Insights on the structure of caseinate particles based on surfactants-induced dissociation

Caseinate is derived from casein, the major protein in milk, and forms aggregates in aqueous solution. Here, the internal structure of caseinate particles based on dissociation induced by the different alkyl chain n-alkyl trimethylammonium bromides (CnTAB, n = 10, 12, 14, 16) was investigated by the combination of calorimetric, spectral, scattering and electron microscopic techniques. The results suggested the hydrophobic environment of, at least, Trp, Tyr, Phe, Leu and Ala residues in caseinate increased with increasing the chain length of CnTAB. The dissociation of caseinate aggregates induced by CnTAB was possibly due to the binding of ammonium bromide groups of CnTAB with opposite charged κ-casein via electrostatic interactions, as well as the binding of CnTAB alkyl chain with the P,Q-rich portion of calcium-sensitive caseins (αs1-,αs2-and β-casein) which provided more energetically favorable non-covalent interactions, such as hydrogen bonding and hydrophobic interactions. These interactions resulted in the disruption of backbone-to-backbone interactions of caseins and thus the dissociation of the caseinate aggregates into smaller caseinate clusters. When 10 mM EDTA was added, the size distribution of caseinate-CnTAB complexes exhibited a greater extent of disruption due to the depletion of calcium ions in the caseinate aggregate, suggesting that the binding of CnTAB with casein may not disrupt the phosphate centers on caseins. According to molecular dynamics (MD) simulations, the “necklace and bead” structure of αs1-casein-CnTAB and the binding number increased with increasing the alkyl chain of CnTAB, providing more hydrophilic headgroups and electrostatic repulsion for αs1-casein-CnTAB complex. Electron microscopy images showed that, upon addition of CnTAB, caseinate nanoparticles lost their initial morphology dissociating into smaller caseinate aggregates with diameters ranging from ~30 to ~100 nm. The caseinate aggregates were made of amorphous denser clusters with a diameter of ~10 nm, and the distance between the clusters was observed as ~20–30 nm. This work provides molecular insights into the structure of caseinate particles by exploiting surfactants-induced dissociation.