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Effect of heat treatment on the circular dichroism spectra of bovine beta-lactoglobulin A, B, and C.

Dilute solutions of beta-lactoglobulin (beta-Lg) A, B, and C were heated in phosphate buffer at temperatures between 40 and 94 degrees C for 10 min, cooled, and analyzed using near-UV and far-UV circular dichroism (CD). The decrease in near-UV CD intensity at 293 nm (Deltaepsilon(293)) could be analyzed in terms of a two-state model, and the stability was beta-Lg C > beta-Lg A > beta-Lg B on the basis of the midpoint temperatures for samples heated at pH 6.7 and 7.4. However, the slopes of the curves at the midpoint temperature for variant A were generally less than those for beta-Lg B and beta-Lg C, indicating that the substitution of Val (beta-Lg A) for Ala (beta-Lg B or beta-Lg C) at position 118 had altered the entropic contribution to unfolding of the protein. The changes in CD at 270 nm (Deltaepsilon(270)), an index of significant alteration to disulfide bond dihedral angles, occurred at higher temperatures than those for the Deltaepsilon(293) results. The far-UV CD showed some small changes as a consequence of heat treatment, and the shifts at 205 nm ([theta](205)) fitted a two-state model. Plotting the changes in both Deltaepsilon(293) and [theta](205) against the loss of nativelike and sodium dodecyl sulfate-monomeric protein (assessed by polyacrylamide gel electrophoresis) showed a strong 1:1 relationship between Deltaepsilon(293) or [theta](205) and the loss of nativelike beta-Lg. These results indicated that the initial irreversible stage in the heat-induced aggregation of beta-Lg (nativelike monomer to unfolded monomer) altered the chirality of the environment of Trp(19) and modified the secondary structure of beta-Lg slightly. The differences in the behavior of variants A-C were explicable on the basis of generalized electrostatic and hydrophobicity effects as well as specific amino acid effects.

Animals↗

Interaction of alkylsulfonate ligands with beta-lactoglobulin AB from bovine milk.

The interaction of alkyl sulfonate ligands (AL) with bovine beta-lactoglobulin AB was measured using Trp fluorescence enhancement. One binding site per protein molecule was observed. The location of this site was related with the dimer formation and could be coincident with the fatty acids and SDS binding site. The apparent binding constants for AL were in the range of 10(-)(6) M, at pH 6.8. At pH 3.0 no binding was observed by this fluorescence method. The strength of the interaction was decreasing in the following way: AL16 > AL12 > AL14 >> AL10. Other sites on the monomer were evidenced by the protective action of the AL toward the urea unfolding of the protein.

Animals↗

Effect of pH on retention of aroma compounds by beta-lactoglobulin.

Interactions between volatile compounds and BLG in aqueous solution were studied using static and dynamic headspace techniques (exponential dilution). The intensity of interactions between methyl ketones (C7-C9), ethyl esters (C6-C9), limonene, myrcene, and beta-lactoglobulin (BLG) were estimated by determination of the relative infinite dilution activity coefficients (gamma(r)). For a constant pH value, the methyl ketones retention by BLG increased significantly with the hydrophobicity of the volatiles, whereas the retention reached a maximum for ethyl octanoate in the ester series, indicating a possible steric hindrance. For limonene and myrcene an unexpected increase in headspace concentration or "salting out" effect was noticed for acid pH. The variations of the retention according to the pH increase of the medium from pH 3 to pH 11 could be related to structural modifications of the BLG. The retention increase observed between pH 3 and pH 9 resulted from the flexibility modification of the protein, allowing better accessibility to the primary or the secondary hydrophobic sites, whereas the dramatic decrease observed at pH 11 was the consequence of the alkaline denaturation of BLG. Electrostatic interactions occurring at pH 7.5 could also explain the observed retention increase.

Acyclic Monoterpenes↗

Phospholipid/fatty acid-induced secondary structural change in beta-lactoglobulin during heat-induced gelation.

Effects of phosphatidylcholine (PC) and the predominant fatty acids (FAs) in milk, butyrate, oleate, and palmitate, on secondary structural changes in beta-lactoglobulin (beta-LG) during heat-induced gelation were analyzed on the basis of circular dichroism (CD) spectra. Small-strain oscillatory measurements were carried out to characterize viscoelastic properties of the heat-induced gels. In the absence of added salt, PC and FAs induced helix formation of beta-LG on heating to 80 degrees C and increased the storage moduli (G') of heat-induced gels. In the presence of 500 mM NaCl, PC did not change the CD spectrum of beta-LG but decreased G'. In contrast, butyrate substantially unfolded beta-LG in 500 mM NaCl on heating, forming very elastic gels with increased G' values. Palmitate and oleate induced beta-LG gel formation at 25 degrees C without heating; heating to 80 degrees C almost completely unfolded beta-LG in 500 mM NaCl.

Fatty Acids↗

Effect of heat treatment on bovine beta-lactoglobulin A, B, and C explored using thiol availability and fluorescence.

Dilute solutions of beta-lactoglobulin (beta-Lg) A, B, and C were heated at temperatures between about 40 and 94 degrees C for 10 min, cooled, and analyzed using Trp fluorescence and extrinsic fluorescence spectra of the probe 1,8-anilinonaphthalene sulfonate (ANS). Thiol availabilities using 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) were determined using a separate set of samples. The normalized ANS fluorescence emission intensity and the thiol availability results showed a 1:1 relationship with the loss of nativelike but not SDS-monomeric protein, as determined by PAGE analysis. The normalized Trp emission intensity results did not show a comparable 1:1 relationship with the loss of nativelike protein, indicating that the Trp intensity arose from consequential disulfide bond reorganization and not the initial unfolding reaction. The results were also analyzed in terms of two-state models, and the midpoint temperatures (T(mid)) for the proteins were generally beta-Lg C > beta-Lg A > beta-Lg B, and the slopes at the midpoint temperatures for the A variant were generally less than those for the B and C variants indicating that beta-Lg A may denature by a different mechanism from that of beta-Lg B or beta-Lg C. The T(mid) parameters derived from the ANS fluorescence intensity results were similar to those for thiol availability and both were lower than the T(mid) values for Trp emission intensity showing that creation of an ANS binding site on a beta-Lg molecule was linked to the irreversible exposure of a thiol group and the loss of native beta-Lg but preceded the decrease in Trp(61) fluorescence quenching. These results for the differences between the behavior of the A and B or the C variants involved the creation of a destabilizing cavity by the Val(118)Ala (A --> B) substitution and the changed charge distribution within the CD loop caused by the Asp(64)Gly (A --> B) substitution.

Animals↗

Identification of peptides in aggregates formed during hydrolysis of beta-lactoglobulin B with a Glu and Asp specific microbial protease.

The purpose of the present study was to identify the peptides responsible for aggregate formation during hydrolysis of beta-lactoglobulin by BLP at neutral pH. Hydrolysates taken at various stages of aggregate formation were separated into a precipitate and a soluble phase and each was analyzed by CE and mass spectrometry. The aggregates consisted of six to seven major peptides of which four were tentatively identified. The peptides were positively charged at neutral pH and had a high charge-to-mass ratio at low pH. The fragment f135-158 seemed to be the initiator of aggregation, since it was present at high concentration in the aggregates at all stages, and the concentration of this peptide remained low in the supernatant. F135-158 contains several basic and acid amino acids alternating with hydrophobic amino acids, which is in accordance with formation of noncovalently linked aggregates, as previously shown.

Amino Acid Sequence↗

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.

Hot Temperature↗

Functional changes in beta-lactoglobulin by conjugation with cationic saccharides.

Bovine beta-lactoglobulin (beta-LG) was conjugated to each of three cationic saccharides [glucosamine (GlcN), chitopentaose (CPO), and chitosan (CHS)] by means of a water-soluble carbodiimide or by the Maillard reaction in an effort to improve the functional properties of beta-LG. The molar ratios of beta-LG to the cationic saccharide in the beta-LG-GlcN, beta-LG-CPO, and beta-LG-CHS conjugates were 2:1, 2:5, and 2:1, respectively. Fluorescence studies indicated that the conformation around Trp had changed in each conjugate and that the surface of each of the conjugates was covered with a saccharide chain. Structural analysis using monoclonal antibodies indicated that the conformation around (15)Val-(29)Ile (beta-sheet region) in beta-LG-GlcN and beta-LG-CPO had changed but that in beta-LG-CHS was maintained, whereas the conformation around (125)Thr-(135)Lys (alpha-helix region) in the conjugates had changed. The emulsifying activity of beta-LG was improved by conjugation with CPO or CHS, and aggregation of beta-LG was suppressed by conjugation with CHS. Reduction of the antigenicity and immunogenicity of beta-LG was achieved by conjugation with CHS.

Animals↗

Effect of milk concentration on the irreversible thermal denaturation and disulfide aggregation of beta-lactoglobulin.

The kinetics of beta-lactoglobulin (beta-LG) denaturation in reconstituted skim milk samples of various concentrations (9.6-38.4% total solids) over a wide temperature range (75-100 degrees C) was studied. The thermal denaturation of beta-LG had a reaction order of 1.5 at all milk solids concentrations and at all temperatures. The rate of denaturation of beta-LG was markedly dependent on the milk solids concentration and the heating temperature. At 75 degrees C, the thermal denaturation of beta-LG was retarded at higher milk solids concentrations. However, this retardation was less pronounced at higher temperatures so that a similar rate of denaturation was observed at all milk solids concentrations at 100 degrees C. From an examination of the level of disulfide-aggregated beta-LG, it was evident that most, but not all, of the denatured beta-LG was involved in disulfide-aggregated complexes, either with other denatured whey proteins or with the casein micelles. As with beta-LG denaturation, the rate of disulfide aggregation of beta-LG was markedly dependent on the milk solids concentration.

Animals↗

Heat treatment of beta-lactoglobulin: structural changes studied by partitioning and fluorescence.

Functional properties of whey protein concentrates (WPC) are primarily dependent on the degree of denaturation of beta-lactoglobulin (beta-LG), the major globular whey protein. Irreversible modifications in the tertiary structure and association state of beta-LG after heat treatment were studied by partition in aqueous two-phase systems and fluorescence quenching. Partitioning of preheated beta-LG in two-phase systems containing 5% (w/w) poly(ethylene glycol) and 7% (w/w) dextran, between pH 6.0 and7.0, are appropriately related with the intensity of heat treatment. An increase in the partition coefficient of beta-LG was observed with increasing temperature of heat treatment. On the other hand, fluorescence quenching of beta-LG by acrylamide was used to study the conformational flexibility of the protein at pH values between 4. 0 and 9.0. The values of bimolecular quenching rate constant (k(q)) obtained showed that beta-LG appears to be more flexible at high pH values, while at low pH the protein assumes a more compact form. The efficiency of acrylamide quenching on preheated beta-LG was substantially more pronounced than for the untreated protein. This difference can be ascribed to the presence of unfolded monomers and aggregates of denatured molecules formed after heat treatment, whose tryptophanyl residues are more exposed to the solvent. In conclusion, the results suggest that partition studies in aqueous two-phase systems and fluorescence quenching are very useful tools to detect changes in conformation and aggregation of beta-LG induced by heat treatment.

Fluorescence↗

Surface dilational rheology of mixed beta-lactoglobulin/surfactant layers at the air/water interface.

The general theoretical model by Garrett and Joos proposed in 1976 for the estimation of the dilational elasticity of mixed surfactant solutions, and also the theoretical model proposed by Joos for the limiting elasticity of such mixtures, demonstrate quite satisfactory agreement with experimental results obtained from the oscillating bubble shape method for mixtures of a nonionic surfactant and a protein, that is, beta-lactoglobuline and decyl dimethyl phosphine oxide, C10DMPO.

Air↗

Alpha-helix formation in melittin and beta-lactoglobulin A induced by fluorinated dialcohols.

Extensive study of the effect of fluorinated alcohols on protein conformations, notably the induction of alpha-helix formation, is important because of its wide range of applications. Circular dichroism (CD) was used to show that the enhancement of helix induction in beta-lactoglobulin A and melittin by the fluorinated diols 2,2,3,3-tetrafluoro-1,4-butanediol (TFBD), 2,2,3,3,4,4-hexafluoro-1,6-pentanediol (HFPD), and 2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol (OFHD) increases in the order TFBD < HFPD < OFHD. For fluorinated diols and monoalcohols the effectiveness of helix induction was found to increase exponentially with increasing number of fluorine atoms per alcohol molecule, and OFHD was found to be more effective than any previously reported fluorinated alcohol. Formation of standard micelles was ruled out as the cause of the enhanced helix induction by the fluorinated diols. The negligible red-edge excitation shift in the fluorescence of melittin indicated that the fluorinated diol/water solvent shell surrounding the tryptophan chromophore is less immobilized than are molecules in a lamellar vesicle.

Alcohols↗

Effects of electrolyte concentration and pH on the coalescence stability of beta-lactoglobulin emulsions: experiment and interpretation.

Experimental results are presented about the effects of ionic strength and pH on the mean drop-size after emulsification and on the coalescence stability of emulsions, stabilized by a globular protein beta-lactoglobulin (BLG). The mean drop-size is determined by optical microscopy, whereas the coalescence stability is characterized by centrifugation. In parallel experiments, the zeta-potential and protein adsorption on drop surface are determined. The experiments are performed at two different BLG concentrations, 0.02 and 0.1 wt%. The electrolyte concentration in the aqueous phase, C(EL), is varied between 1.5 mM and 1 M, and pH is varied between 4.0 and 7.0. The experiments show that the mean drop-size after emulsification depends slightly on C(EL), at fixed protein concentration and natural pH = 6.2. When pH is varied, the mean drop-size passes through a maximum at fixed protein and electrolyte concentrations. A monolayer protein adsorption is registered in the studied ranges of C(EL) and pH at low BLG concentration of 0.02 wt%. In contrast, a protein multilayer is formed at higher BLG concentration, 0.1 wt%, above a certain electrolyte concentration (C(EL) > 100 mM, natural pH). The experimental results for the emulsion coalescence stability are analyzed by considering the surface forces acting between the emulsion drops. The electrostatic, van der Waals, and steric interactions are taken into account to calculate the barriers in the disjoining pressure isotherm at the various experimental conditions studied. The comparison of the theoretically calculated and the experimentally determined coalescence barriers shows that three qualitatively different cases can be distinguished. (1) Electrostatically stabilized emulsions, with monolayer protein adsorption, whose stability can be described by the DLVO theory. (2) Sterically stabilized emulsions, in which the drop-drop repulsion is created mainly by overlapping protein adsorption multilayers. A simple theoretical model is shown to describe emulsion stability in these systems. (3) Sterically stabilized emulsions with a monolayer adsorption on drop surface.

Adsorption↗

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.

Alkanes↗

Influence of free protein on flocculation stability of beta-lactoglobulin stabilized oil-in-water emulsions at neutral pH and ambient temperature.

The influence of protein concentration and order of addition relative to homogenization (before or after) on the extent of droplet flocculation in oil-in-water emulsions stabilized by a globular protein was 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)10 mg/mL beta-Lg added before homogenization; (3) 4 mg/mL beta-Lg added before homogenization and 6 mg/mL beta-Lg added after homogenization. Emulsion 1 contained little nonadsorbed protein (<3%) and underwent extremely rapid and extensive droplet flocculation immediately after homogenization. Emulsion 2 contained a significant fraction of nonadsorbed beta-Lg and exhibited relatively slow droplet flocculation for some hours after homogenization. Measurements on Emulsion 3 showed that the extremely rapid particle growth observed in Emulsion 1 could be arrested by adding native beta-Lg immediately after homogenization. The extent of particle growth in the three types of emulsions was highly dependent on the time that the salt was added to the emulsions, i.e., after 0 or 24 h aging. We postulate that the observed differences are due to changes in droplet surface hydrophobicity caused by differences in the packing or conformation of adsorbed proteins. Our data suggest that history effects have a strong influence on the flocculation stability of protein-stabilized emulsions, which has important implications for the formulation and production of protein stabilized oil-in-water emulsions.

Adsorption↗

pH-Induced structural transitions during complexation and coacervation of beta-lactoglobulin and acacia gum.

pH-Induced structural changes during complex coacervation between beta-lactoglobulin (BLG) and Acacia gum (AG) in aqueous solutions were determined by coupling slow in situ acidification of BLG/AG mixed dispersions and different experimental methods. The combined signal evolution of dynamic light scattering at 90 degrees scattering angle (I(90)), electrophoretic mobility, turbidimetry (tau), circular dichroism, and phase contrast microscopy allowed the distinction of critical structural transitions and the definition of their corresponding pH. The formation of soluble BLG/AG complexes was initiated at pH(sc) (4.90), since I(90) and tau significantly increased from the baseline. In parallel or just following complexation, a conformational change of BLG was detected at pH(pct) (4.8). An increase in positive charge density of BLG induced complex aggregation at pH(ca) (4.7). More efficient charge neutralization of aggregated complexes, especially through the lowering of the number of AG negative charges, promoted initiation of phase separation at pH(psi) (4.4). Mixed dispersions became unstable and phase separation occurred at pH(ps) (4.2). The phase separation of mixed dispersions was suggested by the maximum value of scattered light, by an important acceleration of the dispersion turbidity, by a strong increase of hydrodynamic radii, and by the first appearance of light fluctuations as observed by phase contrast microscopy. At the microscopic level, the first coacervates were observed at pH(coa) (4.0), near the pH of the maximum of turbidity. It was also noticed that, from the onset of interactions between biopolymers, the pH decrease led to (i) a gradual homogenization of particle size in the mixed dispersion as suggested by the decrease of dispersion polydispersity and (ii) conformational transitions of the protein (a loss of alpha-helix structure at pH(pct) and a gain in protein secondary structure near pH(coa), probably involving beta-sheet components).

Acacia↗

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.

Adsorption↗

Modification of beta-lactoglobulin by oligofructose: impact on protein adsorption at the air-water interface.

Maillard products of beta-lactoglobulin (betaLg) and fructose oligosaccharide (FOS) were obtained in different degrees of modification depending on incubation time and pH. By use of a variety of biochemical and spectroscopic tools, it was demonstrated that the modification at limited degrees does not significantly affect the secondary, tertiary, and quaternary structure of betaLg. The consequence of the modification on the thermodynamics of the protein was studied using differential scanning calorimetry, circular dichroism, and by monitoring the fluorescence intensity of protein samples with different concentrations of guanidine-HCl. The modification leads to lowering of the denaturation temperature by 5 degrees C and a reduction of the free energy of stabilization of about 30%. Ellipsometry and drop tensiometry demonstrated that upon adsorption to air-water interfaces in equilibrium modified betaLg exerts a lower surface pressure than native betaLg (16 versus 22 mN/m). Moreover, the surface elastic modulus increased with increasing surface pressure but reached significantly smaller values in the case of FOS-betaLg. Compared to native betaLg, modification of the protein with oligofructose moieties results in higher surface loads and thicker surface layers. The consequences of these altered surface rheological properties are discussed in view of the functional behavior in technological applications.

Adsorption↗