Study of structure and scaling behavior of chemically unfolded β-casein by means of small-angle neutron scattering

Structures of beta-casein formed in water with various concentrations of guanidine hydrochloride (GdmCl) were studied by small-angle neutron scattering (SANS). A transition in structure and properties of the protein seems to occur around a GdmCl concentration of 1.2 mol L-1. The data are interpreted assuming that the protein molecules behave like multi-block copolymers with alternating hydrophilic and hydrophobic sequences. Below the transition, the protein structure is still native and has a fractal dimension larger than that beyond the transition where the beta-casein has the feature of a three-dimensional excluded-volume coil. A possible interpretation of these results is that the presence of salt increases the value of the critical micelle concentration. This allows a comparison of the structure off beta-casein and synthetic macromolecules. We find that, although the protein is short, excluded-volume interactions between monomers are present, and that the structure of beta-casein is very similar to what was found in dilute solutions of linear polymers in a good solvent. Thus, it is reasonable to assume that above the critical salt concentration, the protein is completely isolated, whereas below, some structure remains present. (C) 2006 Society of Chemical Industry.