A theoretical and experimental investigation of the effect of sodium dodecyl sulfate on the structural and conformational properties of bovine β-casein

A predicted three-dimensional structure of bovine beta-casein was constructed using homology modeling with the aid of MODELLER and I-TASSER programs, with the validity and reliability of the models evaluated according to stereochemical qualities and small angle X-ray scattering. By comparing the results obtained from the two models using the CRYSOL program, an optimal model of the beta-casein structure derived from I-TASSER was selected and used in subsequent molecular dynamics (MD) analysis. 300 ns MD simulations of beta-casein in water and in the presence of different SDS concentrations at 300 K were performed. The results of the MD simulations indicated that SDS molecules played a dual role in modifying the conformation of beta-casein at 300 K. Concentrations of SDS below its CMC (1 mM), at which only the monomer form of SDS was present, induced beta-casein to lose its secondary structure by converting helices into random coils; however the conformation of the complex was still comparable with that of native beta-casein. In the presence of 10 mM SDS (above its CMC), the helical content of beta-casein was increased along with reduced random coils, and the structural rearrangement led to a more compact conformation. The latter change is likely related to the hydrophobic interactions that dominate the binding of the C-terminal region, along with the anchoring of sulfate groups of SDS on the positively charged N-terminal portion via electrostatic attraction. Hydrogen bonding supplemented the SDS-induced stabilization of beta-casein. A correlated necklace and bead'' model, in which the micelles nucleate on the protein hydrophobic sites, was proposed for the structure of beta-casein-SDS complexes.