Abstract
The general synthesis methods of bioflavonoid–metal complexes are considered to be unreliable due to the instability of flavonoids in air-saturated alkaline solutions. In this study, dihydromyricetin (DHM), as a representative bioflavonoid, was selected for complexation with various transition metal ions in an air-saturated alkaline solution to form DHM–metal(II) complexes, following the general synthetic procedure. After characterization, the metal complexes were hydrolyzed to observe the stability of DHM under acidic conditions via HPLC. The effects of synthetic conditions (metal ion, alkalinity, and reflux time) on DHM stability were then investigated by UV-vis spectroscopy and HPLC. Finally, using electron paramagnetic resonance, DHM and its analogs were observed with DMPO (5,5-dimethyl-1-pyrroline-N-oxide) to form a relatively stable free radical adduct. Multiple peaks corresponding to unknown compounds appeared in the LC spectra of the DHM–metal(II) complexes after hydrolysis, indicating that some DHM reacted during synthesis. Subsequently, the transition metal ion and solution alkalinity were found to have notable effects on the stability of free DHM. Furthermore, DHM and several of its analogs generated the superoxide-anion radical in air-saturated alkaline solutions. Their capacities for generating the superoxide anion seemed to correspond to the number and/or location of hydroxyl groups or their configurations. Interestingly, DHM can react with the superoxide anion to transform into myricetin, which involves the abstraction of a C3–H atom from DHM by O2−. Therefore, the general synthetic procedure for bioflavonoid–metal complexes in air-saturated alkaline solutions should be improved.
Highlights
Bioflavonoids are naturally occurring phenolic substances that can be isolated from a wide range of vascular plants, with more than 8,000 individual compounds currently identified
The results indicated that the airsaturated alkaline solution used in the general synthetic procedure is not suitable for the synthesis of bioflavonoid–metal complexes
The general synthesis of bioflavonoid–metal complexes involves dissolving bioflavonoids in an air-saturated alkaline solution to undergo the deprotonation of phenolic hydroxyl groups and chelation with metals
Summary
Bioflavonoids are naturally occurring phenolic substances that can be isolated from a wide range of vascular plants, with more than 8,000 individual compounds currently identified. The general procedure involves the deprotonation of the phenolic hydroxyl group(s) of the bioflavonoid into the corresponding phenolate, which chelates the metal ion in an air-saturated alkaline solution These metal complexes have been characterized by FT-IR and UV-vis spectroscopy, elemental analysis, X-ray analyses, and so on. DHM was dissolved in methanol to 0.20 mg/mL, and 100 μL was added to 1 mL of each of the air-, oxygen-, and nitrogensaturated alkaline solutions with continuous stirring for 1 min. To 100 μL of those solutions, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) (100 mg/L in methanol, 10 μL) was added, and the resulting mixtures were transferred into silica tubes and subjected to EPR analysis. DMPO (100 mg/L in methanol, 10 μL) was added to 100 μL of this solution, and the resulting mixture was transferred into a silica tube and subjected to EPR analysis. The resulting mixtures were transferred to silica tubes and subjected to EPR analysis
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