Structural, topographical, and rheological characteristics of beta-casein-dioleoyl phosphatidylcholine (DOPC) mixed monolayers.

The surface pressure (pi)-area (A) isotherms and Brewster angle microscopy (BAM) images of beta-casein-dioleoyl phosphatidylcholine (DOPC) mixed films spread on buffered water at pH 7 and 9 and at 20 degrees C were determined as a function of the mass fraction of DOPC in the mixture (X(DOPC)). The structural characteristics, miscibility, and topography (morphology and reflectivity) of DOPC-beta-casein mixed films were very dependent on surface pressure and monolayer composition. The structure in DOPC-beta-casein mixed monolayers was liquid-expanded-like, as for pure components. The monolayer structure was more expanded as the pH and the DOPC concentration in the mixture were increased. From the concentration and surface pressure dependence on excess area and elasticity (E) it was deduced that DOPC and beta-casein form a practically immiscible monolayer at the air-water interface. The BAM images and the evolution with the surface pressure of the reflectivity of BAM images give complementary information on the interactions and structural characteristics of DOPC-beta-casein mixed monolayers, which corroborate the conclusions derived from the pi-A isotherm. After the spreading or just after the expansion at pi approximately 0 we have observed the presence of 2D-foams, typical topography of DOPC monolayers at low pi. The 2D-foams disappear after the compression of the monolayer and the topography is homogenous and isotropic. From the reflectivity of BAM images it is possible to distinguish between the coexistence of DOPC and beta-casein or beta-casein displacement by DOPC, depending on the surface pressure. The surface dilatational properties of the mixed films corroborate the coexistence of DOPC and beta-casein at pi lower than the equilibrium spreading pressure (pi(e)) of beta-casein and beta-casein displacement by DOPC at pi>pi(e) of beta-casein. The phenomena observed appear to be generic for protein and polar (monoglycerides) and ionizable (phospholipid) mixed monolayers.

Monoglycerides and beta-lactoglobulin adsorbed films at the air-water interface. structure, microscopic imaging, and shear characteristics.

In this work we have analyzed the structural, topographical, and shear characteristics of mixed monolayers formed by adsorbed beta-lactoglobulin (beta-lg) and spread monoglyceride (monopalmitin or monoolein) on a previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm, Brewster angle microscopy (BAM), and surface shear characteristics were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The pi-A isotherm and BAM images deduced for adsorbed beta-lactoglobulin-monoglyceride mixed films at pi lower than the equilibrium surface pressure of beta-lactoglobulin (pi(e)(beta-lg)) indicate that beta-lactoglobulin and monoglyceride coexist at the interface. However, the interactions between protein and monoglyceride are somewhat weak. At higher surface pressures (at pi > or = pi(e)(beta-lg)) a protein displacement by the monoglyceride from the interface takes place. The surface shear viscosity (eta(s)) of mixed films is very sensitive to protein-monoglyceride interactions and displacement as a function of monolayer composition (protein/monoglyceride fraction) and surface pressure. Shear can induce change in the morphology of monoglyceride and beta-lactoglobulin domains, on the one hand, and segregation between domains of the film-forming components on the other hand. In addition, the displacement of beta-lactoglobulin by the monoglycerides is facilitated under shear conditions.

Dynamic properties of soy globulin adsorbed films at the air-water interface.

In this paper we present surface dilatational properties of soy globulins (beta-conglycinin, glycinin, and reduced glycinin with 10 mM of dithiothreitol (DTT)) adsorbed onto the air-water interface, as a function of adsorption time. The experiments were performed at constant temperature (20 degrees C), pH (8.0), and ionic strength (0.05 M). The surface rheological parameters were measured as a function of protein concentration (ranging from 1 to 1x10(-3)% wt/wt). We found that the surface dilatational modulus, E, increases, and the phase angle, phi, decreases with time, theta, which may be associated with protein adsorption. These phenomena have been related to protein adsorption, unfolding, and/or protein-protein interactions (at long-term adsorption) as a function of protein concentration in solution. From a rheological point of view, the surface viscoelastic characteristics of soy globulin films adsorbed at the air-water interface are practically elastic. The main conclusion is that the dilatational properties of the adsorbed films depend on the molecular structure of the protein.