Heat-induced gelation of globular proteins: part 3. Molecular studies on low pH beta-lactoglobulin gels

Protein Fibrils Induce Emulsion Stabilization

The behavior of an oil-in-water emulsion was studied in the presence of protein fibrils for a wide range of fibril concentrations by using rheology, diffusing wave spectroscopy, and confocal laser scanning microscopy. Results showed that above a minimum fibril concentration depletion flocculation occurred, leading to oil droplet aggregation and enhanced creaming of the emulsion. Upon further increasing the concentration of the protein fibrils, the emulsions were stabilized. In this stable regime both aggregates of droplets and single droplets are present, and these aggregates are smaller than the aggregates in the flocculated emulsion samples at the lower fibril concentrations. The size of the droplet aggregates in the stabilized emulsions is independent of fibril concentration. In addition, the droplet aggregation was reversible upon dilution both by a pH 2 HCl solution and by a fibril solution at the same concentration. The viscosity of the emulsions containing fibrils was comparable to that of the pure fibril solution. Neither fibril networks nor droplet gel networks were observed in our study. The stabilization mechanism of emulsions containing long protein fibrils at high protein fibril concentrations points toward the mechanism of a kinetic stabilization.

Genipin crosslinked soy-whey based bioactive material for atorvastatin loaded nanoparticles: preparation, characterization and in vivo antihyperlipidemic study

Lipid lowering potential of soy protein isolate and whey protein concentrate as novel nanomaterial for atorvastatin nanoparticles.

Versatile Cellulose-Based Carbon Aerogel for the Removal of Both Cationic and Anionic Metal Contaminants from Water

Hydrothermal carbonization of cellulose in the presence of the globular protein ovalbumin leads to the formation of nitrogen-doped carbon aerogel with a fibrillar continuous carbon network. The protein plays here a double role: (i) a natural source of nitrogen functionalities (2.1 wt %) and (ii) structural directing agent (S(BET) = 38 m(2)/g). The applicability in wastewater treatment, namely, for heavy metal removal, was examined through adsorption of Cr(VI) and Pb(II) ion solely and in a mixed bicomponent aqueous solutions. This cellulose-based carbogel shows an enhanced ability to remove both Cr(VI) (∼68 mg/g) and Pb(II) (∼240 mg/g) from the targeted solutions in comparison to other carbon materials reported in the literature. The presence of competing ions showed little effect on the adsorption efficiency toward Cr(VI) and Pb(II).

Determination and Quantification of Molecular Interactions in Protein Films: A Review

Protein based films are nowadays also prepared with the aim of replacing expensive, crude oil-based polymers as environmentally friendly and renewable alternatives. The protein structure determines the ability of protein chains to form intra- and intermolecular bonds, whereas the degree of cross-linking depends on the amino acid composition and molecular weight of the protein, besides the conditions used in film preparation and processing. The functionality varies significantly depending on the type of protein and affects the resulting film quality and properties. This paper reviews the methods used in examination of molecular interactions in protein films and discusses how these intermolecular interactions can be quantified. The qualitative determination methods can be distinguished by structural analysis of solutions (electrophoretic analysis, size exclusion chromatography) and analysis of solid films (spectroscopy techniques, X-ray scattering methods). To quantify molecular interactions involved, two methods were found to be the most suitable: protein film swelling and solubility. The importance of non-covalent and covalent interactions in protein films can be investigated using different solvents. The research was focused on whey protein, whereas soy protein and wheat gluten were included as further examples of proteins.

Fibril formation from pea protein and subsequent gel formation

The objective of this study was to characterize fibrillar aggregates made using pea proteins, to assemble formed fibrils into protein-based gels, and to study the rheological behavior of these gels. Micrometer-long fibrillar aggregates were observed after pea protein solutions had been heated for 20 h at pH 2.0. Following heating of pea proteins, it was observed that all of the proteins were hydrolyzed into peptides and that 50% of these peptides were assembled into fibrils. Changes on a structural level in pea proteins were studied using circular dichroism, transmission electron microscopy, and particle size analysis. During the fibril assembly process, an increase in aggregate size was observed, which coincided with an increase in thioflavin T binding, indicating the presence of β-sheet aggregates. Fibrils made using pea proteins were more branched and curly. Gel formation of preformed fibrils was induced by slow acidification from pH 7.0 to a final pH of around pH 5.0. The ability of pea protein-based fibrillar gels to fracture during an amplitude sweep was comparable to those of soy protein and whey protein-based fibrillar gels, although gels prepared from fibrils made using pea protein and soy protein were weaker than those of whey protein. The findings show that fibrils can be prepared from pea protein, which can be incorporated into protein-based fibrillar gels.

The self-assembly, aggregation and phase transitions of food protein systems in one, two and three dimensions

The aggregation of proteins is of fundamental relevance in a number of daily phenomena, as important and diverse as blood coagulation, medical diseases, or cooking an egg in the kitchen. Colloidal food systems, in particular, are examples that have great significance for protein aggregation, not only for their importance and implications, which touches on everyday life, but also because they allow the limits of the colloidal science analogy to be tested in a much broader window of conditions, such as pH, ionic strength, concentration and temperature. Thus, studying the aggregation and self-assembly of proteins in foods challenges our understanding of these complex systems from both the molecular and statistical physics perspectives. Last but not least, food offers a unique playground to study the aggregation of proteins in three, two and one dimensions, that is to say, in the bulk, at air/water and oil/water interfaces and in protein fibrillation phenomena. In this review we will tackle this very ambitious task in order to discuss the current understanding of protein aggregation in the framework of foods, which is possibly one of the broadest contexts, yet is of tremendous daily relevance.

Combined effect of heat treatment and ionic strength on the functionality of whey proteins

A 5% (wt/vol) whey protein isolate (WPI) dispersion (pH 6.5) with different concentrations of NaCl was submitted to dynamic heat treatment. Protein dispersions were characterized as to their rheological properties, particle sizes, morphology, denaturation temperatures, and protein surface hydrophobicity. At low ionic strength (<200 mmol/kg), gel elastic modulus increased and strongest gel stiffness was achieved. High salt concentrations lead to a weaker gel, whereas no gels at all were formed without salt. The gelation temperature was also influenced by ionic strength and an increase in denaturation temperature and thermal stability was also observed by using differential scanning calorimetry. Additionally, heat-induced changes in secondary structures upon salt augmentation were followed by Fourier transform infrared spectroscopy. Secondary structural elements estimations obtained from amide I assignments were correlated with those from amide III assignments. Upon salt increase, no differences in secondary structure were observed without heating, whereas upon heating and without salt increase, the Fourier transform infrared spectroscopy data revealed an increase in intermolecular β-sheets at the cost of β-turns and random coils, with no change in α-helical structures. However, NaCl addition along with dynamic heat treatment of WPI dispersion showed a stabilizing effect on the secondary structural elements of both amide I and amide III bands. Whey protein isolate dispersions in water were also characterized by transmission electron microscopy by a spherical shape with 2 populations (6 and 70 nm). Salt increase alone resulted in the formation of denser aggregates, whereas a transition from spherical/compact protein aggregates to linear ones was observed due to combined salt/heat effect. The important size of these edifices was confirmed by microscopy and light-scattering techniques. Moreover, protein surface hydrophobicity related to the number of hydrophobic sites available decreased significantly. Finally, experimental results demonstrated the strong interaction between ionic strength and dynamic thermal treatment on protein functional properties and their careful adjustment could enable the food industry to effectively use WPI as a gelling agent.

Nanoparticles synthesized from soy protein: preparation, characterization, and application for nutraceutical encapsulation

Nanoparticles were synthesized from soy protein, one of the most abundant and widely utilized plant proteins, for nutraceutical and drug encapsulation. The preparation process consisted of dispersion, desolvation, drug incorporation, cross-linking, and evaporation. The role of each procedure in the formation of nanoparticles was systematically investigated by means of particle size, size distribution, and zeta potential as well as morphology observation. Curcumin as a model drug was encapsulated successfully into the nanoparticles, evidenced by Fourier transform infrared spectroscopy and X-ray diffraction patterns. The average size of the curcumin-loaded nanoparticles was 220.1 to 286.7 nm, and their zeta potential was around -36 mV. The highest encapsulation efficiency and loading efficiency achieved were 97.2% and 2.7%, respectively. The release of curcumin in phosphate buffer saline followed a biphasic pattern. Possible mechanisms of the formation of soy protein nanoparticles as well as the incorporation of curcumin were discussed based on the data obtained from this study.

Influence of protein hydrolysis on the growth kinetics of β-lg fibrils

Recently it was found that protein hydrolysis is an important step in the formation of β-lactoglobulin fibrils at pH 2 and elevated temperatures. The objective of the present study was to further investigate the influence of hydrolysis on the kinetics of fibril formation. Both the hydrolysis of β-lactoglobulin and the growth of the fibrils were followed as a function of time and temperature, using SDS polyacrylamide gel electrophoresis and a Thioflavin T fluorescence assay. As an essential extension to existing models, the quantification of the effect of the hydrolysis on the fibrillar growth was established by a simple polymerization model including a hydrolysis step.

Effect of disulfide interactions and hydrolysis on the thermal aggregation of β-lactoglobulin

The roles of sulfhydryl/disulfide interactions and acid/pepsin hydrolysis on β-lactoglobulin (β-lg) thermal aggregation at acidic pH 3.35 and 2 were studied using rheology, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), transmission electron microscopy (TEM), and Western blotting. Pepsin promoted additional hydrolysis compared to the acid-hydrolyzed control sample based on a 12% increase in free amino groups. Hydrolysis with pepsin also resulted in an increase in the apparent viscosity by 2 logs upon heating 8% β-lg solutions at pH 3.35. Seemingly, hydrolysis promoted thermal aggregation of β-lg, correlating well with viscosity increases. Large microgels were observed in heated pepsin hydrolysates using TEM, supporting the increased viscosities of these dispersions. During thermal aggregation (85 °C, 3 h) of β-lg at pH 3.35, beyond the existence of limited disulfide interactions, acid hydrolysis and noncovalent interactions more likely play a crucial role in defining the functionality of acidified powdered modified whey ingredients.

Fibrillation of β-lactoglobulin at low pH in the presence of a complexing anionic polysaccharide

The influence of electrostatic complexation with κ-carrageenan was tested on the fibrillation process of β-lactoglobulin at pH 2.0. Morphology and structural development were monitored through cross correlation dynamic light scattering, transmission electron microscopy, and atomic force microscopy. Scattering indicated that noncomplexed β-lactoglobulin monomers aggregated to form fibrils after 15-90 min of heating at 90 °C. However, electrostatic protein-carrageenan complexes found in the unheated system were unchanged by the thermal process. Images and scattering results showed that carrageenan complexes slowed fibrillation kinetics, possibly through reduction in available monomer concentration. Complexes adhered to fibrils at ends and junctions in TEM images, indicating interactive affinity with the fibers, presumably as heterogeneous nucleation sites.

Formation and characterization of amyloid-like fibrils from soy β-conglycinin and glycinin

The fibrillar aggregation at pH 2.0 of soy β-conglycinin, glycinin, and the 1:1 mixture thereof, induced by heating at 80 °C for various periods of time, was investigated using Th T and Congo Red spectroscopic techniques. The morphology of the formed fibrillar aggregates was characterized using atomic force microscopy (AFM), whereas the conformational changes and the polypeptide hydrolysis of the proteins upon heating were also evaluated. Th T fluorescence analysis indicated that β-conglycinin had a much higher potential to form heat-induced amyloid-like aggregates than glycinin. AFM analyses showed that all of the soy globulins could form twisted screw-structure fibrils with heights of 1.4-2.2 nm, but the morphology of the amyloid-like fibrils considerably varied among various soy proteins. Significantly lower width at half-heights and higher coil periodicity values were observed for the β-conglycinin fibrils than the glycinin counterpart. Far-UV CD spectral analysis indicated that upon heating, the secondary conformations of the proteins changed considerably, especially during initial heating (e.g., <4 h), and the changes were much more distinct in the β-conglycinin case than in the glycinin case. Furthermore, reducing electrophoresis analyses indicated that progressive polypeptide hydrolysis occurred upon heating, but the polypeptide hydrolysis for the β-conglycinin was much more severe than that of glycinin. The data suggest that soy β-conglycinin exhibited a much higher potential to form thermally fibrillar aggregates than glycinin, and the differences seem to be mainly associated with the differences in their conformational changes and extent of polypeptide hydrolysis by the heating. The results would be of vital importance for the utilization of soy proteins to produce thermally induced fibrillar gels with excellent properties.

Rheological properties of peptide-based hydrogels for biomedical and other applications

Peptide-based hydrogels are an important class of biomaterials finding use in food industry and potential use in tissue engineering, drug delivery and microfluidics. A primary experimental method to explore the physical properties of these hydrogels is rheology. A fundamental understanding of peptide hydrogel mechanical properties and underlying molecular mechanisms is crucial for determining whether these biomaterials are potentially suitable for biotechnological uses. In this critical review, we cover the literature containing rheological characterization of the physical properties of peptide and polypeptide-based hydrogels including hydrogel bulk mechanical properties, gelation mechanisms, and the behavior of hydrogels during and after flow (219 references).

Understanding amyloid aggregation by statistical analysis of atomic force microscopy images

The aggregation of proteins is central to many aspects of daily life, including food processing, blood coagulation, eye cataract formation disease and prion-related neurodegenerative infections. However, the physical mechanisms responsible for amyloidosis-the irreversible fibril formation of various proteins that is linked to disorders such as Alzheimer's, Creutzfeldt-Jakob and Huntington's diseases-have not yet been fully elucidated. Here, we show that different stages of amyloid aggregation can be examined by performing a statistical polymer physics analysis of single-molecule atomic force microscopy images of heat-denatured beta-lactoglobulin fibrils. The atomic force microscopy analysis, supported by theoretical arguments, reveals that the fibrils have a multistranded helical shape with twisted ribbon-like structures. Our results also indicate a possible general model for amyloid fibril assembly and illustrate the potential of this approach for investigating fibrillar systems.

Characterization of beta-lactoglobulin fibrillar assembly using atomic force microscopy, polyacrylamide gel electrophoresis, and in situ fourier transform infrared spectroscopy

The aggregation process of beta-lactoglobulin (beta-lg) from 0 min to 20 h was studied using atomic force microscopy (AFM), scanning transmission electron microscopy (STEM), sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and in situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). Fibril assembly was monitored in real time using AFM up to 20 h. From 0 to 85 min, beta-lg monomers deformed and expanded with some aggregation. After 85 min, fibrillar structures were formed, exceeding 10 mum in length. Fibrillar structures were confirmed by STEM. Secondary structural changes occurring during fibril formation were monitored by ATR-FTIR at 80 degrees C and indicated a decrease in alpha-helix content and an increase in beta-sheet content. SDS-PAGE indicated that fibrils were composed of polypeptides and not intact monomers. In this study, beta-lg and whey protein isolate (WPI)-derived fibrils, including some double helices, in water were observed by AFM under ambient conditions and in their native aqueous environment.

Liquid crystalline phase behavior of protein fibers in water: experiments versus theory

We have developed a new method allowing the study of the thermodynamic phase behavior of mesoscopic colloidal systems consisting of amyloid protein fibers in water, obtained by heat denaturation and aggregation of beta-lactoglobulin, a dairy protein. The fibers have a cross section of about 5.2 nm and two groups of polydisperse contour lengths: (i) long fibers of 1-20 microm, showing semiflexible behavior, and (ii) short rods of 100-200 nm long, obtained by cutting the long fibers via high-pressure homogenization. At pH 2 without salt, these fibers are highly charged and stable in water. We have studied the isotropic-nematic phase transition for both systems and compared our results with the theoretical values predicted by Onsager's theory. The experimentally measured isotropic-nematic phase transition was found to occur at 0.4% and at 3% for the long and short fibers, respectively. For both systems, this phase transition occurs at concentrations more than 1 order of magnitude lower than what is expected based on Onsager's theory. Moreover, at low enough pH, no intermediate biphasic region was observed between the isotropic phase and the nematic phase. The phase diagrams of both systems (pH vs concentration) showed similar, yet complex and rich, phase behavior. We discuss the possible physical fundamentals ruling the phase diagram as well as the discrepancy we observe for the isotropic-nematic phase transition between our experimental results and the predicted theoretical results. Our work highlights that systems formed by water-amyloid protein fibers are way too complex to be understood based solely on Onsager's theories. Experimental results are revisited in terms of the Flory's theory (1956) for suspensions of rods, which allows accounting for rod-solvent hydrophobic interactions. This theoretical approach allows explaining, on a semiquantitative basis, most of the discrepancies observed between the experimental results and Onsager's predictions. The sources of protein fibers complex colloidal behavior are analyzed and discussed at length.

Passive microrheology of solvent-induced fibrillar protein networks

Particle tracking microrheology (PTM) has been used to study the sol-gel transition in solvent-induced fibrillar beta-lactoglobulin gels at room temperature and pH 7. The passive nature of microrheology allowed measurements to be made around and below the critical gelation concentration. The method of superposition introduced by Larsen and Furst (Larsen, T. H.; Furst, E. M. Phys. Rev. Lett. 2008, 100, 146001) was applied to the one-particle mean square displacement (MSD), yielding a critical relaxation exponent of n = 0.58 at concentrations close to the measured critical concentration of 4% (w/v). At a higher concentration of 12% (w/v), n was observed to decrease. The pregel and gel master curves were used to find the viscoelastic moduli over 8 decades of frequency. Combined with the measured shift factors, this allowed cure curves at 1 Hz to be constructed for direct comparison with results from bulk rheology. Time-independent modulus superposition was found for all concentrations. Good agreement for concentration scaling was found between the traditional methods for characterizing gels and the recently described microrheological determination of the gel time and critical behavior.

Multiscale characterization of individualized beta-lactoglobulin microgels formed upon heat treatment under narrow pH range conditions

Aqueous dispersions of demineralized beta-lactoglobulin (beta-lg) were held at 85 degrees C for 15 min at a constant protein concentration of 1 wt % in the pH range of 3.0-7.0. This led to denatured protein content ranging from 20% (pH 3.0) to 90% (pH 5.0). The protein aggregates formed were characterized as to their stability to sedimentation (turbidity), morphology, size, surface charge, ANS surface hydrophobicity, and content in accessible thiol groups. Additionally, the changes in secondary structures of the protein upon heating were followed by Fourier transform infrared spectroscopy (FTIR). Stable dispersions (no sedimentation for 10 min) of individualized beta-lg microgels were obtained at specific pH 4.6 and 5.8, corresponding to an aggregation yield of about 80%. The width of the pH region leading to these microgels was 0.3 pH unit below or above the two specific pH values. Microgels were characterized by a spherical shape and remarkably low polydispersity in size (<0.2). Their z-average hydrodynamic diameter determined by dynamic light scattering (DLS) was between 160 and 220 nm, and their zeta-potential was +30 or -40 mV, depending on the initial pH before heating. Microgels obtained at pH 4.6 displayed a lower binding capacity for ANS and a lower content of accessible thiol groups as compared to those obtained at pH 5.8. Both types of microgels might therefore differ in their internal and interfacial structures. Between pH 4.6 and 5.8, large sedimenting protein particulates were obtained, whereas soluble aggregates were formed at pH <4.6 or >5.8. Interestingly, DLS experiments showed that before heating, beta-lg was mainly present in an oligomeric state at pH 4.6 and 5.8. This result was confirmed by FTIR measurements indicating the stronger contribution of the 1616-1624 cm(-1) spectral band corresponding to intermolecular beta-sheets in the pH range of 4.0-6.0. Upon heating, FTIR spectroscopy revealed that individualized microgels were obtained under pH conditions where a balance between attractive forces arising from protein unfolding leading to the formation of intermolecular beta-sheets (1616-1624 cm(-1 )band) and the repulsive electrostatic forces due to the initial protein net charge was achieved.

Terahertz spectroscopic differentiation of microstructures in protein gels

We demonstrate that terahertz (THz) spectroscopy can be used to differentiate soft protein microstructures. Differentiation of soft microstructures in gels has to date been performed using optical imaging techniques (e.g. electron microscope), but a non-destructive differentiation tool is lacking. Particulate and fine-stranded (fibrillar) soft protein microstructures are of interest, particularly to medical researchers, because they form from naturally occurring proteins that are thought to be involved in several human diseases, such as Alzheimer's disease. In this study, globular beta-lactoglobulin structures with diameters of 2 microm, and fibrillar structures with diameters less than 0.03 microm are observed between 0.8 and 1.5 THz. Results show that the globular structures have a decline in THz transmission when compared to the fibrillar ones. The cause of this decline is possibly due to Rayleigh scattering from the globular microstructures.