Investigations on the Effect of Fatty Acid Additives on Casein Micelles: Role of Ethylenic Unsaturation on the Interaction and Structural Diversity

Casein, one of the major constituent of milk protein, is considered to be a good candidate for oral drug delivery system. Also, milk transports various essential fatty acid to blood through dietary supplements. In this study, we have explored the alteration in the structural characteristic in terms of the modulations in the microenvironment of the protein in the presence of different types of fatty acids. Herein, we have observed that the unsaturation of fatty acids mostly affects the structure of casein micelles (CMs) by impinging upon the hydrophobic force of interaction following a decrease in the electrostatic interaction of various amino acid unit. Alteration of such forces is responsible for the increase in the aggregate size, modification in the protein secondary structure, and different morphology of CMs. Fluorescence behavior of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4<i>H</i>-pyran indicates that the rigidity of the microenvironment is the main characteristic of the fatty acid binding, and the binding constant increases with the fatty acid chain length for saturated fatty acid or with the introduction of unsaturation onto it. Fluorescence lifetime imaging microscopy study indicates that the microstructure of CMs becomes more compact in the presence of unsaturated fatty acids, and this is also responsible for the increase in the diffusion time of the probe. Moreover, decrease in the fluorescence of extrinsic probe 8-anilinonaphthalene-1-sulfonate with the addition of unsaturated fatty acid reveals that these fatty acids alter the electrostatic interaction between casein units, more specifically in case of the surface-bound κ-casein. Therefore, this study provides a very useful information on the binding of fatty acids and helps to evaluate other fatty acid, as well as different small molecules binding in the applicative medicinal purpose