The non-covalent interacting forces and scavenging activities to three free radicals involved in the caseinate–flavonol (kaempferol and quercetin) complexes.

Polyphenolic substances kaempferol and quercetin are major flavonols found in plant foods, while caseins are major protein fractions in milk. In this study, the non-covalent interactions involved in the formation of caseinate–flavonol complexes as well as the resultant scavenging activities to three free radicals were assessed by multi-spectroscopic, molecular docking, and chemical assays. The results revealed that the binding of kaempferol/quercetin to caseinate was through a mode of energy transfer, resulting in fluorescence quenching of caseinate; meanwhile, both hydrophobic interaction and H-bonds were the forces involved in the resultant non-covalent caseinate–flavonol interactions. The results from UV-absorption and three-dimensional fluorescence spectra indicated that the two flavonols induced conformational change of caseinate. Moreover, quercetin with two hydroxyl groups at the carbon-4ʹ and -5ʹ positions of the B-ring exerted higher affinity to caseinate than kaempferol with one hydroxyl group at the carbon 4ʹ-position of the B-ring. In addition, the caseinate–quercetin complexes had higher scavenging activities against the DPPH, ABTS, and hydroxyl radicals than the caseinate–kaempferol complexes. In conclusion, both chemical structures and especially hydroxyl group numbers of flavonols are critical to the non-covalent casein–flavonol interactions and radical scavenging activities of the resultant complexes. [ABSTRACT FROM AUTHOR]