Flavonoids constitute a large group of polyphenolic phytochemicals with antioxidant properties which are overwhelmingly exerted through direct free radical scavenging. Flavonoids also exhibit antioxidant properties through chelating with transition metals, primarily Fe(II), Fe(III) and Cu(II), which participate in reactions generating free radicals. Metal-flavonoid chelates are considerably more potent free radical scavengers than the parent flavonoids and play a prominent role in protecting from oxidative stress. To unravel the origin of their potent biological action extensive physico-chemical studies were undertaken to reveal the chemical structure, chelation sites, assess the impact of the metal/ligand ratio on the structure of the complexes and the capacity of flavonoids to bind metal ions. In spite of such extensive efforts, data on the composition, structure and complex-formation properties are incomplete and sometimes even contradictory. The aim of this paper is to give a personal account on the development of the field through a retrospective evaluation of our own research which covers approximately 40 complexes of flavonoids from different flavonoids subclasses (rutin, quercetin, 3-hydroxyflavone, morin and hesperidin) with several metal ions or groups and suggest directions for future research. Special emphasis will be given to the site of the central ion, the composition of the complexes, the role of pH in complex formation, the stability of metal-flavonoid complexes and their potential application for analytical purposes.
The composition and stability constant of a UO2(II)-rutin complex in 70% ethanol were determined
by suitable spectrophotometric methods and pH measurement. UO2(II) ion and rutin (3,3‘,4‘,5,7-pentahydroxyflavone-3-rhamnoglucoside) form a 1:1 complex in which the UO2(II) ion is linked to
rutin through the carbonyl and 5-hydroxyl group. The concentration stability constant of the
complex, log β1, ranged from 6.57 at pH 4.00 to 4.72 at pH 7.00. Conditions for spectrophotometric
determination of rutin, by complex formation with UO2(II) ion, were investigated. Beer's law was
obeyed in the concentration range from 1.0 × 10-5 to 2.0 × 10-4 M for rutin. Determination of
rutin in Rutinion forte tablets was demonstrated.
Keywords: Complex; rutin; uranilnitrate; stability constant; spectrophotometric methods
A fluorometric method, based on the fluorescence properties of the aluminium(III)-hesperidin complex, for the determination of hesperidin in human plasma and pharmaceutical forms has been developed and validated. The complex shows a strong emission in the presence of the surfactant betain sulphonate SB 12 at 476 nm with excitation at 390 nm. The linearity range for pharmaceutical forms of hesperidin was 0.06-24.4 μg mL -1 with a limit of detection, LOD, of 0.016 μg mL -1 and a limit of quantification, LOQ, of 0.049 μg mL -1 . Recovery values in the range 99.3-99.7 % indicate good accuracy of the method. A linear dependence of the intensity of fluorescence of the complex on the concentration of hesperidin in plasma was obtained in concentration range from 0.1-12.2 μg mL -1 . The LOD was 0.032 μg mL -1 while LOQ was 0.096 μg mL -1 . Recovery values were in the range 98.4-99.8 %. The reliability of the method was checked by an LC-MS/MS method for plasma samples and an HPLC/UV method for tablets with direct determination of hesperidin after separation. Linearity range in determination of hesperidin in pharmaceutical forms was obtained in the range from 0.05 to 10.00 μg mL -1 . The LOD was 0.01 μg mL -1 and the LOQ was 0.03 μg mL -1 . The linearity range for the determination of hesperidin in plasma was 0.02-10.00 μg mL -1 with an LOD 0.005 μg mL -1 and an LOQ of 0.015 μg mL -1 . The good agreement between the two methods indicates the usability of the proposed fluorometric method for the simple, precise and accurate determination of hesperidin in clinical and quality control laboratories.
Composition and stability constant of the Pd(II)-quercetin complex were determined by suitable spectrophotometric methods and pH-metric measurements in 507 Ethanol. It was found that the Pd 2 ion and quercetin form a 1:1 complex in which Pd 2 is linked to quercetin through the carbonyl and the 3-hydroxyl group. The stability constant log 1 ranged from 6.05 at pH 5X00 to 4.96 at pH 6X50X The conditions for the spectrophotometric determination of quercetin by means of complex formation were investigated. Beer's law was obeyed up to 5X00 Â 10 À5 M quercetin.
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