ISSN 0012�5008, Doklady Chemistry, 2011, Vol. 436, Part 2, pp. 31–33. © Pleiades Publishing, Ltd., 2011.Original Russian Text © V.Yu. Tyurin, Wu Yaohuang, A.V. Dolganov, E.R. Milaeva, 2011, published in Doklady Akademii Nauk, 2011, Vol. 436, No. 5, pp. 641–643.

31

Sterically hindered phenols are biologically activecompounds that are widely used as antioxidants andstabilizers of biosubstrates [1, 2]. Lately, these com�pounds have attracted attention due to the possibilityof conjugated electron and proton transfer, which isimportant for biochemistry, in their molecules [3–5].The relationship between the electrochemical proper�ties and antioxidant activities of phenols is also aninteresting issue [6–10]. The antioxidant activities ofcompounds are often estimated using the reactionwith the 2,2�diphenyl�1�pycrylhydrazyl (DPPH) sta�ble radical [11, 12]. However, this method has certainlimitations. For example, it is inapplicable for chro�mophores that have an absorption band close to theband of DPPH (512 nm). In addition, this methodcannot be used with relatively high concentrations of

reacting compounds, because DPPH has high extinc�tion coefficient. Therefore, selection of a new conve�nient method for estimating the antioxidant activity isa topical task.

The purpose of this work is to study the electro�chemical properties and antioxidant activities of 2,6�di�tert�butylphenol derivatives I–V containing phos�phonate groups by cyclic voltammetry (CV) [13,14].The antioxidant activities of these compounds werecompared with the activity of the commercial antioxi�dant ionol (2,6�di�tert�butyl�4�methylphenol). Inorder to block the abstraction of hydrogen atom, thebehavior of these compounds was compared with ananalog, namely, phosphonic acid containing an ani�sole fragment (VI).

HO

Bu

Bu

C P

O

OH

CH

OH NHO

Bu

Bu

C P

O

OH

(CH2)2N

HO

Bu

Bu

C P

O

OH

(CH2)2 N

HO

Bu

Bu

C

OH

P

O CH2CH2N

OHO HO

Bu

Bu

C P

OH

CH2

ON

O

MeO

H

P(OH)2

P(OH)2

O

O

t�

t�

t�

t� t�

t�

t�

t�

t�

t�

2 2 2

22

I II III

IV V VI

H H H

H

Voltammograms were measured in a three�elec�trode cell using an IPC�pro potentiostat (Volta, Rus�sia) under argon. A stationary platinum electrode22 mm in diameter was used as the working electrode

and a platinum wire served as the auxilliary electrode.An (Ag/AgCl/KCl) electrode with a waterproof dia�phragm was used as the reference electrode. Thepotential sweep rate was 0.2 V s–1. The supportingelectrolyte was a 0.5 M solution of Bu4NBF4 (99%,Akros, Russia) twice recrystallized from aqueousEtOH and dried in vacuum for 48 h at 50°С. Acetoni�trile was purified and dried by a known procedure [9].The concentration of the compounds was 1 mM.

CHEMISTRY

Antioxidant Activity Assay of 2,6�Di�tert�Butylphenols with Phosphonate Groups Using Cyclic Voltammetry

V. Yu. Tyurina, Wu Yaohuanga, A. V. Dolganovb, and E. R. Milaevaa

Presented by Academician N.S. Zefirov July 1, 2010

Received August 4, 2010

DOI: 10.1134/S0012500811020042

a Moscow State University, Moscow, 119992 Russiab Nesmeyanov Institute of Organoelement Compounds,

Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia

32

DOKLADY CHEMISTRY Vol. 436 Part 2 2011

TYURIN et al.

The reactions of these compounds with DPPHwere monitored, and their antioxidant activities werequantitatively estimated using CV. Since the voltam�mogram of DPPH in the anodic region shows single�electron reversible waves corresponding to radical oxi�dation and reduction, respectively [15, 16], the peakcurrents of these waves can be found from the Ran�dles–Sevcik equation [14]

Here, z is the charge of the discharging species, S is theelectrode surface area, D is the diffusion coefficient ofthe oxidized and reduced forms, C0 is the concentra�tion of the compound, and v is the potential sweeprate.

It follows from the above equation that at a speci�fied electrode surface area and potential sweep rate,

Imax 2.72 105× z3/2SD1/2C0

v1/2

.=

the ratio of the DPPH oxidation or reduction peakcurrents during the reaction is equal to the concentra�tion ratio

where I is the peak current for a given DPPH concen�tration; I0 is the peak current for the initial DPPHconcentration C0.

Hence, the change in the peak current of reversiblewave is determined by the change of the DPPH con�centration and can serve for estimating the antioxidantactivity of the reacting substance. Using known I0

value at a certain specified DPPH concentration andthe results of monitoring of the time variation of thecurrent I, one can construct the kinetic curve forDPPH concentration vs. time.

Compounds I–III tend to undergo irreversibletwo�electron oxidation localized on the phenolicgroup of sterically hindered phenolic fragment to givephenoxylium cation. Owing to the presence of phos�phonate groups in which fragments of isomericpyridines are attached to phosphorus, the oxidationpotential shifts to the cathodic region with respect toionol (table). The reverse CV branch has a reductionpeak of the proton, which was identified upon theaddition of acids (HClO4, CF3COOH).

A nontrivial electrochemical behavior wasobserved for compounds IV (Fig. 1) and V whose mol�ecules contain two redox�active sites, the phenolicgroup and the nitrogen atom of the pyrrolidone frag�ment connected to the phosphorus atom through anethylene bridge. These redox sites have differentnature and mutually affect each other. The voltammo�gram of compound IV shows two irreversible single�electron waves in the anodic region: one at E = 1.15 Vand the other at E = 1.49 V.

Tertiary amines are known to undergo electrooxi�dation [17, 18]. The first oxidation wave correspondsto the oxidation of pyrrolidone nitrogen to radical cat�ion. The irreversibility of the first wave may be due tothe fact that the radical cation formed, which is cen�tered on nitrogen, undergoes chemical transforma�tions, for example, attacks a second molecule of IV.The second irreversible one�electron wave may corre�spond to the oxidation of the phenolic group to radicalcation followed by fast chemical proton release step.

The anodic behavior of compound V is similar(table).

Thus, depending on the structure of the para�sub�stituent, the oxidation of compounds I–V is accompa�nied by either one or two oxidation waves for which thepotential of the peak, the number of transferred elec�trons and the degree of reversibility depend apprecia�bly on the electronic effects of substituents.

I/I0 C/C0,=

Oxidation potentials (Ea) and antioxidant efficiency values (A)for compounds I–VI

Compound Ea A, %

I 1.38 42

II 1.35 47

III 1.35 47

IV 1.15, 1.55 59

V 1.15, 1.49 52

VI 1.65* –

Ionol 1.52* 12

Note: The A values were determined from the rate of hydrogen atomabstraction from DPPH (CV method). CH3CN, Pt,Ag/AgCl, Bu4NBF4, c = 1 mM, [DPPH] : [additive] = 1 : 1,reaction time 10 min.* Published data [10].

0.020

0.015

0.010

0.005

0

–0.0050 500 1000 1500 2000

Ea, mV

I, mA

Fig. 1. Cyclic voltammograms of compound IV; CH3CN,Pt, Ag/AgCl/KCl, 0.05 M Bu4NBF4, c = 1 mM.

DOKLADY CHEMISTRY Vol. 436 Part 2 2011

ANTIOXIDANT ACTIVITY ASSAY 33

Since all of the studied phenols have an oxidationwave at more anodic potentials than DPPH, they areconvenient objects for the antioxidant activity assaythat we propose. Figure 2 shows the voltammogramsof DPPH in the presence of compound I. It can beseen that as the reaction goes on, the peak current ofboth waves decreases. We constructed kinetic curvesusing the currents I and I0, which correspond to thepeak height of the first wave in the presence and in theabsence of the additive, respectively.

The antioxidant action of compounds I–V wasmeasured using the quantity A (%), which character�izes the percentage of unreacted DPPH (A = ((I0 –Ifin)/I0 × 100%, reaction time 10 min). The table pre�sents the A values for compounds I–V. It can be seenthat the activity varies in the series

.

The highest activity was found for phenols IV and Vfor which the A values were 59 and 52%, respectively.Compounds I–V have much higher antioxidant activ�ity than ionol. Compound VI has no antioxidant activ�ity (A is nearly zero). Note that generally compoundsof similar structure have approximately equal activi�ties. Phenols IV and V are oxidized at the least anodicpotentials. This means that the redox properties and

I III< II V IV< <≈

the inhibitory properties of antioxidants are corre�lated.

ACKNOWLEDGMENTS

Compounds I–VI were synthesized at the Chair ofOrganic Chemistry of the Department of Chemistry,Moscow State University, by A.A. Prishchenko,M.V. Livantsov, O.P. Novikova, and L.I. Livantsovaand kindly provided for investigations.

This work was supported by the Russian Founda�tion for Basic Research, project no. 09–03–00743.

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–0.010

400200 600 800 1000Eа, mV

–0.015

–0.005

0

0.005

0.010

0.015I, mA

Fig. 2. Cyclic voltammograms of DPPH in the presence ofcompound I; CH3CN, Pt, Ag/AgCl/KCl, 0.5 MBu4NBF4, [DPPH] : [additive] = 1 : 1, c = 1 mM, reactiontime 10 min.