Spectrophotometric zinc(II) based determination of quercetin in pharmaceutical formulations

Abstract

Flavonoids are a group of polyphenolic compounds widely present in the herbal world representing an important part of human diet. Quercetin, which is a flavanol, makes 70% of total daily intake of flavonoids. Because of the characteristic chemical structure, quercetin has the ability of complexing metals and antioxidative ability. Using equimolar solution variation method it was determined that quercetin makes a complex with zinc(II)-ion in acidic environment (pH 5.25), in stoichiometric relation quercetin:zinc(II)-ion = 2:1, with absorption maximum on λmax=363 nm. The ability of quercetin to make complex compounds with zinc(II)-ion was used to develop 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 mg/L. LOD and LOQ were derived from the calibration curve and estimated as 0.03 mg/L and 0.1 mg/L respectively. Developed method is reproductive and accurate, as indicated by high value of correlation coefficient R=0.99996 and low value of SD=0.00122. Method was successfully used to determine quercetin content in dietary supplement tablets. Dietary supplements, proscribed for therapeutic and/or profilactic pruposes, usualy content quercetin combined with other flavonoids and ascorbic acid. Therefore, it was necessary to test the selectivity of proposed method. The reliability of the method was checked out by newly developed RP-HPLC/UV method for capsules with direct determination of quercetin after separation. The good agreement between the two methods indicates the applicability of the proposed spectrophotometric method for quercetin determination in dietary supplement tablets with high reproducibility, and enables direct and simple determination without its prior extraction from samples. In addition, the antioxidative ability of quecetin and zinc(II)-quercetin complex was determined using oxidation-reduction standardized methods DPPH and FRAP. The same samples were tested for antimicrobial activity against seven laboratory control strains of bacteria and one yeast. As a result of those tests, there are no obstacles to combine quercetin and zinc in the same formulation.