Flavonoids – natural phenolic complexing agents towards uranium

Flavonoids belong to a class of secondary plant metabolites. They are one of the largest classes of plant phenolics. All flavonoids consist of a 15 carbon skeleton. Flavonoids are most commonly known at least for four main properties: colored (to attract animals), absorbing ultraviolet light (to protect against damage), antimicrobial activity (to protect against bacterial or fungal infection) and antioxidant activity (which may be responsible for anticancer benefits) [1].
The structure of flavonoids is dominated by phenolic hydroxyl groups.

We studied the complex formation of the flavonoid kaempferol with uranium. Kaempferol is a 3,5,7-Trihydroxy-2- (4-hydroxyphenyl)-4H-1- benzopyran-4-on. As stated already flavonoids show strong absorption spectra in the UV-range. The change in their spectroscopic data was used to determine the stability constants. Spectra were evaluated with the factor analysis program Specfit [2]. All spectra of flavonoids show isosbestic points indicating a clear interaction between kaempferol and uranium(VI).
A strong interaction was found for kaempferol. The derived stability constant was assigned to be log β121 = 31.7 ± 0.4 at an ionic strength of 0.1 M for the reaction

UO22+ + C15H5O64- + 2 H+ → UO2C15H7O6 (1)

Data derived from studies with quercetin and hesperetin show an analogue stability constant for the non-charged complex.
We assume that the complex formation occur by a ring formation, whereas the flavonoids form rings between the phenolic group in position 3 or 5 respectively and the ketone oxygen in the benzopyran-4-one ring [3].

[1] Tiaz L, Zeiger E Plant Physiology 2006 4. Edition, Sinauer Associates Inc.
[2] Binstead R A, Jung B, Zuberbühler A D Specfit Global analysis system Spectrum Software Associates 2000-2003
[3] Malesev D, Kuntic V 2007 J. Serb. Chem. Soc. 72 921