Zinc-quercetin complex: From determination to bioactivity

Flavonoids are a group of polyphenolic compounds widely present in the herbal world and playing an important role in the human diet. The flavanol quercetin makes up 70% of the total daily intake of flavonoids. Quercetin is able to complex with many metal ions, and it exhibits potent antioxidative ability. Using the equimolar solution variation method, it was confirmed that quercetin makes a complex with the zinc(II) ion at pH 5.25, in a stoichiometric ratio quercetin:zinc(II) ion = 2:1, with the absorption maximum l=363 nm. This fact was used to develop a simple, precise and accurate assay to determine the content of quercetin in various samples of heterogeneous composition. The proposed indirect spectrophotometric method can selectively determine quercetin in concentrations ranging from 0.1 to 6.0 mgL-1, with LOD and LOQ estimated as 0.03 mgL-1 and 0.1 mgL-1, respectively. The reliability of the proposed method was confirmed by a previously developed RP-HPLC/UV method. The proposed method was successfully used to determine the quercetin content in dietary supplement tablets, capsules and two onion extracts, with high reproducibility. The antioxidative ability of quercetin and the zinc(II)-quercetin complex was determined using DPPH and FRAP methods. The same samples were tested for antimicrobial activity against seven laboratory control strains of bacteria and one strain of yeast. As a result of those tests, there are no obstacles to combine quercetin and zinc in the same supplement formulation.