α-Lactalbumin and sodium dodecyl sulfate aggregates: Denaturation, complex formation and time stability

Abstract

We have combined isothermal titration calorimetry (ITC), spectroscopy and small-angle X-ray scattering (SAXS) to describe the denaturation process of bovine α-lactalbumin (BLA) induced by sodium dodecyl sulfate (SDS). As presented in the article, in the initial binding step, association of ∼6 SDS molecules with one protein molecule leads to a decrease on the hydrophobic environment of tryptophan residues and to an increase on the α-helix content of the protein, showing the endothermic effect of the whole process. Subsequently, binding of 30 SDS molecules with a BLA dimer leads to the highest α-helix content along with an exothermal behavior: at this point the SDS-BLA complexes can be described as core-shell like structures, also known as decorated micelles. At the end, binding of 55 SDS molecules forms a larger decorated micelle bound to a single BLA molecule in a molten globule state. After 600 h incubation, samples with SDS/BLA ratios of 21.7 and 43.2 shows a conversion of α-helix into disordered structures and the formation aggregates, while samples with SDS/BLA ratios of 3.4 and 78.3 exhibit an increased helical conformation with no aggregation. Based on SAXS analysis, the aggregates could be described as a structure of individual core-shell ellipsoidal SDS-BLA complexes connected in a beads-on-a-string structure. The results presented in this work not only provide overall information on SDS/BLA complexation, but also help on a better understanding about the roles of SDS concentration and incubation time on the α-helix content and aggregation/fibrillation process during protein denaturation.