Effect of small molecule surfactants on molecular parameters and thermodynamic properties of legumin in a bulk and at the air–water interface depending on a protein structure in an aqueous medium

This paper presents the effect of fatty acid salts, namely, Na-caprate and Na-palmitate on the legumin (11S globulin of Vicia Faba broad beans) molecular and thermodynamic properties in the bulk aqueous medium and at the air–water interface under different molecular states of the protein. That are the native state of the protein globule (pH 7.2, ionic strength of 0.05 mol dm⁻³), as well as the acidic denatured (pH 3.0, ionic strength of 0.01 mol dm⁻³) and the heat denatured ones (after heating at 90°C for 30 min, pH 7.2, ionic strength of 0.05 mol dm⁻³). In turn, an importance of the state of the small molecule surfactants in a solution in reference to the critical concentrations of micelle formation (CMC), for their effect on the protein properties, was also under our studying. The peculiarities of the legumin structure in the aqueous medium appeared in the different nature of the interactions between the protein and the fatty acid salts, as was indicated by the mixing calorimetry data. So, the hydrophobic contacts provided a basis for interactions between both the native and heat denatured legumin with the fatty acid salts. At the same time, the electrostatic interactions between the oppositely charged functional groups of the fatty acid salts and the acidic denatured protein formed principally a basis of their interactions in an aqueous medium. In response to interactions of the fatty acid salts with legumin the essential changes in the protein conformational stability, depending on both the protein molecular state and concentration of the fatty acid salts, were found using differential scanning calorimetry (DSC). The rather high level of the protein association was observed by light scattering in the bulk aqueous medium in the presence of the fatty acid salts. As this takes place, the surface hydrophilicity of the protein increased under the formation of the associates. The combined data of mixing calorimetry, differential scanning calorimetry and light scattering suggested the complex formation between legumin and the fatty acid salts. The interactions of the fatty acid salts with the protein produced a change in the surface activity for the mixture of the protein with the fatty acid salts. That is a decrease in the protein surface tension at the air–water interface for the mixed solutions in comparison with ones for both the protein and small molecule surfactant alone in the case of Na-caprate, and those are the intermediate values of the surface tension in the case of Na-palmitate. These results were observed independently of the protein state (native or acidic/heat denatured) in an aqueous medium. As this took place, the most dramatic increase in the surface activity was found for the mixtures of the acidic denatured protein with Na-caprate as if the most hydrophobic species were formed in this case. The combined data of mixing calorimetry, DSC, light scattering and tensiometry showed that the effect of the fatty acid salts on the legumin thermodynamic properties in a bulk and at interfaces is governed by a number of the key factors such as: a structure of both the protein and fatty acid salt (a length of the hydrocarbon chain); a degree of the protein association in the bulk aqueous phase (as a result of the interactions with the small molecule surfactants); a change in the protein conformational stability (flexibility) under the influence of the small molecule surfactants; as well as by the nature (hydrophobic, electrostatic) of the protein–small molecule surfactant interactions, determining ultimately the hydrophilic–lipophilic balance of the protein surface.