Spectroscopic characterization of transition metal complexes with quercetin in aqueous solutions

Introduction: Flavonoids are ubiquitous polyphenolic compounds in fruits and vegetables, also referred as Vitamin P, which participate and redox reactions and influence the bioavailability of metals. Quercetin, one of the most abundant dietary flavonoids, has been shown to form a copper complex that binds to DNA and can cause strand cleavage.

Objectives: We want to determine the binding site of transition metals in flavonoids examplified for quercetin. The photreactivity of such complexes and their potential effect on DNA structure is investigated.

Material and methods: Quercetin (3,3’,4’,5,7pentahydroxylflavone) was purchased from SIGMA, salmon testes genomic DNA was from CALBIOCHEM. IR spectra of quercetin films were recorded by attenuated total reflectance (ATR) using a VECTOR 22 FTIR Spectrophotometer (BRUKER). CD spectra were measured with a JASCO 800 instrument.

Results:

The formation of copper–quercetin complexes was observed by the red shift of the quercetin absorption. Zn2+ showed low affinity in the mM range, whereas copper formed complexes already at nM concentratrions. In contrast to Zn, Cu2+ strongly quenches quercetin fluorescence, probably as a result of different ligand to metal charge transfer efficiencies. To address the complexation mode, IR-spectra were recorded from quercetin film is solution. The addition of transition metals affects a band at 1590 cm-1, typical of carbonyl stretching, suggesting that under our experimental conditions, coordination involves the C=O in position 4. Preliminary experiments show structural affects of the interaction of the complex with DNA.

Conclusions:

Copper-Quercetin complexes were synthesized in different solvents and the complex is stable under both aerobic and anaerobic conditions. The carbonyl at C4 of quercetin is the binding site for Cu2+ and affects photooxidative behaviour of the complex. Using ATR-technique highly hydrophbic flavonoids can be studied under fully hydrated conditions.