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

Abstract

Interaction between vitamin D3 with beta-lactoglobulin (β-LG) was studied by turbidimetric measurements covering vitamin concentrations up to 500 μM, 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 D3 in the absence and presence of β-LG (20, 40 and 100 μM) 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 μM vitamin D3 and 20 μM β-LG in 20 mM phosphate buffer at pH 7.0, suggested vitamin D3 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 β-LG properties as a carrier of hydrophobic vitamins or other hydrophobic nutraceuticals in order to enrich non-fat foods.