Stability of Three Distinct Flavonol Binding Sites With Stability of Three Distinct Flavonol Binding Sites With Iron: Implications on the Iron-Binding and Antioxidant Iron: Implications on the Iron-Binding and Antioxidant Properties of FlavonolsProperties of Flavonols

AbstractThe determination of proton and ferric iron stability constants for complexation by the flavonoids 3-hydroxyflavone (flavonol), 5,7-dihydroxyflavone (chrysin), and 3’4’-dyhydroxyflavone are determined by pH potentiometric and spectrophotometric titrations and the linear least squares curve fitting program Hyperquad. The stoichiometry for complex formation was 1:1 in all cases. The three flavonoids were chosen for their hydroxyl substitution pattern—each possessing one of the three most commonly suggested sites for metal binding. The possibility for antioxidant activity by flavonoid chelation of ferric iron is discussed in terms of the measured stability constants and the most likely site for binding.

O

OH

O

OH

OH O

OOH

O

O

OH Quercetin

Chrysin

3'4'-dihydroxyflavonone

Flavonol

OH O

OH

O

OH

OH

OH

1

23

6

5

7

8

4

O1 2

1'

3

6'

2'

3'

4'

5'

Flavan base structure

A

B

C

FlavonoidsFlavonoids used for this study are pictured above with the flavan base structure and conventional ring labeling and numbering patterns indicated. Quercetin is also pictured with the three commonly cited metal binding sites indicated with arrows. The metal binding properties of the flavonoids are likely responsible for much of the observed antioxidant activity.

2003 National Meeting of the American Chemical Society, New York, NYMark D. Engelmann and I. Francis Cheng. Department of Chemistry, University of Idaho, Moscow, ID 84844-2343. Fax: 885-6173, email: ifcheng@uidaho.edu

FeLpro + Lanti FeLanti + Lpro

M + L ML =

[ML]

[M][L]

MLH)

Complex StabilityThe ability of an antioxidant ligand to displace one of a pro-oxidant nature depends on the stability constant for complex formation. In aqueous solution, competition with proton is always important and must be considered. The constants may be obtained by potentiometric or spectrophotometric titrations and Hyperquad least-squares regression analysis.

residuals in pH for selected data. Unweighted rms=3.50e-02

0 20 40 60 80 100 120point number

-0.1

0.0

0.1

Speciation and pH: data from c:\data\final analyses\chrysin\chrysin 052203.ppd

0

10

20

30

40

50

60

70

80

90

100

% fo

rma

tion

re

lativ

e to

Ch

ry

pH

4

5

6

7

8

9

10

chrysin

point number

weighted residual / 1.0659

3

2

1

0

-1

-2

-3

ORJPPQ

10 20 30 40 50 60 70 80 90 100 110 120

Potentiometric Titration

The blue diamonds are experimentally measured pH/titre-volume data pairs, the red line through the points is the model curve after Hyperquad refinement by least-squares regression analysis. The pictured data refers to the flavonoid Chrysin, pictured at left, which as two titratable protons. The stabilities are summarized to the right. The residuals between the experimental and model curves is presented in unweighted units of pH in the above histogram. After the weighting scheme is applied, which lessens the weight given at the inflection points, the residuals appear as indicated below. The units are normalized with respect to sigma, the RMS of the weighted, squared, summed residuals.

Iron and Oxidative stressIron has been implicated in the pathogenesis of many diseases.This is based on its ability to generate reactive oxygen species. Conditions of excess or freely mobile iron may lead to oxidative stress including non-specific oxidation of cellular components and subsequent erythrocyte ageing, ischemia reperfusion injury, and eventually, an exhaustive list of age-related degenerative diseases. Acute iron overload is treated by chelation therapy. Flavonoids are potential antioxidant chelates that are present as dietary non-nutrients. Iron chelation by flavonoids would depend upon the stability constants for complex formation.

0 4 8 12pH

0

20

40

60

80

100

% fo

rmati

on re

lative

to F

e

0 4 8 12pH

0

20

40

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80

100

% fo

rmat

ion re

lative

to F

e

iron 3'4'DHF

430

Wavelength

530

Wavelength

630

Wavelength

730

Wavelength

830

Wavelength

930

Wavelength

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Mo

lar

ab

so

rba

nce

OH O

OH

O

OH

OH

OH

O

OH

O

OH

OH O

OOH

O

O

OH

Fe

Fe

Fe

logK2 = 9.65

logK1 = 8.30

logK1 = 8.39

log110 = 20.91

logK2 = 13.37

logK1= 7.98

logK2 =11.40

log110= = 11.40

logK1 = 9.99

log110 = 13.29

Stability Constants

Eigenvectors, Molar Absorbances

Mathematically, a matrix that is the product of two or more matrices may be solved for the factors of the contributing matrices; these solutions are called eigenvectors. Chemically, the overall absorbance may be solved for the factors of the contributing individual absorbing species. Matrix solutions (smooth eigenvectors) exist for each absorbing species for the measured optical window; additional eigenvectors are manifested as noise—the experimental error involved in the measurement. In this manner, the number of absorbing species in solution may be deduced based on the number of smooth eigenvectors. Once the stability constants are refined, the molar absorbances for absorbing species may be calculated. They may be used to help assess the validity of the refined parameters by the presence of negative absorbances—indicative of mathematical compensation for a poor model, omitted species, or incorrectly measures experimental conditions. Also, the molar absorbances may be known, and compared with those obtained by calculation.

Data from curve HLCDTO

430

Wavelength

530

Wavelength

630

Wavelength

730

Wavelength

830

Wavelength

930

Wavelength

-0.05

0.0

0.05

0.1

0.15

0.2

Eig

enve

ctor

s

1

2

3

Absorbances: data at 715

5 10 15 20 25 30 35Point

0.25

0.3

0.35

0.4

0.45

0.5

0.55

Ab

so

rba

nce

% error in absorbance. ESS = 7.83e00

5 10 15 20 25 30 35point number

-1

0

1

Spectrum: curve no. 1 - HLCDTO point 18

430

Wavelength

530

Wavelength

630

Wavelength

730

Wavelength

830

Wavelength

930

Wavelength

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Ab

so

rba

nce

iron 3'4'DHF

point number

resid

ual / sig

ma 3

2

1

0

-1

-2

-3

680

102030

690

102030

700

102030

710

102030

720

102030

730

102030

740

102030

Spectrophotometric Titration

Speciation Calculations

Acknowledgements

Funding for this work was provided by an NIH R15 GM062777-01.

Thanks to Professor Peter Gans for his assistance with the Hyperquad software.

Spectrophotometry may also be used for the determination of stability constants. It was especially useful for the flavonoids, which absorb strongly in the visible range when complexed with iron. The above data refer to the 1:1 complexation of ferric iron by 3’4’dihydroxyflavone. 1,3-disulfato-4,5-dihydroxybenzene (Tiron) was used as a competing ligand. Blue diamonds are the experimental data, the red line is the refined model curve, the histogram at bottom, left is the % unweighted residuals at 715 nm, the black curve is the overall observed spectrum and the magenta and red curves are the individual contributing spectra of FeIIITiron and FeIII3’4’dihydroxyflavone respectively. The histogram at bottom, right is the weighted residuals, normalized by sigma, at each wavelength used for the refinement.

The stability constants for all flavonoid-metal complexes were determined spectrophotometrically, and are summarized at top, right.

Using the refined stability constants, the above speciation diagrams may be derived. Concentrations for the diagram at right are 1 micromolar each for ferric iron, flavonol, 3’4’dihydroxyflavone, and chrysin. Concentrations for the diagram at left are as follows:

1 M Fe3+ 48 M glutamate5 M each flavonoid 37 M alanine1 M DOPA 10 M ATP 11 M citrate

(Fe(OH)n are omitted for clarity, concentrations are taken from May, Linder and Williams, J. Chem., Dalton Trans. (1977) 588-595.

ConclusionIt can be concluded that of the flavonoids examined, 3’4’dihydroxyflavone, the flavonoid with the catechol binding site, is most important at physiological pH, and that chrysin does not bind strongly enough to be a significant physiological chelator of iron. Flavonol chelation of iron may be important under conditions of acidosis. These findings are consistent with in-vivo literature studies which find that the catechol is a key feature in antioxidant activity.

Christian David Sadik, Helmut Sies and Tankred Schewe“Inhibition of 15-lipoxygenases by flavonoids: structure–activity relations and mode of action” Biochemical Pharmacology Volume 65, Issue 5 , 1 March 2003, 773-781

Matsuda H, Morikawa T, Toguchida I, Yoshikawa M “Structural requirements of flavonoids and related compounds for aldose reductase inhibitory activity” Chem Pharm Bull (Tokyo). 2002 Jun;50(6):788-95

Peng IW, Kuo SM.“Flavonoid structure affects the inhibition of lipid peroxidation in Caco-2 intestinal cells at physiological concentrations.” J Nutr. 2003 Jul;133(7):2184-7.

Hendriks JJ, de Vries HE, van der Pol SM, van den Berg TK, van Tol EA, Dijkstra CD ” Flavonoids inhibit myelin phagocytosis by macrophages; a structure-activity relationship study” Biochem Pharmacol. 2003 Mar 1;65(5):877-85

Fe

FeFlavonol

Fe3’4’DHF

Fe(OH)4

Fe(OH)2

FeFeFlavonol

FeATP

FeCitrateFe3’4’DHF

FeDOPA

Fe(OH)Cit

FeGlutamateFe(Cit)2