Static and Dynamic Scattering of β-Lactoglobulin Aggregates Formed after Heat-Induced Denaturation at pH 2

Structural changes during heat-induced gelation of globular protein dispersions

Macroscopic and molecular structural changes during heat-induced gelation of beta-lactoglobulin, bovine serum albumin, ovalbumin, and alpha-lactalbumin aqueous dispersions were probed by the mechanical and CD spectroscopy, respectively. Aqueous solutions of the native globular proteins, except for alpha-lactalbumin, exhibited solid-like mechanical spectra-namely, the predominant storage modulus G' over the loss modulus G" in the entire frequency range examined (0.1-100 rad/s), suggesting that these protein solutions were highly structured even before gelation, possibly due to strong repulsions among protein molecules. Such solid-like structures were susceptible to nonlinearly large shear but recovered almost immediately at rest. During gelation by isothermal heating, major changes in the secondary structure of the globular proteins completed within a few minutes, while values of the modulus continued to develop for hours with maintaining values of tandelta (= G"/G') less than unity. As a result, a conventional criterion for mechanically defining the gelation point, such as a crossover between G' and G", was inapplicable to these globular protein systems. beta-Lactoglobulin gels that had passed the gelation point satisfied power laws (G' approximately G" approximately omega(n)) believed to be valid only at the gelation point, suggesting that fractal gel networks, similar to those of critical gels (i.e., gels at the gelation point), were formed.

Constitutive equations for an elastic material with anisotropic rigid particles

In this paper we have derived constitutive equations for an elastic material with anisotropic rigid particles. We have included a dependence on the Finger tensor B and the orientation tensor Q in the expression for the free energy of the system. With this expression for the free energy we have derived an expression for the stress tensor up to second order in both these variables. We have shown that the elastic modulus in this expression depends on Q, and this dependence leads to an effective elastic modulus that depends on the strain. We have calculated the explicit form of the equation for the stress tensor for a deformation in the xy plane with a strain equal to -gamma. For fully isotropic materials with Q=0 this expression reduces to an equation containing only odd powers of gamma. The inclusion of a non-zero value for the orientation tensor leads to an additional set of terms in the equation, all proportional to Qxy (the xy component of the tensor Q), and all proportional to even powers of gamma. We have qualitatively compared these expressions with Fourier transform (FT) rheological measurements of xanthan gels, at concentrations above and below the order-disorder transition. In FT rheometry an oscillatory deformation is applied in the nonlinear regime, and the resulting stress response is analyzed in Fourier space. In the 2% (w/w) xanthan system (disordered state) only odd harmonics were found in the stress response, whereas in the 4% (w/w) xanthan gel (ordered state) even harmonics could be detected. As predicted by our theory, the intensity of these even harmonics first increased with increasing gamma, until a maximum value was reached. Beyond this maximum the intensity decreased continuously with increasing gamma.

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 2θ=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 2θ=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.

Characterization and isolation of intermediates in beta-lactoglobulin heat aggregation at high pH

The early stages of heat induced aggregation at 67.5 degrees C of beta-lactoglobulin were studied by combined static light scattering and size exclusion chromatography. At all conditions studied (pH 8.7 without salt and pH 6.7 with or without 60 mM NaCl) we observe metastable heat-modified dimers, trimers, and tetramers. These oligomers reach a maximum in concentration at about the time when large aggregates (1000-4000 kg/mol) appear, after which they decline in concentration. By isolating the oligomers it was demonstrated that they rapidly form aggregates upon heating in the absence of monomeric protein, showing that these species are central to the aggregation process. To our knowledge this is the first time that intermediates in protein aggregation have been isolated. At all stages of aggregation the dominant oligomer was the heat-modified dimer. Whereas the heat-modified oligomers are formed at a higher rate at pH 8.7 than at pH 6.7, the opposite is the case for the formation of aggregates from the metastable oligomers indicating cross-linking via disulfide bridges for the oligomers and noncovalent interaction in the formation of the aggregates. The data suggest that an aggregate nucleus is formed from four oligomers. For protein concentrations of 10 or 20 g/l a heat-modified monomer can be observed until about the time when the maximum in concentration appears of the heat-modified dimer. The disappearance of this heat-modified monomer correlates to the formation of dimers (trimers and tetramers).