Association behavior of β-casein

Abstract

The association behavior of β-casein, a protein with a distinct amphipathic character, was studied. β-Casein exhibits markedly temperature-dependent association behavior; at low temperatures (<10–15 °C), monomers predominate, but as the temperature is increased, monomers associate, via hydrophobic bonding, into micelles. β-Casein micelles have a hydrodynamic radius of ∼12 nm, a radius of gyration of ∼8.3 nm, and an interaction radius of ∼15 nm. These data are fully consistent with a pervious fluffy particle. The association behavior of β-casein is also strongly affected by concentration and solvent quality. At low concentrations β-casein exhibits a critical micelle concentration (CMC) of approximately 0.05%, w/v, at 40 °C. In the presence of 6 M urea the temperature dependence of β-casein's association behavior is eliminated, leaving monomers predominantly. Temperature-dependent transformations in micelle morphology can be explained by changes in solvent quality, i.e., the temperature–protein hydrophobicity and temperature–voluminosity profiles of β-casein. The results obtained are consistent with the shell model as developed by Kegeles, in which a distribution of micelle sizes is formed. They contrast with the traditional description of the micellization of β-casein by a two-state model or by the closed-association model, i.e., monomers ⇔ micelles.