UV–Vis spectroscopy combined with chemometric study on the interactions of three dietary flavonoids with copper ions

•The pKa values of three dietary flavonoids quercetin, hyperin and rutin were determined.•Quercetin, hyperin and rutin were observed to form 1:1 Cu2+-flavonoid complex at pH 7.4.•2:1 Cu2+-flavonoid complexes were formed for hyperin and rutin at pH 6.0.•Glycoside moieties decrease the logK between flavonoids and Cu2+. The complex formation between a copper ion and the dietary flavonoid quercetin (QU) and its two glycosides hyperin (HY) and rutin (RU) was studied by the combined use of spectroscopic measurement and the chemometric method. The spectral changes of pH titration revealed two successively formed deprotonated species of QU: the first formed species was proposed to be the 3-hydroxyl group deprotonated QU, and the second was the quinone form QU, which was formed by oxidation after the hydroxyl groups in the B-ring were deprotonated at high pH values. Similar results were obtained for HY and RU with two deprotonated species forming at high pH values. UV/visible spectroscopy showed successive formation of CuL2 and CuL species of QU at pH 6.0, while only Cu2L was formed for HY and RU at this pH. Glycoside moieties in the C-ring of flavonoids decrease the conditional associated constants between flavonoids and Cu2+.