Characterization of the interaction between tartrazine and β-lactoglobulin under different pH conditions: multispectral analysis and computer simulation

Unraveling the interaction mechanism between the food azo dye tartrazine (Tz) and β -lactoglobulin ( β -LG) under varying pH conditions is crucial for understanding their safe application in food systems. Molecular docking indicated that Tz primarily bound to the β -hydrophobic barrel of β -LG. The protonation of lysine residues diminished with increasing pH, which reduced hydrogen bond formation. The binding energy decreased from 8.77 kcal/mol (pH 2.0) to 8.43 kcal/mol (pH 7.4). Molecular dynamics simulations indicated that Tz bound exerts the greatest structural influence on β -LG at pH 2.0. Fluorescence and thermodynamic analyses confirmed that Tz quenched β -LG fluorescence through a static mechanism, through their interaction was a spontaneous process driven mainly by hydrophobic forces. The protonation level of β -LG gradually decreased with increasing pH, and electrostatic repulsion between β -LG and Tz was observed at pH 7.4. As a result, the binding constant between Tz and β -LG decreased from 6.669 × 10 5  L/mol (pH 2.0) to 3.596 × 10 5  L/mol (pH 5.2) and 0.956 × 10 5  L/mol (pH 7.4), respectively. Furthermore, Tz induced changes in the secondary structure content of β -LG and reduced its surface hydrophobicity under all pH conditions.