Conformation of beta-Lactoglobulin Studied by FTIR: Effect of pH, Temperature, and Adsorption to the Oil-Water Interface

Effect of thermal treatment, ionic strength, and pH on the short-term and long-term coalescence stability of beta-lactoglobulin emulsions

We present experimental results about the effects of thermal treatment, ionic strength, and pH on the protein adsorption and coalescence stability of freshly prepared (2 h after emulsification) and 6-day-stored emulsions, stabilized by the globular protein beta-lactoglobulin (BLG). In all emulsions studied, the volume fraction of the dispersed soybean oil is 30% and the mean drop diameter is d(32) approximately 40 microm. The protein concentration, C(BLG), is varied between 0.02 and 0.1 wt %, the electrolyte concentration, C(EL), between 1.5 mM and 1 M, and pH between 4.0 and 7.0. The emulsion heating is performed at 85 degrees C, which is above the denaturing temperature of BLG. The results show that, at C(BLG) > or = 0.04 wt %, C(EL) > or = 150 mM, and pH > or = 6.2, the heating leads to higher protein adsorption and to irreversible attachment of the adsorbed molecules, which results in enhanced steric repulsion between the protein adsorption multilayers and to higher emulsion stability. At low electrolyte concentration, C(EL) < or = 10 mM, the emulsion stability is determined by electrostatic interactions and is not affected significantly by the emulsion heating. The latter result is explained by electrostatic repulsion between the adsorbed protein molecules, which keeps them separated from each other and thus precludes the formation of disulfide covalent bonds in the protein adsorption layer. The coalescence stability of heated and nonheated emulsions is practically the same and does not depend on C(EL), when pH is around the isoelectric point (IEP) of the protein molecules. This is explained with the adsorption of uncharged BLG molecules, in compact conformation, which stores the reactive sulfhydryl groups hidden inside the molecule interior, thus preventing the formation of covalent intermolecular bonds upon heating. We studied also the effect of storage time on the stability of heated and nonheated emulsions. The stability of nonheated emulsions (C(BLG) = 0.1 wt %, C(EL) > or = 150 mM, and pH = 6.2) significantly decreases after 1 day of storage (aging effect). In contrast, no aging effect is observed after emulsion heating. FTIR spectra of heated and nonheated, fresh and aged emulsions suggest that the aging effect is caused by slow conformational changes of the protein molecules in the adsorption layer, accompanied with partial loss of the ordered secondary structure of the protein and with the formation of lateral noncovalent bonds (H-bonds and hydrophobic interactions) between the adsorbed molecules. After thermal treatment of the BLG emulsions, the molecules preserve their original secondary structure upon storage, which eliminates the aging effect.

Enzymatic Cross-Linking of β-Lactoglobulin: Conformational Properties Using FTIR Spectroscopy

In this study, we use FTIR spectroscopy to probe the conformational changes of beta-lactoglobulin (beta-LG)-the main constituent of whey proteins-as subjected to enzymatic cross-linking by transglutaminase. We investigate both the amide I region (1600-1700 cm(-1)) and the C-H stretching region (2800-3100 cm(-1)). In the amide I region, spectra of denatured conformations of beta-LG, known to be necessary for cross-linking, differ according to the denaturation procedure, i.e., chemical or thermal treatment. Denaturation by chemical denaturants, dithiothreitol (DTT) or beta-mercaptoethanol, show no effect on the alpha-helix, while shifting the monomer dimer equilibrium toward higher monomer concentration. On the other hand, denaturing by thermal treatment dissociates the beta-sheets in the native structure, leading to new intermolecular beta-sheets being formed. Preheated then enzyme cross-linked beta-LG molecules show very similar spectra in the amide I region to the molecules with no cross-linking, indicating minimal effects of the cross-links on the carbonyl stretching mode. However, chemically denatured (using beta-mercaptoethanol) then enzyme cross-linked beta-LG molecules show noticeable diminution in the alpha-helix band and formation of strong hydrogen-bonded intermolecular beta-sheets. In the C-H stretching region, preheated then enzyme cross-linked beta-LG molecules exhibit a different degree of exposure of aliphatic amino acids due to the enzyme action. The same behavior is observed for DTT-treated then enzyme cross-linked beta-LG molecules. Generally, the changes in the C-H stretching region clearly indicate that hydrophobic interactions are altered upon enzymatic cross-linking.

Thermostability and in vitro digestibility of a purified major allergen 2S albumin (Ses i 1) from white sesame seeds (Sesamum indicum L.)

A major 2S albumin allergen, Ses i 1, from white sesame seeds was purified to homogeneity, characterized and identified using proteomic techniques. Ses i 1 exhibited a molecular weight of 12062 Da, although an extensive C-terminal clipping of the small subunit was observed. In addition, the N-terminal glutamine of the small subunit had been converted to pyroglutamate and a variant of the large subunit which had lost the N-terminal glutamine was also detected. The protein was thermo-stable up to 90 degrees C at neutral and acid pH, retaining its monomeric state and showing minimal alterations, which were reversible on cooling, in a predominantly alpha-helical secondary structure, as shown by circular dichroism and Fourier transform-infrared spectroscopy. Ses i 1 was also highly resistant to digestion using a physiologically relevant in vitro gastrointestinal model system. After 2 h of gastric digestion, the allergen remained completely intact and only the small subunit was cleaved during 2 h of subsequent duodenal digestion, leaving a major IgE epitope region of this protein intact. Neither prior heating of the Ses i 1 nor the presence of the physiological surfactant phosphatidylcholine affected the pattern of proteolysis. These findings are consistent with those found for the 2S albumin allergen from Brazil nut, Ber e 1, and suggest that Ses i 1 may preserve its structure from the degradation in the gastrointestinal tract, a property thought to be crucial for both a protein to sensitise the mucosal immune system and provoke an allergic reaction in a sensitised individual.

Fluorescence spectroscopy as a tool for monitoring solubility and aggregation behavior of β-lactoglobulin after heat treatment

Denaturation and aggregation of whey proteins are of interest to the food and pharmaceutical industry due to the importance of final structure in functionality, impact on food texture, and the chemical stability of the final product. In this study, we demonstrate the potential of fluorescence spectrometry combined with multivariate chemometric methods for quantifying solubility and aggregation behavior of beta-lactoglobulin (beta-LG); a major whey protein and a frequent food ingredient. Heat-induced aggregation of beta-LG was studied under different conditions including pH, temperature and heating durations. Results showed very good agreement between the fluorescence-based predictions and measurements obtained by HPLC and gravimetric analysis regardless of the conditions. Standard normal variate (SNV), a signal preprocessing and filtering tool, was found to enhance the predictive accuracy and robustness of the fluorescence-based model.

Cross linking and rheological characterization of adsorbed protein layers at the oil-water interface

The dilatational rheological properties of cross-linked protein layers adsorbed at the oil-water interface were investigated with help of a modified drop tensiometer allowing successive replacements of the external phase. This setup enables one to perform cross-linking reactions at the interface only, that is, without any contact between the cross-linking agent and protein molecules in solution, under continuous monitoring of the interfacial tension. The mechanical properties of the resulting interface were investigated with dilatational large strain experiments. Measured rheological properties were related to the expected stability of an emulsion against disproportionation by considering the ratio of the interfacial elasticity to the interfacial tension. In an attempt to increase this ratio to improve the resistance against disproportionation, experiments were performed with densified protein layers obtained via reduction of the droplet area prior to cross linking. To highlight the influence of the protein morphology on the dilatational rheological properties of the cross-linked adsorbed layers, experiments were performed with random coil (beta-casein) as well as globular (beta-lactoglobulin) proteins. Glutaraldehyde was used as a cross-linking agent. Experiments were performed at 55 degrees C and pH 7.0 in 20 mM imidazole buffer for later comparison with enzymatically cross-linked adsorbed protein layers. The present work demonstrated substantial qualitative and quantitative differences in the interfacial rheological properties of cross-linked random coil and globular proteins.

Study of protein-lipid interactions at the bovine serum albumin/oil interface by Raman microspectroscopy

The interface of 10 or 25% (w/v) bovine serum albumin (BSA), pH 7, buffered solution against mineral or corn oil was studied with a Raman microscope. A gradient of distribution of protein and oil at the interface was observed. The difference spectrum obtained by subtracting the spectrum of mineral or corn oil from that of the BSA/oil interface indicated interactions involving different functional groups of the BSA and the oil molecules. Against mineral oil, the BSA spectrum showed reduced intensity of the tryptophan band at 750 cm(-1) and reduced intensity ratio of the tyrosine doublet at 850-830 cm(-1), indicating changes in the microenvironment of these hydrophobic residues. A negative band at 2850 cm(-1) indicated the involvement of the CH groups in the mineral oil. However, the amide regions, normally assigned to protein secondary structure, were not significantly changed. When the spectrum of BSA was subtracted from the BSA/mineral oil interface spectrum, the resultant difference spectrum showed changes of symmetric and antisymmetric CCC stretches at 980 and 1071 cm(-1), respectively. In contrast, the difference spectrum of BSA/corn oil interface - BSA showed a decrease of CH(2) symmetric stretching at 2850 cm(-1) and a decrease of unsaturated fatty acid hydrocarbon chain stretch at 3010 cm(-1). Raman spectroscopy is a useful tool to study the nature of protein-lipid interactions.

Influence of protein concentration and order of addition on thermal stability of beta-lactoglobulin stabilized n-hexadecane oil-in-water emulsions at neutral pH

The influence of protein concentration and order of addition relative to homogenization (before or after) on the extent of droplet flocculation in heat-treated oil-in-water emulsions stabilized by a globular protein were examined using laser diffraction. n-Hexadecane (10 wt%) oil-in-water emulsions (pH 7, 150 mM NaCl) stabilized by beta-lactoglobulin (beta-Lg) were prepared by three methods: (1) 4 mg/mL beta-Lg added before homogenization; (2) 4 mg/mL beta-Lg added before homogenization and 6 mg/mL beta-Lg added after homogenization; (3) 10 mg/mL beta-Lg added before homogenization. The emulsions were then subjected to various isothermal heat treatments (30-95 degrees C for 20 min), with the 150 mM NaCl being added either before or after heating. Emulsion 1 contained little nonadsorbed protein and exhibited extensive droplet aggregation at all temperatures, which was attributed to the fact that the droplets had a high surface hydrophobicity, e.g., due to exposed oil or extensive protein surface denaturation. Emulsions 2 and 3 contained a significant fraction of nonadsorbed beta-Lg. When the NaCl was added before heating, these emulsions were relatively stable to droplet flocculation below a critical holding temperature (75 and 60 degrees C, respectively) but showed extensive flocculation above this temperature. The stability at low temperatures was attributed to the droplets having a relatively low surface hydrophobicity, e.g., due to complete saturation of the droplet surface with protein or due to more limited surface denaturation. The instability at high temperatures was attributed to thermal denaturation of the adsorbed and nonadsorbed proteins leading to increased hydrophobic interactions between droplets. When the salt was added to Emulsions 2 and 3 after heating, little droplet flocculation was observed at high temperatures, which was attributed to the dominance of intra-membrane over inter-membrane protein-protein interactions. Our data suggests that protein concentration and order of addition have a strong influence on the flocculation stability of protein-stabilized emulsions, which has important implications for the formulation and production of many emulsion-based products.

Inhibition of heat-induced aggregation of a β-lactoglobulin-stabilized emulsion by very small additions of casein

Heat stability has been studied in model systems of oil-in-water emulsions (3 wt.% total protein, 45 vol.% n-tetradecane, pH 6.8, ionic strength 30-50 mM) with pure beta-lactoglobulin (beta-lg) as the main emulsifier. The effect of small additions of sodium caseinate, beta-casein or alpha s1-casein prior to emulsion preparation has been investigated. Samples heated for 3 min at 90 degrees C were monitored with respect to changes in viscosity and particle-size distribution. As expected, the pure beta-lg-stabilized emulsions were susceptible to heat-induced changes. But the replacement of just 1% of the beta-lg by sodium caseinate (0.03 wt.% caseinate in the total emulsion) led to complete elimination of any heat-induced viscosity or particle size increase. These findings show that a very small proportion of casein can inhibit the susceptibility of a beta-lg-based emulsion to heat-induced destabilization. The magnitude of the effect is dependent on the type of casein, with the order of effectiveness being beta-casein>sodium caseinate>alpha s1-casein. This work has potential implications for the development of milk protein-stabilized emulsions of improved shelf life.

Comparison of droplet flocculation in hexadecane oil-in-water emulsions stabilized by beta-lactoglobulin at pH 3 and 7

The influence of surface and thermal denaturation of adsorbed beta-lactoglobulin (beta-Lg) on the flocculation of hydrocarbon oil droplets was measured at pH 3 and compared with that at pH 7. Oil-in-water emulsions (5 wt % n-hexadecane, 0.5 wt % beta-Lg, pH 3.0) were prepared that contained different levels of salt (0-150 mM NaCl) added immediately after homogenization. The mean particle diameter (d43) and particle size distribution of diluted emulsions were measured by laser diffraction when they were either (i) stored at 30 degrees C for 48 h or (ii) subjected to different thermal treatments (30-95 degrees C for 20 min). In the absence of salt, little droplet flocculation was observed at pH 3 or 7 because of the strong electrostatic repulsion between the droplets. In the presence of 150 mM NaCl, a progressive increase in mean particle size with time was observed in pH 7 emulsions during storage at 30 degrees C, but no significant change in mean particle diameter with time (d43 approximately 1.4 +/- 0.2 microm) was observed in the pH 3 emulsions. Droplet aggregation became more extensive in pH 7 emulsions containing salt (added before thermal processing) when they were heated above 70 degrees C, which was attributed to thermal denaturation of adsorbed beta-Lg leading to interdroplet disulfide bond formation. In contrast, the mean particle size decreased and the creaming stability improved when pH 3 emulsions were heated above 70 degrees C. These results suggest that the droplets in the pH 3 emulsions were weakly flocculated at temperatures below the thermal denaturation temperature of beta-Lg (T < 70 degrees C) but that flocs did not form so readily above this temperature, which was attributed to a reduction in droplet surface hydrophobicity due to protein conformational changes. The most likely explanation for the difference in behavior of the emulsions is that disulfide bond formation occurs much more readily at pH 7 than at pH 3.

Mass spectrometry and structural characterization of 2S albumin isoforms from Brazil nuts (Bertholletia excelsa)

Proteomic approaches have been used to characterise the main 2S albumin isoforms from Brazil nuts (Bertholletia excelsa). Whilst most isoforms ( approximately 10 discrete protein species) exhibited molecular masses of around 12 kDa with a high amino acid sequence homology, important charge heterogeneity was found, with pIs varying between 4.6 and 6.6, with one >or=7.0. Proteomic analysis showed that these corresponded to a total of six National Center for Biotechnology Information (NCBI) accessions and that three isoforms had been purified to homogeneity corresponding to gi/384327, 112754 and 99609. The latter sequence corresponds to an isoform, previously only identified at the nucleotide sequence level, had a slightly higher molecular weight (13.4 kDa), and with noticeable differences in the primary structure. Proteins corresponding to six different NCBI accessions were identified, the heterogeneity of which had been increased by posttranslational processing. Evidence was found of cyclization of the N-terminal glutamine residue in two isoforms, together with ragged C-termini, indicative of carboxypeptidase activity within the vacuole following posttranslational processing. No evidence of glycosylation was found. Circular dichroism (CD) and Fourier transform-infrared (FT-IR) spectroscopy indicated all the studied isoforms were predominantly alpha-helical in nature, but that the Mr 13400 species was structurally distinct, with a higher proportion of alpha-helical structure.

Proteins and emulsifiers at liquid interfaces

The interfacial properties of proteins and emulsifiers have been studied extensively in the field of food colloid research. Emulsions form the basis of a huge range of food products and are generally stabilised by either protein and/or emulsifiers. Proteins have been shown to stabilise emulsions by forming a viscoelastic, adsorbed layer on the oil droplets, which form a physical barrier to coalescence. Emulsifiers can be oil or water soluble, forming a fluid, close-packed layer at the interface with a low interfacial tension. This results in an emulsion with a small droplet size distribution, stabilised by the fluid Gibbs-Marangoni mechanism or weak electrostatic repulsion. In real food emulsions, there is usually a mixture of proteins and emulsifiers competing for the interfacial area. This can produce a finer emulsion, however, the emulsifiers break down the viscoelastic protein-adsorbed layer, resulting in an emulsion with reduced stability. We present a review recent work that aims to characterise the composition, structure and physical properties of mixed protein-emulsifier interfaces, in an effort to understand the mechanisms behind the stability behaviour of food emulsion systems.

Modification of IgE binding during heat processing of the cow's milk allergen beta-lactoglobulin

The effect of heat treatment on the IgE binding ability of beta-lactoglobulin, as pure protein or in whole milk, was studied by inhibition of IgE antibody binding using FEIA-CAP inhibition. A slight but significant decreased IgE binding was seen between unheated and heat-treated beta-lactoglobulin solution at 74 degrees C (IC(50) = 2.03 and 3.59 microg/mL, respectively, p = 0.032). A more pronounced decrease was found at 90 degrees C with an IC(50) of 8.45 microg/mL (p = 0.014). The inhibition of IgE binding of milk after heat treatment at 90 degrees C was also significantly decreased (p = 0.007). However, at all heat treatments, a similar total amount of IgE antibodies could be inhibited at a sufficiently high concentration of beta-lactoglobulin. The inhibiting ability of beta-lactoglobulin was significantly impaired in some fermented acidified milk products such as yogurt as compared to that in nonfermented milk (p < 0.001). There was only a small difference of IgE binding between the native forms of genetic variants A and B.

Incorporation of β-lactoglobulin in a lipid/porphyrin monolayer at the air–water interface

A catanionic lipid/porphyrin monolayer was formed at the air-water interface by the tetra-anionic porphyrin, tetra-sodium-meso-tetra(4-sulfonatophenyl)porphine (TSPP), mixed with the cationic lipid dioctadecyldimethylammonium bromide (DODAB) in a 1:4 molar ratio. This binary mixture (TSPP/4DODAB) was used as the incorporation matrix of beta-lactoglobulin (betaLG). Binary and ternary systems (TSPP/4DODAB/zbetaLG, where z stands for the number of protein residues per TSPP) were characterized by surface pressure versus area (pi-A) measurements and by Brewster angle microscopy (BAM) observation at the air-water interface. Pi-A measurements and BAM images show that protein is incorporated in the expanded regime of the monolayer and is gradually expelled upon compression at high surface pressures. The successive compression-expansion cycles indicate that the protein under adsorbed to the floating film is reincorporated after the expansion of the monolayer. At low subphase pH, TSPP tends to aggregate decreasing the interaction with DODAB molecules. Electrostatic and hydrophobic interactions are responsible for the presence of betaLG at the interfacial film.

Formation of intermolecular beta-sheet structures: a phenomenon relevant to protein film structure at oil-water interfaces of emulsions

Oil-in-water emulsions stabilized with beta-lactoglobulin (beta-lg) were made using a homogenizer or a high-speed blender. The protein was studied by Fourier transform infrared (FTIR) spectroscopy in the raw emulsion, in the bulk phase, and at the interface, as a function of pH, oil content, and homogenizing pressure. Results show that the amount of adsorbed protein varies with the available interfacial area. The protein that remains in the aqueous phase exhibit no spectral change, which suggests that homogenization causes no conformational modification or reversible ones. Strong and irreversible changes were observed in the adsorbed protein. Our findings reveal the formation of intermolecular antiparallel beta-sheets upon adsorption due to the protein self-aggregation. As deduced from transmission electronic microscopy, this surface aggregation leads to the formation of continuous and homogeneous membranes coating the globules. The structure of the adsorbed proteins is unaffected by the homogenizing pressures used in our study and slightly modified by the pH. FTIR spectroscopy allows to characterize the type of aggregates formed at the interface. An analysis of the spectra of beta-lg heat-induced gels shows that the aggregates at the interface are very close at a molecular scale to those that constitute particulate gels near the protein's isoelectric point. Since the type of aggregates is similar when the emulsion water phase is pure D(2)O and D(2)O at pD 4.4, the interface not only seems to induce aggregation, but seems to determine the type of aggregation as well. The mechanism that drives the formation of particulate aggregates (rather than fine-stranded ones) may reside in strong protein-protein interactions that are promoted by adverse oil-protein interactions.

Front-face fluorescence spectroscopy study of globular proteins in emulsions: displacement of BSA by a nonionic surfactant

The displacement of a globular protein (bovine serum albumin, BSA) from the surface of oil droplets in concentrated oil-in-water emulsions by a nonionic surfactant (polyoxyethylene sorbitan monolauarate, Tween 20) was studied using front-face fluorescence spectroscopy (FFFS). This method relies on measurement of the change in intensity (I(MAX)) and wavelength (lambda(MAX)) of the maximum in the tryptophan emission spectrum. A series of oil-in-water emulsions (21 wt % n-hexadecane, 0.22 wt % BSA, pH 7.0) containing different molar ratios of Tween 20 to BSA (R = 0-131) were prepared. As the surfactant concentration was increased, the protein was progressively displaced from the droplet surfaces. At R > or = 66, the protein was completely displaced from the droplet surfaces. There was an increase in both I(MAX) and lambda(MAX) with increasing Tween 20 concentration up to R = 66, which correlated with the increase in the ratio of nonadsorbed to adsorbed protein. In contrast, there was a decrease in I(MAX) and lambda(MAX) with Tween 20 concentration in protein solutions and for R > or = 66 in the emulsions, which was attributed to binding of the surfactant to the protein. This study shows that FFFS is a powerful technique for nondestructively providing information about the interfacial composition of droplets in concentrated protein-stabilized emulsions in situ. Nevertheless, in general the suitability of the technique may also depend on protein type and the nature of the physicochemical matrix surrounding the proteins.

Identification of the wheat seed protein CM3 as a highly active emulsifier using a novel functional screen

Lyophilized albumin protein fractions were prepared from flour of four varieties of wheat: Triticum aestivum cvs. Mercia and Riband, Triticum aestivum var. spelta, and Triticum turgidum var. durum (Kamut). The dry powders were redissolved in sodium phosphate buffers at pH 3.0, 6.5, or 8.0 and at ionic strengths of 0.1 or 1.0 M to a concentration of 0.1% (w/v). Emulsions formed by sonication of protein solutions with n-hexadecane were aged at room temperature and separated into aqueous, interstitial, and interfacial phases. The distinct emulsion components were lyophilized and analyzed by RP-HPLC. A protein was observed to be preferentially located in the interfacial component and subsequently purified from a total albumin fraction and identified by N-terminal sequencing as CM3, an alpha-amylase inhibitor subunit. Measurement of the equilibrium surface tension of CM3 as a function of protein concentration demonstrated that it was at least as active as bovine beta-lactoglobulin, an established protein emulsifier. Furthermore, measurement of the surface dilational elastic modulus at an air/water interface demonstrated the formation of a viscoelastic film, while fluorescence and FT-IR spectroscopic measurements on adsorbed and nonadsorbed CM3 suggest that the secondary structure is essentially unchanged upon adsorption to an oil/water interface. It is concluded that functional screening is a valid approach to identify novel protein emulsifiers in complex mixtures.

Influence of sucrose on droplet flocculation in hexadecane oil-in-water emulsions stabilized by beta-lactoglobulin

The influence of sucrose on the flocculation stability of hydrocarbon oil-in-water emulsions stabilized by a globular protein was examined using laser diffraction. Salt (150 mM NaCl) and sucrose (0-40 wt %) were added to n-hexadecane oil-in-water emulsions stabilized by beta-lactoglobulin (beta-Lg, pH 7.0) either before or after isothermal heat treatment (30-95 degrees C for 20 min). When salt was added to emulsions before heat treatment, appreciable droplet flocculation was observed below the thermal denaturation temperature of the adsorbed beta-Lg (T(m) approximately 70 degrees C), and more extensive flocculation was observed above T(m). On the other hand, when salt was added to emulsions after heat treatment, appreciable droplet flocculation still occurred below T(m), but little flocculation was observed above T(m). Addition of sucrose to the emulsions increased T(m) and either promoted or suppressed droplet flocculation depending on whether it was added before or after heat treatment. These results are interpreted in terms of the influence of sucrose on protein conformational stability, protein-protein interactions, and the physiochemical properties of aqueous solutions. This study has important implications for the formulation and production of protein stabilized oil-in-water emulsions.

Impact of protein surface denaturation on droplet flocculation in hexadecane oil-in-water emulsions stabilized by beta-lactoglobulin

The influence of globular protein denaturation after adsorption to the surface of hydrocarbon droplets on flocculation in oil-in-water emulsions was examined. n-Hexadecane oil-in-water emulsions (pH 7.0) stabilized by beta-lactoglobulin (1-wt % beta-Lg) were prepared by high-pressure valve homogenization. NaCl (0-150 mM) was added to these emulsions immediately after homogenization, and the evolution of the mean particle diameter (d) and particle size distribution (PSD) was measured by laser diffraction during storage at 30 degrees C for 48 h. No change in d or PSD was observed in the absence of added salt, which indicated that these emulsions were stable to flocculation. When 150 mM NaCl was added to emulsions immediately after homogenization, d increased rapidly during the following few hours until it reached a plateau value, while the PSD changed from monomodal to bimodal. Addition of N-ethylmaleimide, a sulfhydryl blocking agent, to the emulsions immediately after homogenization prevented (at 20 mM NaCl) or appreciably retarded (at 150 mM NaCl) droplet flocculation. These data suggests that protein unfolding occurred at the droplet interface, which increased the hydrophobic attraction and disulfide bond formation between droplets. In the absence of added salt, the electrostatic repulsion between droplets was sufficient to prevent flocculation, but in the presence of sufficient salt, the attractive interactions dominated, and flocculation occurred.

Temperature-induced beta-aggregation of fibronectin in aqueous solution

Fibronectin structural reorganization induced by temperature has been investigated by Fourier-transform infrared (FT-IR) spectroscopy and light-scattering experiments. At 20 degrees C, from resolution enhanced by FT-IR spectra, 43% of beta sheet, 31% of turn and 26% of unordered structures were estimated. Static and quasi-elastic light-scattering results do not change significantly between 20 and 34 degrees C. Just below 50 degrees C, a decrease of 1/3 of beta sheet structures contents is observed, concomitantly with a corresponding increase of turn. The contribution of disordered structures is found to be temperature-independent. Above 50 degrees C, our data reveals the formation of intermolecular hydrogen bonding leading to the formation of intermolecular beta sheet structures. The IR band absorption at 1618 cm(-1) increases strongly as a function of temperature. The scattered intensity increases and becomes strongly q(2)-dependent. The dynamic structure factor is not a single exponential decay and becomes strongly dependent on the scattering angle. These results demonstrate that aggregation occurs in fibronectin solution. When temperature decreases, this aggregation is found irreversible. Fibronectin aggregation is driven by the formation of intermolecular hydrogen bonds responsible for intermolecular beta sheet structures.

Adsorbed protein secondary and tertiary structures by circular dichroism and infrared spectroscopy with refractive index matched emulsions

The secondary structure of protein adsorbed at the emulsion interface has been studied in refractive index matched emulsions using the techniques of circular dichroism (CD) and Fourier transform infrared spectroscopy. Bovine serum albumin (BSA) and bovine beta-lactoglobulin (betalg) stabilized emulsions were studied, and the refractive index was altered by the addition of glycerol or polyethylene glycol. The effect of additive on the solution and adsorbed protein structure in addition to the effect of adsorption time was considered. Both adsorption and glycerol addition alter protein secondary structure; however, the majority of secondary structure remains. Small changes are observed in the secondary structure of adsorbed protein with time. Near-ultraviolet CD studies showed the effect of glycerol and adsorption on the aromatic groups. BSA showed small changes both upon the addition of glycerol to protein in solution and upon adsorption. betalg showed slightly larger changes upon the addition of glycerol to protein in solution and a larger change upon adsorption.

Interfacial shear rheology of aged and heat-treated beta-lactoglobulin films: displacement by nonionic surfactant

Interfacial shear rheology of adsorbed beta-lactoglobulin films (bulk protein concentration 10(-)(3) wt %) has been studied over the temperature range 20-90 degrees C using a two-dimensional Couette-type viscometer. Effects of the type of interface (air-water, triolein-water, and n-dodecane-water), the pH (2.0, 5.6, 6.0, 7.0, and 9.0), and the extent of the heat treatment have been assessed via measurements of changes in the apparent interfacial shear viscosity and elasticity before and after the addition of increasing amounts of nonionic surfactant Tween 20 (polyoxyethylene sorbitan monolaurate). The highest interfacial viscosities were obtained at the n-dodecane-water interface and the lowest at the air-water interface. Competitive displacement of protein from the interface by Tween 20 was easier at the air-water and n-dodecane-water interfaces as compared to the triolein-water interface. The surface shear viscosity was higher and the displacement by Tween 20 more difficult as the isoelectric point of the protein was approached, which is in agreement with the presence of a more strongly cross-linked protein network at the interface. The effect of heat treatment was dependent on the pH of the aqueous solution. No simple relationship between the surface rheological characteristics and the ease of displacement by Tween 20 could be inferred.

Oil/Alkanethiol Layers for the Study of Emulsified Protein Conformation by Surface Plasmon Resonance Using Monoclonal Antibodies

A method combining surface plasmon resonance and epitope mapping was developed to study the protein conformation at the oil/water interface of an emulsion. The conformation of beta-lactoglobulin stabilizing dodecane/water and miglyol/water interfaces was investigated using five anti-beta-lactoglobulin monoclonal antibodies. The developed method allows us to specifically recognize the emulsified beta-lactoglobulin at the surface of a sensor chip with good repeatability; i.e., standard deviations range between 0.7 and 3.6%. Considering that the monoclonal antibodies, recognizing conformational epitopes, still bind to beta-lactoglobulin at oil/water interfaces, it is concluded that the protein retains a globular conformation. It is shown that the inhibition-binding values of two pairs of Mabs are different for beta-lactoglobulin stabilizing dodecane/water and miglyol/water interfaces. This indicates that the conformations of emulsified beta-lactoglobulin are slightly different according to the nature of the oil phase. Copyright 2000 Academic Press.

Relationships between conformation of beta-lactoglobulin in solution and gel states as revealed by attenuated total reflection Fourier transform infrared spectroscopy

Attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR) has been used to compare the structure of beta-lactoglobulin, the major component of whey proteins, in solution and in its functional gel state. To induce variation in the conformation of beta-lactoglobulin under a set of gelling conditions, the effect of heating temperature, pH, and high pressure homogenization on the conformation sensitive amide I band in the infrared spectra of both solutions and gels has been investigated. The results showed that gelification process has a pronounced effect upon beta-lactoglobulin secondary structure, leading to the formation of intermolecular hydrogen-bonding beta-sheet structure as evidenced by the appearance of a strong band at 1614 cm(-1) at the expense of other regular structures. These results confirm that this structure may be essential for the formation of a gel network as it was previously shown for other globular proteins. However, this study reveals, for the first time, that there is a close relationship between conformation of beta-lactoglobulin in solution and its capacity to form a gel. Indeed, it is shown that conditions which promote predominance of intermolecular beta-sheet in solution such as pH 4, prevent the formation of gel in conditions used by increasing thermal stability of beta-lactoglobulin. On the basis of these findings, it is suggested that by controlling the extent of intermolecular beta-structure of the protein in solution, it is possible to modify the ability of protein to form a gel and as a consequence to control the properties of gels.

The adsorption-induced secondary structure of beta-casein and of distinct parts of its sequence in relation to foam and emulsion properties

Changes in the secondary structure upon adsorption of beta-casein (betaCN) and of distinct parts of its sequence were investigated by far-ultraviolet circular dichroism in order to find suggested relationships with foam and emulsion-forming and -stabilising properties of the same protein/peptides. A teflon/water interface was used as a model system for foam and emulsion interfaces. The maximum surface loads of beta-casein and its derived peptides were investigated. The main secondary structure element of all samples in solution was the unordered random coil, but upon adsorption ordered structure, especially alpha-helix, was induced. At lower pH more ordered structure was induced, just as at lower ionic strength. Apparently, both hydrophobic and hydrophilic groups influence the change of secondary structure induced at a hydrophobic interface. The results suggest that the hydrophobic C-terminal half of betaCN accounted for the high maximum surface load on teflon, while the N-terminal half of betaCN seemed to be responsible for the secondary structure induction upon adsorption. A relation between the maximum surface load and the foam-stabilising properties was found, but an influence of the secondary structure properties on the foam and emulsion-forming and -stabilising properties was not observed.