Thermodynamic and structural insight into the underlying mechanisms of the phosphatidylcholine liposomes - casein associates co-assembly and functionality.

The combination of data obtained from isothermal mixing calorimetry and light scattering allowed us to reveal the relationships between the character of the interactions of casein (ß-casein and sodium caseinate (SCN) particles) with phosphatidylcholine (PC) liposomes and their specific properties, such as, the hydrophilic-lipophilic balance of the surface properties, the size, and the architecture. The size distribution diagrams, obtained by dynamic light scattering for the particles of pure PC, ß-casein and their complexes, indicated the involvement of both the protein and liposome particles in the complex formation at pH 5.5 and low ionic strength (0.001 M). As this took place, the data of the extraction of the free PC from the aqueous solutions of the complex particles by diethyl ether showed about 90% of the binding of the PC liposomes by the ß-casein particles. As a result of the interactions, the complex particles showed markedly higher values of their density, calculated on the basis of the light scattering data, as compared to the ones, which were inherent to the pure protein particles. The generality of the importance of such a structural parameter as the density of the complex (protein + PC) particles for their function as protectors against oxidation for the unsaturated PC was supported by the data obtained for the complex particles formed between SCN and PC liposomes at pH 5.5, 40 °C and the low ionic strength (0.001 M). In addition, the importance of both the density and the architecture of the complex (protein + PC) particles for their susceptibility to the proteolysis in the gastrointestinal tract was illustrated by the example of the proteolysis of the complex (ß-casein + PC) particles in the simulated gastrointestinal conditions in vitro.

Structural and thermodynamic features of covalent conjugates of sodium caseinate with maltodextrins underlying their functionality.

The sodium caseinate (SCN)-maltodextrin (MD) covalent conjugates were prepared by a food-grade process involving the first step of the Maillard reaction. The covalent conjugates were prepared with different weight ratios of biopolymers (R(MD : SCN) = 0.4; 1; 2; 5) in the system using maltodextrins of strongly different dextrose equivalents (DE), i.e., DE = 2 and 10. We have observed that the covalent conjugation of SCN with MD, in contrast to their simple mixing, improved the protein solubility in an aqueous medium in a wide pH range that was more pronounced in the vicinity of the SCN isoelectric point (pH 3.8-4.4). The extent of SCN solubility was mainly governed by the weight/molar ratio of the biopolymers in the covalent conjugates, R(MD : SCN). Data of static multiangle laser light scattering showed that the revealed increase in the solubility of the conjugates could be predominantly attributable to the dramatic increase in their thermodynamic affinity for an aqueous medium. Which was most pronounced for the maltodextrin with the higher DE (DE = 10). The direct relationship between the increase in the solubility of the conjugates and the increase in their foaming ability, as compared against SCN, has been revealed as a rule both at neutral pH and at the pI. In addition, the found improvement in the protein foaming ability was also defined by both the weight/molar ratio (R(MD : SCN)) and the dextrose equivalent of the maltodextrins attached to the protein.

Surface activity at the planar interface in relation to the thermodynamics of intermolecular interactions in the ternary system: maltodextrin-small-molecule surfactant-legumin.

We report on the effect of potato maltodextrins with variable dextrose equivalent (Paselli SA-2, SA-6 and SA-10) on the surface behavior at the air-water interface of the mixture: legumin+small-molecule surfactant. Distinct in nature small-molecule surfactants (model: sodium salt of capric acid, Na-caprate; and commercially important: a citric acid ester of monoglyceride, CITREM) have been under our consideration. The role of the structure of both of the maltodextrins and the small-molecule surfactants in the effect studied has been elucidated by measurements in a bulk aqueous medium of the enthalpy of their interaction from mixing calorimetry, value of weight average molecular weight of the maltodextrins and the thermodynamics of the pair maltodextrin-solvent and maltodextrin-protein interactions from laser static light scattering. The combined data of mixing calorimetry and light scattering suggest some complex formation between the small-molecule surfactants and the maltodextrins. Predominantly hydrophobic interactions along with hydrogen bonding form the basis of the complexes. The effect of the maltodextrins on the thermodynamics of the protein heat denaturation and thereby on the protein conformational stability in the presence of the small-molecule surfactants has been studied by differential scanning calorimetry. The interrelation between the thermodynamics of intermolecular interactions in a bulk and the surface behavior at the planar air-water interface of the ternary systems (maltodextrin+legumin+small-molecule surfactant) has been elucidated by tensiometry. The effect of the maltodextrins on the surface activity of mixtures of legumin with the small-molecule surfactants is governed by the competitive in relation to the protein interactions with the small-molecule surfactants and a subsequent change in the thermodynamic properties of the both biopolymers, which are favorable to the ternary complex formation.

On relationships between molecular structure, interaction and surface behavior in mixture: small-molecule surfactant+protein.

We report on the effect of distinct in nature small-molecule surfactants (model, a sodium salt of capric acid, Na-caprate; and commercially important, a citric acid ester of monoglyceride, CITREM; a sodium salt of stearol-lactoyl lactic acid, SSL (Na(+)); polyglycerol ester, PGE (080)) on molecular properties in a bulk and at the air-water interface of globular legumin and random-coiled micellar sodium caseinate. The role of the structure of both proteins and small-molecule surfactants in the effect studied has been elucidated by measurements in a bulk aqueous medium of the enthalpy of their interaction from mixing calorimetry, the change in value of weight average molecular weight of the proteins and the thermodynamics of the pair protein-protein interactions from laser static light scattering as well as, in addition, by measurements of the change in hydrodynamic radius for micellar sodium caseinate from laser dynamic light scattering. The effect of the small-molecule surfactants on the thermodynamics of the protein heat denaturation and thereby on the protein conformational stability has been studied by differential scanning calorimetry in the case of globular legumin. The interrelation between the effects of the small-molecule surfactants on the properties of the proteins in a bulk and at the planar air-water interface has been elucidated by tensiometry. The combined data of mixing calorimetry, differential scanning calorimetry and laser light scattering suggest some complex formation between the small-molecule surfactants and the proteins in a bulk aqueous medium. Predominantly hydrophobic interaction along with electrostatic and hydrogen bonding form the basis of the complex formation. The found effect of the small-molecule surfactants on the surface activity of their mixtures with proteins is governed primarily by both the extent of the protein association, resulting in specific hydrophobicity/hydrophilicity of the surface of the protein associates, and the specific protein conformational stability, for the globular protein, produced by the interaction between the proteins and the small-molecule surfactants.