Study on Interactions between Cyanidin-3-Glucoside and Four Kinds of Milk Proteins

The interactions between cyanidin-3-glucoside (C3G) and α-casein, β-casein, whey protein and β-lactoglobulin were studied by fluorescence spectroscopy, fourier transform infrared spectroscopy and circular dichroism. The results showed that C3G had fluorescence static quenching activity on the four kinds of milk proteins, and the combination of C3G with each milk protein was in a molar ratio of 1:1 approximately in the solution. The thermodynamic parameters of the milk protein showed the major binding force between C3G and α-casein was van der Waals forces and hydrogen bonding. The electrostatic attraction played a major role in the interaction between C3G and β-casein/whey protein/β-lactoglobulin. By the comparison of fluorescence quenching rate (84%, 74%, 77%, 75%), binding constant on 298, 318, 338 K (423.448, 362.994, 28.655×10⁴ L/mol.9.524, 8.056, 8.308×10⁴ L/mol.9.262, 6.940, 7.889×10⁴ L/mol.30.440, 11.830, 17.262×10⁴ L/mol) and binding distance (2.17, 2.66, 2.18, 2.19 nm) of the interaction on C3G and α-casein, β-casein, whey protein, β-lactoglobulin, it showed that the binding of α-casein and C3G was the most closely. Fourier transform infrared spectrometer and circular dichroism revealed that the addition of C3G increased α-helix, decreased β-sheet and turn angle of α-casein and increased α-helix, β-sheet and turn angle of β-casein. There was no significant changes in the whey protein. The addition of C3G decreased α-helix, increased β-sheet and turn angle of β-lactoglobulin.The results indicated that the interaction between C3G and four kinds of milk proteins was strong, which caused a conformational change of milk proteins.