Association behavior of beta-casein.
The association behavior of beta-casein, a protein with a distinct amphipathic character, was studied. beta-Casein exhibits markedly temperature-dependent association behavior; at low temperatures (<10-15 degrees C), monomers predominate, but as the temperature is increased, monomers associate, via hydrophobic bonding, into micelles. beta-Casein micelles have a hydrodynamic radius of approximately 12 nm, a radius of gyration of approximately 8.3 nm, and an interaction radius of approximately 15 nm. These data are fully consistent with a previous fluffy particle. The association behavior of beta-casein is also strongly affected by concentration and solvent quality. At low concentrations beta-casein exhibits a critical micelle concentration (CMC) of approximately 0.05%, w/v, at 40 degrees C. In the presence of 6 M urea the temperature dependence of beta-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 beta-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 beta-casein by a two-state model or by the closed-association model, i.e., monomers if micelles.
O'Connell
J E
JE
NIZO Food Research, 6710 BA Ede, The Netherlands.
Grinberg
V Ya
VY
de Kruif
C G
CG
D002364:Caseins / Q000737:chemistry* / Q000378:metabolism*; D055598:Chemical Phenomena; D002621:Chemistry; D004305:Dose-Response Relationship, Drug; D008823:Micelles; D009502:Neutrons; D017434:Protein Structure, Tertiary; D012542:Scattering, Radiation; D013696:Temperature; D013997:Time Factors; D014508:Urea / Q000494:pharmacology; D014783:Viscosity
D002364:Caseins; D008823:Micelles; D014508:Urea