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

• Published in: Biomacromolecules 2008-09, Vol.9 (9), p.2477-2486
• Main Authors: Jung, Jin-Mi, Savin, Gabriela, Pouzot, Matthieu, Schmitt, Christophe, Mezzenga, Raffaele
• Format: Article
• Language: English
• Subjects: Applied sciences; Biological and medical sciences; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; Hot Temperature; Hydrogen-Ion Concentration; In solution. Condensed state. Thin layers; Lactoglobulins - chemistry; Molecular biophysics; Natural polymers; Particle Size; Physico-chemical properties of biomolecules; Physicochemistry of polymers; Protein Conformation; Protein Denaturation; Protein Structure, Secondary; Proteins; Sodium Dodecyl Sulfate - chemistry; Surface Properties; Surface-Active Agents - chemistry
• ISSN: 1525-7797; 1526-4602
• DOI: 10.1021/bm800502j

We report on the conformation of heat-induced bovine β-lactoglobulin (βlg) 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 βlg 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 βlg 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.