soluble soybean polysaccharide, resulting from the combined electrostatic and steric repulsions. The encapsulated thymol showed the significantly improved antilisterial activity in milk with different fat levels when compared to thymol crystals, resulting from the quicker mixing and increased solubility in the milk serum. The transparent thymol nanodispersions have promising applications to improve microbiological safety and quality of foods.

Microscopic and thermodynamic analysis of PEG–β-lactoglobulin interaction

We report the binding of milk β-lactoglobulin (β-LG) with PEG-3000, PEG-6000 and methoxypoly(ethylene glycol) anthracene (mPEG-anthracene) in aqueous solution at pH 7.4, using multiple spectroscopic methods, thermodynamic analysis, transmission electron microscopy (TEM) and molecular modeling.

Interactions of β-lactoglobulin variants A and B with Vitamin A. Competitive binding of retinoids and carotenoids

β-Lactoglobulin (β-Lg) is the major whey protein of bovine milk present at a concentration of 2-3 g L(-1). Its biological role is still not well-known. However, many studies have suggested that β-Lg may play either nutritional or specific transporter role. The high affinity of β-Lg for retinol and other retinoids was reported. The results of interaction studies of β-Lg with carotenoids, that is, β-carotene, β-cryptoxanthin, and α-carotene, which display similar structures are reported in this study. The affinities of β-Lg for binding of retinoids and carotenoids were compared, providing more information about the binding site(s) of these molecules by β-Lg. Interactions were followed by the measurements of quenching of β-Lg tryptophan fluorescence and retinol fluorescence. The obtained results indicate that carotenoids are bound by β-Lg with high affinity of the order of 10(-8) M. Measurement of retinol competition with carotenoids for binding by β-Lg suggests that the binding of these two ligands occurs at two different sites of β-Lg.