Zinc-quercetin complex: From determination to bioactivity

Abstract

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.

Antioxidant capacity and antimicrobial effects of zinc complexes of flavonoids – Does synergism exist?

“…Actually, some of the bio-effectiveness of flavonoids is based on their metal-complex formation. On the other hand, thanks to this characteristic, it was possible to develop the low-cost and efficient spectrophotometric determinations of flavonoids [32,33,36,37]. The most important analytical parameters of the reported methods for the determination of certain flavonoids based on zinc complexes are given in Table 1.…”

“…Experimental procedures, including sample preparations and conditions for testing the antioxidative ability of the flavonoids and their zinc complexes, were based on the proscriptions for the DPPH and ferric reducing antioxidant power (FRAP) tests previously reported for quercetin and morin [32,33]. Solutions of flavonoids at a concentration of 5•10 -6 mol l -1 were used for DPPH assay and to test for antioxidant capacity through the FRAP test.…”

“…where AABTS + is the absorbance of ABTS + solution + 5 ml of PBS buffer, and Asample is the absorbance of solution in the presence of sample after 3, 6, or 20 min of reaction. Further, the antimicrobial activities of rutin, hesperidin, and morin and their zinc complexes were determined as minimal inhibitory concentration (MIC), according to the procedures already reported for quercetin [32].…”

Comparative spectrophotometric determination of 3-hydroxyflavone based on zinc and aluminium complexes and their antioxidative profiles

“…Experimental measures and conditions for testing the antioxidative ability of 3HF and corresponding zinc and aluminium complexes were performed according to the procedures for the DPPH tests previously reported for chosen flavonoids (25)(26)(27), with the solutions of 3HF at a concentration of 5 × 10 -6 mol L -1 and concentrations of Zn 2+ and Al 3+ -ion c = 2.5 × 10 -6 mol L -1 .…”

Bioconversión de flavonoides cítricos utilizando células microbianas enteras

“…Los flavonoides son un grupo de compuestos fenólicos de bajo peso molecular que ejercen una amplia gama de actividades funcionales como, antimutagénicos, antibacterianos, antiinflamatorios, acciones antialérgicas, antivirales, antitrombóticos, vasodilatadores y fundamentalmente como antioxidante (Markovic, 2007). Su estructura se compone de tres anillos con actividad antioxidante (Figura 1.2).…”