Differences between the pressure- and temperature-induced denaturation and aggregation of β-lactoglobulin A, B, and AB monitored by FT-IR spectroscopy and small-angle X-ray scattering

We examined the temperature- and pressure-induced unfolding and aggregation of beta-lactoglobulin (beta-Lg) and its genetic variants A and B up to temperatures of 90 degrees C in the pressure range from 1 bar to 10 kbar. To achieve information simultaneously on the secondary; tertiary, and quaternary structures, we have applied Synchrotron small-angle X-ray diffraction and Fourier transform infrared spectroscopy. Upon heating a beta-Lg solution at pH 7.0, the radius of gyration R-g first decreases, indicating a partial dissociation of the dimer into the monomers, the secondary structures remaining essentially unchanged. Above 50 degrees C, the infrared spectroscopy data reveal a decrease in intramolecular beta-sheet and alpha-helical structures, whereas the contribution of disordered structures increases; Within the temperature range from 50 to 60 degrees C, the appearance of the pair distance distribution function is not altered significantly, whereas the amount of defined secondary structures declines approximately by 10%. Above 60 degrees C the aggregation process of 1% beta-Lg solutions is clearly detectable by the increase in R-g and intermolecular beta-sheet content. The irreversible aggregation is due to intermolecular S-H/S-S interchange reactions and hydrophobic interactions. Upon pressurization at room temperature, the equilibrium between monomers and dimers is also shifted and dissociation of dimers is induced. At pressures of approximately 1300 bar, the amount of beta-sheet and alpha-helical structures decreases and the content of disordered structures increases, indicating the beginning unfolding of the protein which enables aggregation. Contrary to the thermal denaturation process, intermolecular beta-sheet formation is of less importance in pressure-induced protein aggregation and gelation. The spatial extent of the resulting protein clusters is time- and concentration-dependent. The aggregation of a 1% (w/w) solution of A, B, and the mixture AB results in the formation of at least octameric units as can be deduced from the radius of gyration of about 36 Angstrom. No differences in the pressure stability of the different genetic variants of beta-Lg are detectable in our FT-IR and SAXS experiments. Even application of higher pressures (up to 10 kbar): does not result. in complete unfolding of all beta-Lg variants.