ABRASIVE STRIPPING SQUARE-WAVE VOLTAMMETRY OF SOME NATURAL ANTIOXIDANTS

ABRASIVE STRIPPING SQUARE-WAVE VOLTAMMETRY OF SOME NATURAL ANTIOXIDANTS

Šebojka Komorsky-Lovrić1 & Ivana Novak Jovanović2

1Ruđer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia2Institute for Medical Research and Occupational Health, Ksaverska cesta 2, HR-10000 Zagreb, Croatia

AIMEstimating oxidation potentials and antioxidant activity of microparticles of several flavonoids, including anthocyanidins (delphinidin, cyanidin, pelargonidin), catechins (epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG)) and myricetin.

PROCEDUREThe microparticles of flavonoids are mechanically immobilized on the surface of a paraffin-impregnate graphite electrode (PIGE), immersed into aqueous electrolyte (0.1 M KNO3) and studied by square-wave voltammetry.

2

3

456

78 1`

2`

3`4`

5`6`

+O

R1

R2

R3

OH

OH

HO

Cl-

A

B

C2

345

6

7

8 1`

2`

3`

4`

5`6`

R1

R2

R3

R4

OH

HO O

A C

B

2

345

6

7

8 1`

2`

3`4`

5`6`

O

OH

OH

OH

OOH

HO

A C

B

OH

anthocyanidinscyanidin: R1 = R2 = OH; R3 = Hdelphinidin: R1 = R2 = R3 = OHpelargonidin: R1 = R3 = H; R2 = OH

catechinsEGCG: R1 = R2 = R3 = OH; R4 = galloyl groupEGC: R1 = R2 = R3 = R4 = OHECG: R1 = R2 = OH; R3 = H; R4 = galloyl group

myricetin

Table 1. Net peak potentials of voltammograms of immobilized microparticles at pH 2.

Compound EP,1 / V vs. Ag/AgCl EP,2 / V vs. Ag/AgCl

delphinidin 0.327

EGCG 0.365 0.486

EGC 0.373

cyanidin 0.403

myricetin 0.415

pelargonidin 0.440

ECG 0.480

Effect of the nuber of hydroxyl groups on the B ring

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-0.1

0.0

0.1

0.2

0.3

ib

if

i

i / m

A

E / V vs. Ag/AgCl

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-0.05

0.00

0.05

0.10

0.15

0.20

ib

if

i

i / m

A

E / V vs. Ag/AgCl

Figure 1. Abrasive stripping square-wave voltammetry of EGCG (A) and ECG (B) on the paraffin-impregnated graphite electrode in 0.1 M KNO3 at pH 2. A net response DI and its forward (If) and backward (Ib) components are shown. The frequency is 100 Hz, the pulse amplitude is 50 mV and the potential increment is 2 mV.

(A) (B)

- the response of EGCG consists of two peaks: peak 1 - electrooxidation of pyrogallol moiety in the B ring,peak 2 - electrooxidation of galloyl moiety.

Pyrogallol group in EGCG is more easily oxidized than the catechoil group in ECG.

- the voltammogram of ECG is characterized by a single, broad peak.

- ECG peak originates primarly from the oxidation of ortho-hydroquinone group (catechol group) in the b ring of the molecule, while the oxidation of galloyl group constitutes only a minor component of the response.

Figure 2. Square-wave voltammograms of immobilized microparticles of delphinidin (A) and myricetin (B). The frequency is 8 Hz (A) and 100 Hz (B). All other experimental conditions are as in Fig. 1.

Effect of 4-oxo group in conjuction with a 2,3 – double bond in the C ring

E / V

0.0 0.2 0.4 0.6 0.8

I / m

A

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

I

If

Ib

(A)

-0.25 0.00 0.25 0.50 0.75 1.00 1.25

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

I / m

A

E / V vs. Ag/AgCl

IIf

Ib

(B)

The oxidation potential is particularly low if the hydroxyl group on the position 3 in the C ring of molecule is conjugated to pyrogallol group in the B ring.

The ketone group on the position 4 in the C ring inhibits oxidation.

CONCLUSIONS

- all investigated electrode reacrions were reversible- SWV peak potentials of immobilized microparticles are consistent with available

knowledge, indicating that ASSWV may be used for the estimation of antioxidant activity of solids

- the advantage of ASSWV is that there is no influence of adsorption of oxidation products on the electrode surface