Interaction of vitamin D3 with beta-lactoglobulin at high vitamin/protein ratios: Characterization of size and surface charge of nanoparticles

Interaction between vitamin D-3 with beta-lactoglobulin (beta-LG) was studied by turbidimetric measurements covering vitamin concentrations up to 500 Mu M, within a wide range of vitamin concentration and at high vitamin/protein ratios, both conditions that have not been assayed in previous studies. Turbidity of vitamin D-3 in the absence and presence of beta-LG (20, 40 and 100 Mu M) in 20 mM phosphate buffer at pH 7.0 proved the expected vitamin-protein interaction as well as the effect of protein concentration. In order to estimate the proportion of bound vitamin in the vitamin-protein complex, a binding parameter (BP) was defined and its dependence on protein concentration was analysed. Fluorescence quenching with acrylamide for 100 Mu M vitamin D-3 and 20 Mu M beta-LG in 20 mM phosphate buffer at pH 7.0, suggested vitamin D-3 interact in the hydrophobic calix in the protein. Circular dichroism experiments showed the binding of the vitamin causes conformational changes in the secondary protein structure. Particle size and zeta potential determinations were also carried out in order to establish possible conformational models of interaction vitamin-protein. The higher the vitamin concentration, the greater the bound vitamin proportion was; which could be due to a cooperative phenomenon and/or a stacking process. These studies would be useful for a better understanding of the beta-LG properties as a carrier of hydrophobic vitamins or other hydrophobic nutraceuticals in order to enrich non-fat foods.