Properties Of Β-casein Research Articles

Micellar properties of β-casein–cationic surfactant solutions

The micellar properties of b-casein in solution with a homologous series of cationic surfactants have been investigated using turbidity, fluorescence spectroscopy, and quenching fluorescence measurements. Turbidity mea- surements indicated that monomeric cationic surfactants initially bind cooperatively with a site on the b-casein micelle in negatively charged amino acids. This leads to the formation of low-soluble b-casein micelle-monomeric surfactants complexes. Further addition of surfactant leads to the formation of more highly soluble complexes of b- casein micelle-cationic surfactant. The fluorescence experiments were also used to study the formation of b- casein-cationic surfactant complexes and to determine critical micelle concentration of surfactants and b-casein. The mean aggregation number of b-casein and cationic surfactant micelles at various experimental conditions and the dependence of aggregation phenomenon on the chain length of cationic surfactants were investigated. The mean aggregation number for b-casein micelle has been deter- mined to be 45.70 using quenching fluorescence.

Secondary structural studies of bovine caseins: temperature dependence of β-casein structure as analyzed by circular dichroism and FTIR spectroscopy and correlation with micellization

To obtain a molecular basis for the similarities and dissimilarities in the functional, chemical, and biochemical properties between β-casein and the other caseins, three-dimensional models have been presented. Secondary structural prediction algorithms and molecular modeling techniques were used to predict β-casein structure. The secondary structure of bovine β-casein was re-examined using Fourier transform infrared and circular dichroism spectroscopies to test these predictions. Both methods predict a range of secondary structures for β-casein (28–32% turns, 32–34% extended) at 25°C. These elements were highly stable from 5 to 70°C as viewed by circular dichroism. More flexible conformational elements, tentatively identified as loops, helix and short segments of polyproline II, were influenced by temperature, increasing with elevated temperatures. Another view is that as temperature decreases, these elements are lost (cold denaturation). Several distinct transitions were observed by circular dichroism at 10, 33 and 41°C, and another transition, extrapolated to occur at 78°C. Calculations from analytical ultracentrifugation indicate that the 10, 33 and 41°C transitions occur primarily in the monomeric form of the protein. As β-casein polymers are formed, and increase in size, the transitions at higher temperature may reflect changes in the more flexible conformational elements as they adjust to changes in surface charge during polymer formation. The transition at 10°C may represent an actual general conformational change or cold denaturation. Over the range of temperatures studied, the sheet and turn areas remain relatively constant, perhaps forming a supporting hydrophobic core for the monomers within the micelle-like polymer. This interpretation is in accord with the known properties of β-casein, and those predicted from molecular modeling.

Properties of β-Casein at the Air/Water Interface As Supported by Surface Rheological Measurements

The properties of air/water adsorbed and spread monolayers of native and dephosphorylated β-casein were monitored using surface pressure (Langmuir trough) and surface rheology (ring trough) techniques. Two stages of rearrangement are observed for native β-casein at surface areas of about 1.0-1.3 and 0.7 m 2 mg. The first accounts for distinct surface elasticity changes in the film, which are probably due to the expulsion of the most hydrophilic segments of the protein chain. The second accounts for the collapse of the monolayer. The experiments on dephosphorylated β-casein monolayers show that dephosphorylation changes the surface elasticity behavior of the monolayer, in particular between 1 and 1.3 m 2 mg -l . We calculated a two-dimensional Flory exponent, v, for both proteins. This exponent is constant over a (semi-) dilute range of surface tensions, maximally up to a surface area of around 1.3 m 2 mg. The adsorption of native β-casein is shown to be diffusion-limited up to a surface area of around 1 m 2 mg -1 . Experiments at high ionic strength show the importance of charge on the typical surface elasticity behavior of β-casein. Experiments with enzymatically treated β-casein show the importance of the presence of hydrophilic section in the molecule on the surface elasticity behavior. It is assumed and shown that, at surface concentrations below monolayer collapse and at given solvent conditions, native /Lcasein and dephosphorylated β-casein show irreversible (air/water) adsorption behavior. Furthermore, the proteins in the monolayer are very flexible (i.e. quick relaxations).

Interaction of .BETA.-casein and .KAPPA.-carrageenan. I. Viscosity and turbidity under non-gelling conditions.

The relationship between gelling and casein stabilizing ability of κ-carrageenan was investigated using viscosity and turbidity measurements. At low ionic strength, 0.5% β-casein decreased the viscosity of 0. 1 % κ-carrageenan solution in the absence of calcium. In the presence of calcium, the viscometric behavior of κ-carrageenan-β-casein mixture was governed by the properties of β-casein rather than that of κ-carrageenan. A 0.1% κ-carrageenan solution formed a structure above 5 mM.CaCl2 at 5°C. Under the same conditions, the mixture began to gel at lower concentration of calcium than 5 mM. These phenomena suggested the participation of β-casein in the formation of the κ-carrageenan structure.

Preparation and properties of β-casein from buffalo's milk

Beta-casein from individual buffalo's milk was found to be homogeneous by starch-gel electrophoresis. Beta-Casein was separated from buffalo's milk by the method of Warner (1944) and purified by DEAE-cellulose chromatography. Buffalo beta-casein possesses identical end-groups to those of cow beta-casein; namely N-terminal arginine and assuming a single polypeptide chain a possible C-terminal sequence of Ile-Ile-Val. However, the amino-acid composition and the trypic peptide patterns of the 2 proteins are not the same.

Acylated β-Caseins: Electrostatic Interactions and Aggregation

β-Casein was both acetylated and succinylated, to investigate alterations of selected properties brought about by removal of positive charges and, in the case of succinylated β-casein, addition of negative charges. The results were used to evaluate the role of electrostatic interactions in the aggregation of β-casein. Mobility during alkaline polyacrylamide gel electrophoresis of both derivatives was greater than β-casein, succinylated β-casein having the greatest mobility. Succinylated β-casein and acetylated β-casein, in that order, required a higher concentration of NaCl than did β-casein for elution from DEAE-cellulose at pH 7.0. The calcium ion senstivity of acetylated β-casein was decreased in comparison to β-casein. Succinylated β-casein was insensitive to calcium ions at pH 7 in 0.1 m CaCl2 at a 0.3% concentration. Sedimentation patterns at pH 6.86, 20C, revealed that succinylated β-casein did not form a fast-sedimenting peak, usually associated with aggregation. The fast-sedimenting peak of acetylated β-casein had a lower sedimentation coefficient than did β-casein. These alterations of the above properties of β-casein are interpreted to result from an increase in the net negative electrostatic charge of the modified protein in neutral solution. For succinylated β-casein, the net negative electrostatic charge is sufficient to prevent precipitation by calcium ions and to prevent aggregation under conditions that favor aggregation of β-casein.

Tensile Properties of Bristle Fibers Made from α- and β-Casein

The tensile properties of fibers made from α- and β-casein and unfractionated casein have been compared. β-Casein fiber, oriented by stretching, was considerably better than the other casein fibers, and values for its wet and dry tenacity compared favorably with corresponding values for wool. The better fiber properties of β-casein are ascribed to its molecular structure.