Structure of Heat-Induced β-Lactoglobulin Aggregates and their Complexes with Sodium-Dodecyl Sulfate

Abstract

We report on the conformation of heat-induced bovine beta-lactoglobulin (betalg) aggregates prepared at different pH conditions, and their complexes with model anionic surfactants such as sodium dodecyl sulfate (SDS). The investigation was carried out by combining a wide range of techniques such as ultra small angle light scattering, static and dynamic light scattering, small angle neutron scattering, small-angle X-ray scattering, electrophoretic mobility, isothermal titration calorimetry (ITC) and transmission electron microscopy. Three types of aggregates were generated upon heating betalg aqueous dispersions at increasing pH from 2.0 to 5.8 to 7.0: rod-like aggregates, spherical aggregates, and worm-like primary aggregates, respectively. These aggregates were shown not only to differ for their sizes and morphologies, but also for their internal structures and fractal dimensions. The main differences between aggregates are discussed in terms of the ionic charge and conformational changes arising for betalg at different pHs. The formation of complexes between SDS and the various protein aggregates at pH 3.0 was shown to occur by two main mechanisms: at low concentration of SDS, the complex formation occurs essentially by ionic binding between the positive residues of the protein and the negative sulfate heads of the surfactant. At complete neutralization of charges, precipitation of the complexes is observed. Upon further increase in SDS concentration, complex formation of SDS and the protein aggregates occurs primarily by hydrophobic interactions, leading to (i) the formation of an SDS double layer around the protein aggregates, (ii) the inversion of the total ionic charge of each individual protein aggregate, and (iii) the complete redispersion of the protein aggregate-SDS complexes in water. Remarkably, the SDS double layer around the protein aggregates provides an efficient protective shield, preventing precipitation of the aggregates at any possible pH values, including those values corresponding to the isoelectric pH of the aggregates.

Understanding amyloid aggregation by statistical analysis of atomic force microscopy images

“…The powder was dialysed and freeze-dried before use, as described in literature 16 . A 2 wt% solution was prepared by dissolving the powder in Milli-Q water at room temperature over at least 1 h. The solution was centrifuged at 10,800g over a period of 1 h at 20 8C using a Sorvall RC3C Plus centrifuge (DuPont), adjusted to pH 2, and filtered through a 0.45-mm Millipore filter before heat treatment.…”

“…b-Lactoglobulin has been particularly well studied, because it represents both a relevant model system and a major whey protein of interest to the food industry [14][15][16][17][18][19][20][21][22] .…”

Fibrillation of transferrin

“…The solution had a polydispersity index (PI or PDI) of 0.707, corresponding to a polydispersity of 84% PD. PD vaues of <20% are often taken to imply the presence of single protein molecules whereas values > 20% indicate aggregation [47,48] Unchromatographed transferrin and monomeric rTf were too polydisperse to allow measurement of their particle sizes, showing a propensity for ongoing aggregation. DLS measurements are therefore consistent with a tendency towards higher order protein aggregation.…”

How to tailor heat-induced whey protein/κ-casein complexes as a means to investigate the acid gelation of milk—a review

“…Risso et al (2000) for instance reported that the net charge of heated casein micelles resuspended in aqueous Tris-HCl-CaCl 2 buffer at pH 6.4 could be increased by the binding of anionic fluorescent probes. Jung et al (2008) further reported that the double-coating of β-LG heat-induced complexes by SDS in water through successive electrostatic and hydrophobic interactions could eventually reverse their net charge. However, application of the method to whey protein/κ-casein complexes may have consequences on their structure, especially in the case of surfactants, because they are partially built on hydrophobic interactions .…”