Effects of High Intensity Ultrasound on Physiochemical and Structural Properties of Goat Milk β-Lactoglobulin

This study aimed to compare the effects of high intensity ultrasound (HIU) applied at various amplitudes (20~40%) and for different durations (1~10 min) on the physiochemical and structural properties of goat milk β-lactoglobulin. No significant change was observed in the protein electrophoretic patterns by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Deconvolution and second derivative of the Fourier transform infrared spectra (FTIR) showed that the percentage of β-sheet of goat milk β-lactoglobulin was significantly decreased while those of α-helix and random coils increased after HIU treatment The surface hydrophobicity index and intrinsic fluorescence intensity of samples was enhanced and increased with increasing HIU amplitude or time. Differential scanning calorimetry (DSC) results exhibited that HIU treatments improved the thermal stability of goat milk β-lactoglobulin. Transmission electron microscopy (TEM) of samples showed that the goat milk β-lactoglobulin microstructure had changed and it contained larger aggregates when compared with the untreated goat milk β-lactoglobulin sample. Data suggested that HIU treatments resulted in secondary and tertiary structural changes of goat milk β-lactoglobulin and improved its thermal stability.

Confinement-induced liquid crystalline transitions in amyloid fibril cholesteric tactoids

Chirality is ubiquitous in nature and plays crucial roles in biology, medicine, physics and materials science. Understanding and controlling chirality is therefore an important research challenge with broad implications. Unlike other chiral colloids, such as nanocellulose or filamentous viruses, amyloid fibrils form nematic phases but appear to miss their twisted form, the cholesteric or chiral nematic phases, despite a well-defined chirality at the single fibril level. Here we report the discovery of cholesteric phases in amyloids, using β-lactoglobulin fibrils shortened by shear stresses. The physical behaviour of these new cholesteric materials exhibits unprecedented structural complexity, with confinement-driven ordering transitions between at least three types of nematic and cholesteric tactoids. We use energy functional theory to rationalize these results and observe a chirality inversion from the left-handed amyloids to right-handed cholesteric droplets. These findings deepen our understanding of cholesteric phases, advancing their use in soft nanotechnology, nanomaterial templating and self-assembly.

Modulating materials by orthogonally oriented β-strands: composites of amyloid and silk fibroin fibrils

Amyloid fibrils and silk fibroin (SF) fibrils are proteinaceous aggregates occurring either naturally or as artificially reconstituted fibrous systems, in which the constituent β-strands are aligned either orthogonally or parallel to the fibril main axis, conferring complementary physical properties. Here, it is shown how the combination of these two classes of protein fibrils with orthogonally oriented β-strands results in composite materials with controllable physical properties at the molecular, mesoscopic, and continuum length scales.

Optimizing the selective recognition of protein isoforms through tuning of nanoparticle hydrophobicity

We demonstrate that ligand hydrophobicity can be used to increase affinity and selectivity of binding between monolayer-protected cationic gold nanoparticles and β-lactoglobulin protein isoforms containing two amino acid mutations.

Systematic Analysis of Aggregates from 38 Kinds of Non Disease-Related Proteins: Identifying the Intrinsic Propensity of Polypeptides to Form Amyloid Fibrils

The ability to form amyloid fibrils from a wide range of proteins would open up the opportunity to augment studies of the molecular basis of amyloid fibril formation. We investigated 36 different conditions with respect to four model proteins to evaluate their ability to form amyloid fibrils. In a 5% ethanol solution at pH 2 at 57 degrees C, hen egg white lysozyme, bovine beta-lactoglobulin, and bovine trypsinogen formed mature-type fibrils, while only histone H2A formed immature-type fibrils. Under these conditions, 25 of the 38 proteins formed amyloid fibrils. In addition, three additional proteins formed fibrils in a solution containing 5% trifluoroethanol instead of 5% ethanol. In summary, a total 28 proteins formed amyloid fibrils. Under these extreme conditions, chemical fragmentation was observed. Destabilization of the native structure, strengthening of hydrogen bonds, and chemical fragmentation are thought to play important roles in the formation of amyloid fibrils.

On the crucial importance of the pH for the formation and self-stabilization of protein microgels and strands

Stable suspensions of protein microgels are formed by heating salt-free β-lactoglobulin solutions at concentrations up to about C = 50 g·L(-1) if the pH is set within a narrow range between 5.75 and 6.1. The internal protein concentration of these spherical particles is about 150 g·L(-1) and the average hydrodynamic radius decreases with increasing pH from 200 to 75 nm. The formation of the microgels leads to an increase of the pH, which is a necessary condition to obtain stable suspensions. The spontaneous increase of the pH during microgel formation leads to an increase of their surface charge density and inhibits secondary aggregation. This self-stabilization mechanism is not sufficient if the initial pH is below 5.75 in which case secondary aggregation leads to precipitation. Microgels are no longer formed above a critical initial pH, but instead short, curved protein strands are obtained with a hydrodynamic radius of about 15-20 nm.

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.

Native beta-lactoglobulin self-assembles into a hexagonal columnar phase on a solid surface

Using electron scanning microscopy, we have studied the protein deposit left on silicon and mica substrates by dried droplets of aqueous solutions of bovine beta-lactoglobulin at low concentration and pH = 2-7. We have observed different self-assembled structures: homogeneous layers, hexagonal platelets and flower-shaped patterns laying flat on the surface, and rods formed by columns. Homogeneous layers covered the largest area of the droplet deposit. The other structures were found in small isolated regions, where the protein solution dried in the form of microdroplets. The presence of hexagonal platelets, flower-shaped patterns and columnar rods shows that beta-lactoglobulin self-assembles at the surface in a hexagonal columnar phase, which has never been observed in solution. A comparison with proteins showing similar aggregates suggests that beta-lactoglobulin structures grow from hexagonal germs composed of discotic nanometric building blocks, possibly possessing an octameric structure. We propose that discotic building blocks of beta-lactoglobulin may be produced by the anisotropic interaction with the solid surface.

The effect of physiological conditions on the surface structure of proteins: setting the scene for human digestion of emulsions

Understanding and manipulating the interfacial mechanisms that control human digestion of food emulsions is a crucial step towards improved control of dietary intake. This article reports initial studies on the effects of the physiological conditions within the stomach on the properties of the film formed by the milk protein (β-lactoglobulin) at the air-water interface. Atomic force microscopy (AFM), surface tension and surface rheology techniques were used to visualize and examine the effect of gastric conditions on the network structure. The effects of changes in temperature, pH and ionic strength on a preformed interfacial structure were characterized in order to simulate the actual digestion process. Changes in ionic strength had little effect on the surface properties. In isolation, acidification reduced both the dilatational and the surface shear modulus, mainly due to strong repulsive electrostatic interactions within the surface layer and raising the temperature to body temperature accelerated the rearrangements within the surface layer, resulting in a decrease of the dilatational response and an increase of surface pressure. Together pH and temperature display an unexpected synergism, independent of the ionic strength. Thus, exposure of a pre-formed interfacial β-lactoglobulin film to simulated gastric conditions reduced the surface dilatational modulus and surface shear moduli. This is attributed to a weakening of the surface network in which the surface rearrangements of the protein prior to exposure to gastric conditions might play a crucial role.

Amyloid fibril-like structure underlies the aggregate structure across the pH range for beta-lactoglobulin

The protein beta-lactoglobulin aggregates into two apparently distinct forms under different conditions: amyloid fibrils at pH values away from the isoelectric point, and spherical aggregates near it. To understand this apparent dichotomy in behavior, we studied the internal structure of the spherical aggregates by employing a range of biophysical approaches. Fourier transform infrared studies show the aggregates have a high beta-sheet content that is distinct from the native beta-lactoglobulin structure. The structures also bind the amyloidophilic dye thioflavin-T, and wide-angle x-ray diffraction showed reflections corresponding to spacings typically observed for amyloid fibrils composed of beta-lactoglobulin. Combined with small-angle x-ray scattering data indicating the presence of one-dimensional linear aggregates at the molecular level, these findings indicate strongly that the aggregates contain amyloid-like substructure. Incubation of beta-lactoglobulin at pH values increasingly removed from the isoelectric point resulted in the increasing appearance of fibrillar species, rather than spherical species shown by electron microscopy. Taken together, these results suggest that amyloid-like beta-sheet structures underlie protein aggregation over a much broader range of conditions than previously believed. Furthermore, the results suggest that there is a continuum of beta-sheet structure of varying regularity underlying the aggregate morphology, from very regular amyloid fibrils at high charge to short stretches of amyloid-like fibrils that associate together randomly to form spherical particles at low net charge.

Preparation of sub-100-nm β-lactoglobulin (BLG) nanoparticles

Sub-100-nm nanoparticles were prepared from beta-lactoglobulin (BLG) with a narrow size distribution by a desolvation method using glutaraldehyde for cross-linking. With pre-heating of the BLG solution to 60 degrees C and subsequent pH readjustment to 9.0, nanoparticles of 59 +/- 5 nm were obtained with improved uniformity. Bovine serum albumin (BSA) nanoparticles, prepared under similar conditions for comparison, were larger and less uniform. The half-width of 80% particle distribution was used to compare the uniformity of particle size distribution. The stability of the nanoparticles was investigated by degradation tests at neutral and acidic pHs with and without proteolytic enzymes, trypsin and pepsin. The degradation time, determined by a graphical approach, was used to compare the relative stabilities of BLG and BSA nanoparticles. The particles of BLG were more stable than those of BSA in acidic and neutral media with and without added enzymes.

Interfacial characterization of beta-lactoglobulin networks: displacement by bile salts

The competitive displacement of a model protein (beta-lactoglobulin) by bile salts from air-water and oil-water interfaces is investigated in vitro under model duodenal digestion conditions. The aim is to understand this process so that interfaces can be designed to control lipid digestion thus improving the nutritional impact of foods. Duodenal digestion has been simulated using a simplified biological system and the protein displacement process monitored by interfacial measurements and atomic force microscopy (AFM). First, the properties of beta-lactoglobulin adsorbed layers at the air-water and the olive oil-water interfaces were analyzed by interfacial tension techniques under physiological conditions (pH 7, 0.15 M NaCl, 10 mM CaCl2, 37 degrees C). The protein film had a lower dilatational modulus (hence formed a weaker network) at the olive oil-water interface compared to the air-water interface. Addition of bile salt (BS) severely decreased the dilatational modulus of the adsorbed beta-lactoglobulin film at both the air-water and olive oil-water interfaces. The data suggest that the bile salts penetrate into, weaken, and break up the interfacial beta-lactoglobulin networks. AFM images of the displacement of spread beta-lactoglobulin at the air-water and the olive oil-water interfaces suggest that displacement occurs via an orogenic mechanism and that the bile salts can almost completely displace the intact protein network under duodenal conditions. Although the bile salts are ionic, the ionic strength is sufficiently high to screen the charge allowing surfactant domain nucleation and growth to occur resulting in displacement. The morphology of the protein networks during displacement is different from those found when conventional surfactants were used, suggesting that the molecular structure of the surfactant is important for the displacement process. The studies also suggest that the nature of the oil phase is important in controlling protein unfolding and interaction at the interface. This in turn affects the strength of the protein network and the ability to resist displacement by surfactants.

Theoretical modeling of the kinetics of fibrilar aggregation of bovine beta-lactoglobulin at pH 2

The authors propose a kinetic model for the heat-induced fibrilar aggregation of bovine beta-lactoglobulin at pH 2.0. The model involves a nucleation step and a simple addition reaction for the growth of the fibrils, as well as a side reaction leading to the irreversible denaturation and inactivation of a part of the protein molecules. For the early stages of the aggregation reaction, the authors obtain an analytical solution of the model. In agreement with their experimental results, the model predicts a critical protein concentration below where almost no fibrils are formed. The model agrees well with their experimental data from in situ light scattering. By fitting the experimental data with the model, the authors obtain the ionic strength dependent kinetic rate constants for beta-lactoglobulin fibrilar aggregation and the size of the critical nucleus.

Microstructure of β-Lactoglobulin/Pectin Coacervates Studied by Small-Angle Neutron Scattering

Small-angle neutron scattering (SANS) has been used to investigate the microstructure of beta-lactoglobulin/pectin coacervates prepared by different initial protein/polysaccharide weight ratio (r), sodium chloride concentration (C(NaCl)), and pectin charge density. The higher r and higher pectin charge density lead to higher scattering intensity at small q range (0.007 Angstrom(-1) < q < 0.02 Angstrom(-1)), suggesting that the charges of pectin chains are screened significantly by the binding of oppositely charged protein molecules, leading to a tighter aggregation of pectin chains. On the other hand, the appearance of a shoulder peak at intermediate q range (0.04 Angstrom(-1) < q < 0.2 Angstrom(-1)) is used to interpret the formation of protein domains in beta-lactoglobulin/pectin coacervates. At C(NaCl) = 0.1 M, the coacervate of beta-lactoglobulin and pectin A does not show a shoulder peak at intermediate q range at r = 10:1, suggesting that protein molecules are separately bound on pectin chains. However, a shoulder peak appears at intermediate q range at r = 20:1 and 30:1, and the average protein domain size estimated from the shoulder peak position is 7.2 and 8.5 nm, respectively, for these two coacervates. When C(NaCl) increases from 0.05 to 0.2 M, the shoulder peak shifts toward smaller q and becomes broader, indicating that the addition of a higher amount of salt leads to a more heterogeneous coacervate structure. Pectin B with a lower linear charge density favors the formation of larger protein domains. The formation of protein domains in beta-lactoglobulin/pectin coacervates is partially ascribed to the self-aggregation of beta-lactoglobulin molecules. Two kinds of microstructures of beta-lactoglobulin/pectin coacervates with and without observable protein domains have been proposed.

Comparison of bovine and porcine beta-lactoglobulin: a mass spectrometric analysis

Nano-electrospray-ionization mass spectrometry (nano-ESI-MS) is applied to comparison of bovine and porcine beta-lactoglobulin (BLG and PLG). The conformational and oligomeric properties of the two proteins under different solvent and experimental conditions are analyzed. The pH-dependence of dimerization is described for the pH range 2-11. The results indicate maximal dimer accumulation at pH 6 for BLG and pH 4 for PLG, as well as a lower stability of the PLG dimer at pH 4 compared to BLG at pH 6. Conformational stability appears to be higher for BLG at acidic pH, but higher for PLG at basic pH. The higher stability of BLG at low pH is revealed by means of either chemical or thermal denaturation. Equilibrium folding intermediates of both proteins are detected. Finally, conditions are found that promote dissociation of the BLG dimer at pH 6 into folded monomers.

Microstructure of β-lactoglobulin-stabilized emulsions containing non-ionic surfactant and excess free protein: Influence of heating

The influence of the non-ionic surfactant Tween 20 on the microstructure of beta-lactoglobulin-stabilized emulsions with substantial excess free protein present was investigated via confocal microscopy. The separate distributions of oil droplets and protein were determined using two different fluorescent dyes. In the emulsion at ambient temperature the excess protein and protein-coated oil droplets were associated together in a reversibly flocculated state. The pore-size distribution of the initial flocculated emulsion was found to depend on the surfactant/protein ratio R, and at higher values of R the system became more inhomogeneous due to areas of local phase separation. Evidence for competitive displacement of protein from the oil-water interface by surfactant was obtained only on heating (from 25 to 85 degrees C) during the process of formation of a heat-set emulsion gel. By measuring fluorescence intensities of the protein dye inside and outside of the oil-droplet-rich areas, we have been able to quantify the evolving protein distribution during the thermal processing. The results are discussed in relation to previous work on the competitive adsorption of proteins and surfactants in emulsions and the effect of emulsion droplets on the rheology of heat-set protein gels.

Aggregation across the length-scales in β-lactoglobulin

The protein beta-lactoglobulin (BLG) has been widely studied, in large part because of its importance to the food industry. Following denaturation during heating, under different conditions of pH it has been found to form either particulate (around the isoelectric point at pH 5.1) or fibrillar gels. The nature of the fibrils has recently been suggested to be the same as that identified with amyloid fibrils known for a wide-range of different proteins and implicated in many disease states. We confirm that the BLG fibrils show all the classical signatures of amyloid fibrils. In addition, the fibrils are capable themselves of aggregating further to form large-scale (many microns in size) spherulites. Polarized light microscopy and Environmental scanning electron microscopy (ESEM) have been used to explore the internal structure of these spherulites under conditions in which the solvent has not been dried off. The factors which determine whether or not the spherulites form have also been considered, together with implications for other amyloid-containing systems.

Gelation of chicken pectoralis major myosin and heat-denatured beta-lactoglobulin

Thermal, rheological, and microstructural properties of myosin (1 and 2% protein) were compared to mixtures of 1% myosin and 1% heat-denatured beta-lactoglobulin aggregates (myosin/HDLG) and 1% myosin and 1% native beta-lactoglobulin (myosin/beta-LG) in 0.6 M NaCl and 0.05 M sodium phosphate buffer, pH 6.0, 6.5, and 7.0 during heating to 71 degrees C. Thermal denaturation patterns of myosin and myosin/HDLG were similar except for the appearance of an endothermic peak at 54-56 degrees C in the mixed system. At pH 7.0, 2% myosin began to gel at 48 degrees C and had a storage modulus (G') of 500 Pa upon cooling. Myosin/HDLG (2% total protein) had a gel point of 48 degrees C and a G' of 650 Pa, whereas myosin/beta-LG had a gel point of 49 degrees C but the G' was lower (180 Pa). As the pH was decreased, the gel points of myosin and myosin/HDLG decreased and the G' after cooling increased. The HDLG was incorporated within the myosin gel network, whereas beta-LG remained soluble.

Fractal analysis of structure of beta-lactoglobulin preparations

Beta-lactoglobulin (beta-Lg) preparations obtained from whey retentate through (a) salting out at high NaCl concentration in a strongly acid medium (30% NaCl, pH 2.0), (b) alfa-lactalbumin (alfa-La) removal at mild conditions (55 degrees C/30 min, pH 3.9) and (c) alfa-La removal with 6% TCA solutions (20 degrees C/40 min) were characterized by high values (1.31 to 1.33) for fractal dimension (D(L)), indicating their developed surface. At the same time, a small differentiation between these values allows assuming that the methods of the protein isolation from whey retentate did not influence the surface development. High values for correlation coefficients indicated a great accuracy of the method used for the measurements of the protein particles.

Effect of electrostatic interactions on the percolation concentration of fibrillar beta-lactoglobulin gels

The effect of electrostatic interactions on the critical percolation concentration (c(p)) of fibrillar beta-lactoglobulin gels at pH 2 was investigated using rheological measurements, transmission electron microscopy (TEM), and performing conversion experiments. A decreasing c(p) with increasing ionic strength was found. The fraction of nonaggregated beta-lactoglobulin was independent of ionic strength in the regime of 0.01-0.08 M. TEM experiments showed long fibrils (2-7 microm) for ionic strengths between 0.01 and 0.08 M. Since both the conversion of monomers and the contour length of the fibrils were independent of ionic strength (0.01-0.08M), the linear increase of c(p) with the Debye length can be attributed purely to an increase of electrostatic repulsion between the fibrils. This increase is explained in terms of an adjusted random contact model which takes into account the charge and semiflexibility of the fibrils.

Molecular differences in the formation and structure of fine-stranded and particulate beta-lactoglobulin gels

In order to reveal at a molecular level differences between fine-stranded and particulate gels, we present an Fourier transform infrared spectroscopic study of the thermal behavior of beta-lactoglobulin (beta-lg) in salt-free D(2)O solutions and low ionic strength at different pDs. Differences are found in the denaturation mechanism, in the unfolded state of the protein, in the aggregate formation, and in the strength of the intermolecular interactions. For fine-stranded gels (pD 2.8 and 7.8), heating induces the dissociation of the dimers into monomers. The protein undergoes extensive structural modifications before aggregation begins. Aggregation is characterized by the appearance of a new band attributed to intermolecular beta-sheets which is located in the 1613-1619 cm(-1) range. For particulate gels (pD 4.4 and 5.4), the protein structure is almost preserved up to 75-80 degrees C with no splitting of the dimers. The band characteristic of aggregation originates from the component initially located at 1623 cm(-1), suggesting that at the beginning of aggregation, globular beta-lg in the dimeric form associate to constitute oligomers with higher molecular mass. Aggregation may result in the association of globular slightly denatured dimers, leading to the formation of spherical particles rather than linear strands. The aggregation band is always located in the 1620-1623 cm(-1) range for particulate gels showing that hydrogen bonds are weaker for these aggregates than for fine-stranded ones. This has been related to a more extensive protein unfolding for fine-stranded gels that allows a closer alignment of the polypeptide chains, and then to the formation of much stronger hydrogen bonds. Small differences are also found in protein organization and in intermolecular hydrogen bond strength vs pD within the same type of gel. Protein conformation and protein-protein interactions in the gel state may be responsible of the specific macroscopic properties of each gel network. A coarse representation of the different modes of gelation is described.

Effect of partial hydrolysis with an immobilized proteinase on thermal gelation properties of beta-lactoglobulin B

We have investigated the influence of partial hydrolysis with an immobilized proteinase from Bacillus licheniformis on the thermal gelation of isolated beta-lactoglobulin B. Gelation behaviour was determined by dynamic rheological measurements (small deformation) and the gels were characterized with respect to microstructure and water-holding properties. A fine-stranded gel with a complex modulus of approximately 2000 Pa was formed from beta-lactoglobulin (50 g/l in 75 mM-Tris-HCl, pH 7.5). Limited hydrolysis prior to thermal gelation resulted in coarser gels with thicker protein strands and larger pores. Gel structure correlated with its permeability, proton mobility and water-holding capacity. Total stiffness gel increased with low degrees of hydrolysis, but decreased after prolonged hydrolysis. Maximal gel stiffness was 1.5-fold that gels made from of unhydrolysed beta-lactoglobulin. This was much lower than the stiffening effect obtained after partial hydrolysis of whey protein isolate, showing that the gel strengthening effect of partial hydrolysis was depedent on the protein composition and/or the hydrolysis and gelatin conditions. A mechanism to explain the observed effects of hydrolysis on gelation and gel properties is presented.

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

Heat-set gels and aggregates from beta-lactoglobulin (beta-Lg), one of the major globular proteins from milk, have been studied on a molecular distance scale using negative-staining transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), and Fourier transform infrared spectroscopy (FTIR). The microscopy showed long linear aggregates forming in solutions at pH 2 (and sometimes 2.5) after prolonged heating. While there appeared to be no differences in aggregates formed under these conditions in H(2)O as compared with D(2)O, at all other pH and pD values, and in the presence of added salt, much shorter linear aggregates were formed. These became slightly more extended the further the pH was removed from pI. Wide-angle X-ray diffraction (WAXD) showed a diffuse beta-sheet halo at 2&theta;=19 degrees in patterns for both dried native and aggregated protein (irrespective of pH) with only a small change (sharpening) of this feature on heat treatment. Solution FTIR spectra, measured at pD=2, 2.5, 3, and 7, during heating, indicated shoulder development at 1612 cm(-1) in the carbonyl-stretching Amide I region diagnostic of a modest increase in intermolecular beta-sheet. In terms of the shoulder size, no distinctions could be made between acid and neutral aggregate structures. At all pHs, beta-lactoglobulin showed only limited secondary and tertiary structural changes in aggregation, in contrast to previous studies of insulin aggregation, where highly ordered crystalline fibrils were indicated. The current work has implications both in structural studies of food biopolymers and in ongoing studies of pathological protein self-assembly in disease states, such as spongiform encephalopathies.

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

Heat-set gels and aggregates from beta-lactoglobulin (beta-Lg), one of the major globular proteins from milk, have been studied on a molecular distance scale using negative-staining transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), and Fourier transform infrared spectroscopy (FTIR). The microscopy showed long linear aggregates forming in solutions at pH 2 (and sometimes 2.5) after prolonged heating. While there appeared to be no differences in aggregates formed under these conditions in H(2)O as compared with D(2)O, at all other pH and pD values, and in the presence of added salt, much shorter linear aggregates were formed. These became slightly more extended the further the pH was removed from pI. Wide-angle X-ray diffraction (WAXD) showed a diffuse beta-sheet halo at 2&theta;=19 degrees in patterns for both dried native and aggregated protein (irrespective of pH) with only a small change (sharpening) of this feature on heat treatment. Solution FTIR spectra, measured at pD=2, 2.5, 3, and 7, during heating, indicated shoulder development at 1612 cm(-1) in the carbonyl-stretching Amide I region diagnostic of a modest increase in intermolecular beta-sheet. In terms of the shoulder size, no distinctions could be made between acid and neutral aggregate structures. At all pHs, beta-lactoglobulin showed only limited secondary and tertiary structural changes in aggregation, in contrast to previous studies of insulin aggregation, where highly ordered crystalline fibrils were indicated. The current work has implications both in structural studies of food biopolymers and in ongoing studies of pathological protein self-assembly in disease states, such as spongiform encephalopathies.

Comparison of heat and pressure treatments of skim milk, fortified with whey protein concentrate, for set yogurt preparation: effects on milk proteins and gel structure

Heat (85 degrees C for 20 min) and pressure (600 MPa for 15 min) treatments were applied to skim milk fortified by addition of whey protein concentrate. Both treatments caused > 90 % denaturation of beta-lactoglobulin. During heat treatment this denaturation took place in the presence of intact casein micelles; during pressure treatment it occurred while the micelles were in a highly dissociated state. As a result micelle structure and the distribution of beta-lactoglobulin were different in the two milks. Electron microscopy and immunolabelling techniques were used to examine the milks after processing and during their transition to yogurt gels. The disruption of micelles by high pressure caused a significant change in the appearance of the milk which was quantified by measurement of the colour values L*, a* and b*. Heat treatment also affected these characteristics. Casein micelles are dynamic structures, influenced by changes to their environment. This was clearly demonstrated by the transition from the clusters of small irregularly shaped micelle fragments present in cold pressure-treated milk to round, separate and compact micelles formed on warming the milk to 43 degrees C. The effect of this transition was observed as significant changes in the colour indicators. During yogurt gel formation, further changes in micelle structure, occurring in both pressure and heat-treated samples, resulted in a convergence of colour values. However, the microstructure of the gels and their rheological properties were very different. Pressure-treated milk yogurt had a much higher storage modulus but yielded more readily to large deformation than the heated milk yogurt. These changes in micelle structure during processing and yogurt preparation are discussed in terms of a recently published micelle model.

High-pressure treatment of milk: effects on casein micelle structure and on enzymic coagulation

High isostatic pressures up to 600 MPa were applied to samples of skim milk before addition of rennet and preparation of cheese curds. Electron microscopy revealed the structure of rennet gels produced from pressure-treated milks. These contained dense networks of fine strands, which were continuous over much bigger distances than in gels produced from untreated milk, where the strands were coarser with large interstitial spaces. Alterations in gel network structure gave rise to differences in rheology with much higher values for the storage moduli in the pressure-treated milk gels. The rate of gel formation and the water retention within the gel matrix were also affected by the processing of the milk. Casein micelles were disrupted by pressure and disruption appeared to be complete at treatments of 400 MPa and above. Whey proteins, particularly beta-lactoglobulin, were progressively denatured as increasing pressure was applied, and the denatured beta-lactoglobulin was incorporated into the rennet gels. Pressure-treated micelles were coagulated rapidly by rennet, but the presence of denatured beta-lactoglobulin interfered with the secondary aggregation phase and reduced the overall rate of coagulation. Syneresis from the curds was significantly reduced following treatment of the milk at 600 MPa, probably owing to the effects of a finer gel network and increased inclusion of whey protein. Levels of syneresis were more similar to control samples when the milk was treated at 400 MPa or less.

Properties of the iron-binding site of the N-terminal lobe of human and bovine lactotransferrins. Importance of the glycan moiety and of the non-covalent interactions between the N- and C-terminal lobes in the stability of the iron-binding site

The recent determination by X-ray diffraction of the tridimensional structure of human lactotransferrin has underlined the presence of two lobes, each composed of two domains, I and II, as well as the involvement of five ligands in the binding of iron. Only one of the ligands (Asp-61) is located in domain I (residues 1-90 and 252-320), while the others [two tyrosine, one histidine and one (bi)carbonate ion linked to an arginine residue] belong to domain II (residues 91-251). On the basis of these data and of our previous results concerning the isolation of the 30 kDa N-tryptic fragment (residues 4-281) and the 20 kDa N2-glycopeptide (N-terminal domain II; residues 91-253) from human and bovine lactotransferrins, we have compared the iron-binding properties of these two fragments. The results demonstrate that Asp-61, which is missing from domain II, does not take part in the stability upon protonation of the iron complex of both human and bovine lactotransferrins. Furthermore, by comparing the iron-binding properties of human and bovine lactotransferrins to those of isolated 30 kDa N-tryptic and 50 kDa C-tryptic fragments and of the reassociated N,C-tryptic complex of both proteins, it has been shown that the non-covalent interactions which occurred between the two lobes of lactotransferrins and in the reassociated N,C-tryptic complex can explain in part the high affinity of lactotransferrins for iron. Finally, deglycosylation experiments on the 30 kDa N-tryptic fragment and N-terminal domain II from human and bovine lactotransferrins demonstrate that full removal of the glycan moiety leads to the loss of iron-binding capacity and so underlines the importance of the glycan moiety in the stability upon protonation of the N-terminal iron-binding site of both lactotransferrins.