The adsorption of /3-and K-casein at the airlwater interface from solutions containing concentrations ranging from lo-' to 10-1 weight % has been monitored using both radiotracer and ellipsometric techniques. The surface pressure and dilatational modulus have also been measured both during the adsorption process and at steady state conditions. Differences between the surface properties of ,8-and K-casein are related to the structures of the molecules. The adsorption data cannot be described by Gibbs' adsorption equation. The adsorption isotherms and film thicknesses are discussed in terms of the distribution of amino-acid residues between trains and loops. At low surface coverage where loop formation is not pronounced, theories based on polymer statistics describe the relationships between film pressure, dilatational modulus and surface coverage satisfactorily. At low surface concentrations of protein the dilatational modulus is purely elastic whereas at higher surface concentrations it has both elastic and viscous components.

SynopsisThe conformation of (3-casein A in the monomeric and thermally aggregated states has been investigated by a range of techniques. P-Casein exists as a monomer in solution a t 4°C and a t concentrations up to a t least 3 g/dl. The molecule is flexible and exhibits a lot of segmental motion, but its secondary structure is not wholly random coil; about one-third of the polypeptide chain is ordered and the likely locations of these regions are discussed. The radius of gyration, representing the time-average distribution of the flexible chain, is 46 A. Increasing temperature leads to aggregation of the P-casein molecules. The degree of association is very sensitive to experimental conditions, and under our conditions a 14-mer exists a t 20°C. The aggregate is spherical with a radius of about 100 A. The interior of the aggregate is relatively disordered, and the P-casein molecules remain in a largely flexible, hydrated conformation. The volume restriction of the protein molecules which occurs on association leads tvsome immobilization of the hydrophobic C-terminal region, which is packed toward the center of the aggregate.

SynopsisA sedimentation analysis has been used to determine the proportion of protein present as monomer and aggregate in 0.5 and 1.0 g/dl solutions of 0-casein A in pH 7 phosphate buffer over the temperature range 10-40°C. The amount and molecular weight of the aggregate increase with temperature; under the conditions used, the aggregation number ( n ) of &casein is given approximately by n = 0.6t + 2 with t in degrees centigrade. The concentration of p-casein in monomeric and aggregated states a t different temperatures is used to calculate the standard enthalpy of aggregation AH" (Van't Hoff) by assuming that p-casein undergoes a cooperative, two-state, micellization process; aggregation is an endothermic process and AH" = 66.0 f 2.6 kJ mol-'. Combination of this AHo with the amount of protein calculated to dissociate when 1 g/dl solutions are diluted isothermally to 0.5 g/dl gives the heat of dilution a t various temperatures. These calculated heats of dilution are compared with the experimental values obtained by carrying out the same dilutions in a microcalorimeter. The heat of dilution decreases linearly with &casein concentration, but the extrapolated zero-concentration value of 65.8 f 1.6 kJ mol-' is the same as the Van't Hoff enthalpy. This agreement in the enthalpy values indicates that the micellization of &casein occurs cooperatively. The effect of modifying the hydrophobic/hydrophilic balance of the system on the micellization of 6-casein A has been investigated. The hydrophobic interaction between the protein molecules is decreased by removing the three C-terminal residues (Ileu Ileu Val) with carboxypeptidase-A. This modification drastically reduces the ability of the p-casein molecule to form micelles. Substitution of 2H20 for HzO at constant temperature perturbs the monomer-micelle equilibrium in favor of micelles because of enhanced hydrophobic interactions in the former solvent. The results are consistent with P-casein micellization involving a delicate balance of the hydrophobic forces favoring aggregation and electrostatic forces opposing it.

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