Characterization of flavonoid-protein interactions using fluorescence spectroscopy: Binding of pelargonidin to dairy proteins

In this study, the interaction between the flavonoid pelargonidin and dairy proteins: β-lactoglobulin (β-LG), whey protein (WPI), and caseinate (CAS) was investigated. Fluorescence experiments demonstrated that pelargonidin quenched milk proteins fluorescence strongly. However, the protein secondary structure was not significantly affected by pelargonidin, as judged from far-UV circular dichroism. Analysis of fluorescence data indicated that pelargonidin-induced quenching does not arise from a dynamical mechanism, but instead is due to protein−ligand binding. Therefore, quenching data were analyzed using the model of independent binding sites. Both β-LG and CAS, but not WPI, showed hyperbolic binding isotherms indicating that these proteins firmly bound pelargonidin at both pH 7.0 and 3.0 (binding constants ca. 1.0 × 105 at 25.0 °C). To investigate the underlying thermodynamics, binding constants were determined at 25.0, 35.0, and 45.0 °C. These results pointed to binding processes that depend on the structural conformation of the milk proteins.