[Spectral properties of interaction between caffeic acid and milk protein and the change in antioxidant capacity].

The interaction between caffeic acid and milk protein (alpha-casein, beta-casein, kappa-casein, alpha-lactalbumin, beta-lactoglobulin) were studied in this work. The binding constant K(A), binding force, binding distance r(0) and transfer efficiency E were evaluated by UV-absorption and fluorescence spectroscopy. The antioxidant capacity of the combination was determined using both 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay and ferric reducing antioxidant power (FRAP) assay. The results showed that the interaction between milk protein and caffeic acid resulted in the endogenous fluorescence quenching of milk protein, which belongs to a static quenching mechanism. The negative sign of free energy meant that the interaction process was spontaneous. The strength between caffeic acid and alpha-casein was electrostatic attraction (deltaH < 0, deltaS > 0), and that between beta-casein and alpha-Lactalbumin was hydrogen bonding (deltaH < 0, deltaS < 0). In addition, the strength of caffeic acid interacting with kappa-casein and beta-lactoglobulin was hydrophobic interaction (deltaH > 0, deltaS > 0). The binding distance (r(0) < 7 nm) proved that caffeic acid lead to a static quenching mechanism of milk protein. The antioxidant capacity of caffeic acid was inhibited by milk protein to different degrees.