Combined inhibitory effect of sulfur-containing phenol SO-4 with natural and synthetic antioxidants in the oxidation of methyl oleate

Article abstract of DOI:10.1007/s11172-006-0429-2

In a model system of initiated oxidation of methyl oleate, the antioxidant activities of 3-hydroxy-2-ethyl-6-methylpyridinesuccinate (mexidol) and bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propyl] disulfide (SO-4) were studied and compared with those of α-tocopherol and 1-hydroxy-2,6-di-tert-butyl-4- methylbenzene (dibunol). A linear pattern of dependence of the inhibitory effect on the concentration of compounds was established. The ability of antioxidants to decompose hydroperoxides and inhibit their accumulation was revealed. The combined inhibitory effects of SO-4 with mexidol, α-tocopherol, and phospholipids were described for the first time. The rate constant for disproportionation of the SO-4 phenoxyl radicals, k ₉ = 0.90.10 ³ L mol^-1 s^-1, was determined by steady-state photolysis. The rate constant k ₁₀ ^eff for the reactions of SO-4 phenoxyl radicals with lipids characterized by different unsaturation degrees were determined for methyl oleate, linolic and arachidonic acids containing one, two, and four multiple bonds, and phospholiopids containing polyunsaturated fatty acids.

Full text of DOI:10.1007/s11172-006-0429-2

1
380
Russian Chemical Bulletin, International Edition, Vol. 55, No. 8, pp. 1380—1387, August, 2006
Combined inhibitory effect
of sulfurꢀcontaining phenol SOꢀ4 with natural and synthetic
antioxidants in the oxidation of methyl oleate
M. G. Perevozkina,aꢀ N. M. Storozhok, A. P. Krysin, and V. E. Borisenko
a
b
c
aTyumen State Medical Academy,
5
2 ul. Odesskaya, 625023 Tyumen, Russian Federation.
Fax: +7 (345 2) 20 6200. Eꢀmail: tgma@tgma.info
b
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry,
Siberian Branch of the Russian Academy of Sciences,
9
prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383 2) 34 4752
c
Tyumen State University,
1
0 ul. Semakova, 625003 Tyumen, Russian Federation
In a model system of initiated oxidation of methyl oleate, the antioxidant activities of
3
ꢀhydroxyꢀ2ꢀethylꢀ6ꢀmethylpyridinesuccinate (mexidol) and bis[3ꢀ(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀ
hydroxyphenyl)propyl] disulfide (SOꢀ4) were studied and compared with those of αꢀtocoꢀ
pherol and 1ꢀhydroxyꢀ2,6ꢀdiꢀtertꢀbutylꢀ4ꢀmethylbenzene (dibunol). A linear pattern of depenꢀ
dence of the inhibitory effect on the concentration of compounds was established. The ability
of antioxidants to decompose hydroperoxides and inhibit their accumulation was revealed. The
combined inhibitory effects of SOꢀ4 with mexidol, αꢀtocopherol, and phospholipids were
described for the first time. The rate constant for disproportionation of the SOꢀ4 phenoxyl
radicals, k9 = 0.90•103 L mol
–1
s
–1
, was determined by steadyꢀstate photolysis. The rate
eff
constant k10 for the reactions of SOꢀ4 phenoxyl radicals with lipids characterized by different
unsaturation degrees were determined for methyl oleate, linolic and arachidonic acids containꢀ
ing one, two, and four multiple bonds, and phospholiopids containing polyunsaturated fatty
acids.
Key words: αꢀtocopherol, sulfurꢀcontaining antioxidants, 3ꢀhydroxypyridine derivatives,
mexidol, phospholipids.
The oxidation of food and cosmetic oils or fatty bases
disulfide (SOꢀ4) has been synthesized at the Novosibirsk
Institute of Organic Chemistry, Siberian Branch of
the RAS. This compound does not have any local or
general toxicity, does not affect the embryogeny or develꢀ
opment of the descendants, which allows its use in cliniꢀ
cal practice.6
Thus, the purpose of this work was to study the antiꢀ
radical and antioxidant activities of mexidol and the sulꢀ
furꢀcontaining phenol SOꢀ4 in comparison with known
inhibitors, and to study the prospects of combined use of
AOx of different classes as additional drugs for nonspeꢀ
cific therapy of many diseases and for stabilization of
model and natural unsaturated lipids against oxidation.
of pharmaceuticals is often inhibited using synergistic
compositions, either consisting of several oxidation inꢀ
hibitors that enhance the action of one another or includꢀ
ing an antioxidant (AOx) and a synergist. As a rule, the
latter does not exert a selfꢀsustained inhibitory action, but
its presence markedly enhances the efficiency of the inꢀ
hibitor (phospholipids). The range of nontoxic AOx has
markedly extended in recent years. Despite the wide use
of 3ꢀhydroxypyridine derivatives, viz., mexidol (3ꢀhydrꢀ
oxyꢀ2ꢀethylꢀ6ꢀmethylpyridinesuccinate) and emoxipin, in
clinical practice where they showed good therapeutic efꢀ
fect, the mechanism of the antioxidant effect of these
compounds is little studied.1 Sulfurꢀcontaining phenols
also arouse considerable interest because they efficiently
retard oxidation by several mechanisms including the reꢀ
action with peroxyl radicals and destruction of hydroperꢀ
—3
Experimental
The antiradical activity of the AOx was tested in the system
of initiated oxidation of ethylbenzene using chemilumineꢀ
4
—6
oxides to give molecular products.
A sulfurꢀcontaining
7
phenol, bis[3ꢀ(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxyphenyl)propyl]
scence (CL). The luminescence intensity was measured on a
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1330—1337, August, 2006.
066ꢀ5285/06/5508ꢀ1380 © 2006 Springer Science+Business Media, Inc.
1

Inhibitory effect of phenol SOꢀ4
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 8, August, 2006
1381
photometric setup designed at the Institute of Chemical Physics
of the RAS. The oxidation of ethylbenzene was carried out in a
glass cell located in a lightꢀtight chamber of the photometric
setup equipped with a FEUꢀ29 photoelectron multiplier. The
cell has a thermostatically controlled shell. Dustꢀ and water
vaporꢀfree air was passed through the cell. The substance under
study was injected into the oxidation cell in the course of the
reaction by means of a syringe unit. The emitted light was foꢀ
cused on the photoelectron multiplier by a system of spherical
phenols was carried out in specially designed chamber equipped
by two 250ꢀW mercury lamps.
Commercial αꢀTP and dibunol (Serva, Germany) were used.
Methyl oleate synthesized at the Novosibirsk Institute of Orꢀ
ganic Chemistry of the Siberian Branch of the RAS and purified
by vacuum distillation in an argon flow at 105 °C was used as the
oxidation substrate. The purity of phospholipids (egg phosphatiꢀ
dylcholine) was determined by TLC. Mexidol was synthesized
at the N. M. Emanuel´ Institute of Biochemical Physics (IBCP),
SOꢀ4 was prepared at the N. N. Vorozhtsov Institute of Organic
Chemistry, Siberian Branch of the RAS. The purity of antioxiꢀ
dants was determined by UV and IR spectroscopy and HPLC on
a Milichrome Aꢀ02 chromatograph with a spectrophotometric
detector composed of a doubleꢀbeam UV spectrophotometer
and a Nucleosil 100ꢀ5 column. The chromatograms were reꢀ
corded using gradient elution by water, methanol, and acetoniꢀ
dishes. The oxidation was initiated by azobisisobutyronitrile
–1
(
AIBN) (concentration 3•10–3 mol L ) at 60 °C. The rate of
free radical generation measured experimentally using a referꢀ
ence inhibitor, chromane C, was 2.3•10– mol L
8
–1 –1
s . The
luminescence was enhanced by the luminophore 9,10ꢀdibromoꢀ
anthracene (concentration 5•10–4 mol L ), which does not
affect the oxidation kinetics. The concentration of the inhibitor
was (1—5)•10–4 mol L . The resulting kinetic curves have a
typical Sꢀshape. The key kinetic characteristic of the CL curve is
the slope of the tangent line in the inflection point, which
is proportional to the maximum rate of AOx consumption
–1
–1
–1
trile (supply flow rate 100 µL min , cell volume 1.2 µL). The
content of the major AOx was ≥99.9%.
Results and Discussion
[
d(I /I )/dt] . This value was used to calculate k (see Ref. 7)
0 max 7
taking into account the equation
The rate constant of the elementary reaction of AOx
with peroxyl radicals k was determined by CL (according
to the generally accepted scheme, this is reaction (7)):
7
[
d(I /I )/dt]
= (0.22±0.02)k7
,
0
max
7
where k6 is the rate constant for recombination of peroxide
8
8a
InH + RO2•
ROOH + In•,
(7)
radicals (for ethylbenzene, k = 4.1•10 exp[–2100/(RT )]).
6
The oxidation kinetics was measured by monitoring the abꢀ
sorption of oxygen in Warburg type gage setup during oxidation
of a model substrate, methyl oleate, in the presence of an inert
where InH is the oxidation inhibitor, In• is the inhibitor
radical, and RO2 is the peroxyl radical. The results are
summarized in Table 1.
•
9
solvent (chlоrobenzene). The process was initiated by thermal
decomposition of AIBN at 60 °C, the initiation rate under the
The antiradical activity (k7) was compared with that
experimental conditions was 4.2•10– mol L
8
–1 –1
s
. The inducꢀ
for αꢀTP and dibunol. The rate constants k for SOꢀ4 and
7
4
4
tion period (τ ) was found as the intercept on the xꢀaxis made by
dibunol were similar, being equal to 1.3•10 and 1.4•10
i
the perpendicular from the point of intersection of the tangents
to the kinetic curve.8b
The inhibitory effect was estimated based on the antioxidant
activity (A) determined quantitatively by the formula A =
–1 –1
10
L mol
s , respectively. According to published data,
4
–1 –1
the k value for dibunol is 2.0•10 L mol
of mexidol, k = 2.8•10 L mol
s
. In the case
7
4
–1 –1
s
, which is consistent
with published data. The k value for αꢀTP is equal to
7
1
1
7
(
τ – τ )/τ (τS and τ are the induction periods of substrate
i S S i
6
–1 –1
3
.6•10 L mol
s , which is 250 times higher than those
oxidation in the absence and in the presence of the AOx under
study, respectively) and was compared with the action of the
reference inhibitor by using the τ /τ ratio where τ is the
for SOꢀ4 or dibunol.
The kinetic curves for the oxidation of methyl oleate
in the presence of equal concentrations of various AOx
are shown in Fig. 1. The introduction of the AOx not only
i
ref
ref
9
induction period for the reference inhibitor. The accumulation
kinetics of hydroperoxide was studied by iodometric back titraꢀ
tion with methyl oleate autoxidation at 60 °C in chlоrobenzene.8
The reference inhibitors used included 6ꢀhydroxyꢀ2,5,7,8ꢀ
tetramethylꢀ2ꢀphytylchromane (α ꢀtocopherol, αꢀTP) and
c
Table 1. Kinetic characteristics of antioxidants of various chemiꢀ
cal structuresa
1
ꢀhydroxyꢀ2,6ꢀdiꢀtertꢀbutylꢀ4ꢀmethylbenzene (dibunol) present
in comparable concentrations. The efficiency of combined inꢀ
hibitory effect of the mixture was either characterized quantitaꢀ
tively by the absolute value (∆τ) of the difference between the
induction times of methyl oleate oxidation in the presence of AOx
–
1
4
τindb
/min
Ac
Antioxidant
k7•10
/
L mol– s–1
Mexidol
SOꢀ4
Dibunol
αꢀTocopherol
2.80
1.30
1.40
110
240
190
160
3.2
8.2
6.3
5.2
(
τ ) and the simple sum of the components (Στ , additive action,
Σ
i
∆
τ = τ – Στ ) or expressed in relative units (∆τ/Στ )•100%.
Σ
i
i
The phenoxyl radicals were obtained by steadyꢀstate phoꢀ
360.00
tolysis and identified by UV spectroscopy using a Specord Мꢀ40
spectrophotometer with automated recording and computer proꢀ
cessing of the spectra. The amount of phenols was estimated by
IR spectroscopy (at 3620 cm–1) on a Specord IRꢀ75 spectrophoꢀ
tometer; the same method was used to study the reaction of
phenoxyl radicals with oxidation substrates. The photolysis of
a
Here and in Tables 2—4, the significance of the differences was
evaluated using the Student criterion; an error probability less
than 0.05 was considered to be acceptable.
b
The AOx concentration was 2•10 mol L–1
–4
.
c
The oxidation conditions are presented below Fig. 1, T = 60 °C.

1382
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 8, August, 2006
Perevozkina et al.
VO2/mm3
The pattern of hydroperoxide transformations has a
substantial effect on the kinetics and mechanism of oxiꢀ
dation as a whole; therefore, studies of the effect of inꢀ
hibitors on the behavior of peroxides receives much atꢀ
1
1
20
00
1
2
3
4
5
1
2—15
16,17
tention.
It was shown
that sulfurꢀcontaining
8
6
4
2
0
0
0
0
compounds that combine different functional groups in
one molecule promote heterolytic cleavage of the perꢀ
oxide bond and, under certain conditions, they can exꢀ
hibit substantial synergistic effects in the antioxidant acꢀ
tion. Thus, these AOx can efficiently terminate the oxidaꢀ
tion chains upon the reaction with the peroxyl radicals
and also prevent the secondary initiation of the proꢀ
cess through hydroperoxide destruction by a molecular
route.
0
5
0
100
150
200
250
t/min
Fig. 1. Kinetic curves for the absorption of oxygen by methyl
oleate in chlоrobenzene in the presence of various AOx:
In recent years, it has been shown18—20 that for some
natural AOx, the direct proportion between the induction
times and the concentration of compounds holds only for
low doses, while with an increase in the AOx concentraꢀ
tion, the efficiency may decrease. Therefore, for these
AOx it is important to study the variation of the induction
time vs their content in the substrate.
(
1) reference, (2) mexidol, (3) αꢀTP, (4) dibunol, (5) SOꢀ4;
CAOx = const = 1•10–4 mol L , W = 4.2•10 mol L
–1
–8
–1 –1
s
,
i
T = 60 °C.
increases the induction period with respect to the control
experiment but also decreases the initial and the highest
oxidation rates. When the compound concentration was
1
1
respectively. For αꢀTP and dibunol, this phenomenon
was not observed. Presumably, SOꢀ4 and mexidol can
destroy hydroperoxides.
This hypothesis was verified by measuring the accuꢀ
mulation kinetics of hydroperoxides in the autoxidation
of methyl oleate. In a specified instant, AOx was added,
in a concentration of 2•10 mol L , to relatively oxiꢀ
dized substrate. It can be seen from Fig. 2 that the hydroꢀ
peroxide concentration decreases during the first hour
and remains low for the subsequent 8 h, whereas in the
blank experiment (in the absence of AOx), hydroperoxides
continue to accumulate. The most pronounced hydroperꢀ
oxide destruction was found for SOꢀ4.
–
3
–1
•10 mol L , the highest oxidation rate decreased
.3ꢀ and 8.3ꢀfold in the presence of SOꢀ4 and mexidol,
The inhibitory effect of AOx was studied over a broad
concentration range. It was shown that for dibunol, SOꢀ4,
and mexidol, the induction time increases in direct proꢀ
portion to their concentration in the model system,
whereas for αꢀTP this dependence passes through a maxiꢀ
1
9,20
mum as described previously
and reproduced in this
study (Fig. 3). Under our experimental conditions, the
–
3
maximum was observed at a concentration of 2.5•10
–
4
–1
–1
mol L . The antioxidant activity of mexidol is commenꢀ
surable with the efficiency of αꢀTP. It can be seen in
Fig. 3 that SOꢀ4 ensures longer retardation times than
dibunol or αꢀTP.
The efficiency of the AOx is also related to the high
reactivity of the phenoxyl radicals formed on their oxidaꢀ
tion by reactions (8), (8´), (9), (9´) and on the reaction
with the oxidation substrate (10).
[
ROOH]/g of I •(100 g of lipids)–1
2
τind/min
1
2
0
0
0
0
0
0
.7
.6
.5
.4
.3
.2
1
1
200
000
1
4
2
3
8
00
3
600
4
2
00
00
1
00
300
500
t/min
Fig. 2. Destruction kinetics of hydroperoxides during autoxidaꢀ
tion of methyl oleate in the presence of equal concentrations
0
4
–1
5
10
15
20
C•10 /mol L
of AOx: (1) reference, (2) mexidol, (3) SOꢀ4. The arrow shows
Fig. 3. Plot for the induction times vs. the concentration of AOx:
the injection point of AOx; CAOx = соnst = 2•10–4 mol L
T = 60 °C.
–1
,
(1) SOꢀ4, (2) dibunol, (3) αꢀTP, (4) mexidol; W = 4.2•10
–8
i
mol L–
1
s , T = 60 °C.
–1

Inhibitory effect of phenol SOꢀ4
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 8, August, 2006
1383
E•10–2/dm mol cm–1
3
–1
In• + R•
P
(8)
5
4
3
2
1
a
In• + RO2•
1
P
(8´)
2
In• + In•
In• + RH
In—In + InH
InH + R•
(9), (9´)
3
4
(10)
RH is hydrocarbon (lipid), RO • is peroxyl radical,
InH is inhibitor, In—In is dimer, In is the inhibitor radical,
P are molecular products
2
•
ν/cm–1
3
600
3620
3640
3660
3680
3
–1
The activation energy of the reaction of phenoxyl radiꢀ
cals with peroxyl radicals (reaction (8)) is most often
C•10 /mol L
8
7
6
5
4
3
b
close to zero. The rate constants k for sterically hindered
8
phenols vary in a narrow range, depending only slightly
1
2
•
•
on the nature of In or reactive radical RO , and are
2
usually ~3•108 mol L–1
s
–1 21
.
Thus, the crucial factor
3
contributing to the antioxidant activity of phenols is played
by the reactivity of the radicals they produce by reacꢀ
tion (9), which is recognized as the major route of the
decay of the phenoxyl radicals under freeꢀradical oxidaꢀ
tion conditions.
4
5
2
0
40
60
80
100
tphoto/s
The reactivity of SOꢀ4 phenoxyl radicals with respect
to model substrates with increasing degree of unsaturation
was studied. The research was performed for the following
series of phenolic AOx: hydroxybenzene, αꢀTP, SOꢀ4,
dibunol. The reactivity of the radicals of the most imporꢀ
tant natural AOx (αꢀTP) containing two Me groups in the
orthoꢀposition to the OH group was tested. The curve for
AOx consumption was measured by IR spectroscopy in
Fig. 4. a. IR absorprion spectra of solutions of dibunol in hexane
depending on the photoirradiation time: (1) 0, (2) 20, (3) 40,
–1
(
4) 60 s; CAOx = 8•10–3 mol L , T = 20 °C; b. Variation of the
concentration of SOꢀ4 during photoirradiation of its solutions in
hexane: (1) SOꢀ4 + arachidonic acid; (2) SOꢀ4 + phosphoꢀ
lipids; (3) SOꢀ4 + linolic acid; (4) SOꢀ4 + methyl oleate;
(
8
5) SOꢀ4. Concentrations of SOꢀ4 and oxidation substrates
•10 mol L , T = 20 °C.
–3
–1
–
1
the phenolic hydroxyl region (3600 cm ) after exposure
of samples to hard UV light. The kinetics of phenol transꢀ
formations in two systems were compared, one system
containing no substrate and the other containing lipids
with different degrees of unsaturation. In the former sysꢀ
tem, the inhibitor was consumed due to photolysis to give
radicals, involved mainly in reactions (9), while in the
latter system, this route was supplemented by reacꢀ
tion (10), namely, phenoxyl reduction to phenols.
Figure 4, a shows the IR absorption spectra of dibunol
solutions before and after irradiation for a specified peꢀ
riod of time. The intensity of the OH absorption band
decreased appreciably upon an increase in the time of
photolysis due to the formation of phenoxyl radicals. The
IR spectra of solutions of AOx with different concentraꢀ
tions allow one to determine the concentration of the
inhibitor that did not reacted over a particular time of
photolysis using the Bouguer—Lambert—Beer law. The
resulting data were employed to plot the kinetic curves for
AOx consumption.
tion was determined from the initial segments of the kiꢀ
netic curves (Table 2). Based on the AOx consumption
eff
rate (W ), the effective constants k9 were calculated
d
taking into account the equation
eff
• 2
W = k [In ] .
d
9
•
The steadyꢀstate concentration [In ] was determined
by UV spectroscopy at λ = 410—425 nm from the relation
•
[In ] = D/(lε), which takes into account the cell length l,
the observed absorbance D, and the molar extinction coꢀ
efficient ε, which was taken to be 4000 L mol–
1
s
–1
by
analogy with the extinction coefficient of the model radiꢀ
2
2
eff
cals derived from αꢀTP. The k9 values that we found
for all inhibitors were of the same order of magnitude (see
Table 2). The published value for αꢀTP is also of the same
3
–1 –1
order of magnitude, namely, 3.3•10 L mol
s
(25 °C)
3
–1 –1
and 2.2•10 L mol
radicals, k9 = 4.7•10 L mol
s
(50 °C). For dibunol phenoxyl
eff
3
–1 –1 23
s
.
It can be seen that
the disproportionation constants for tocopheroxyl radiꢀ
cals are comparable with published data. Hence, the apꢀ
proach we used appears to be wellꢀposed enough to estiꢀ
mate the phenoxyl dimerization constant. The lower rate
The curves for SOꢀ4 consumption during photoꢀ
irradiation of solutions with equal initial concentrations
of AOx are shown in Fig. 4, b. The rate of AOx consumpꢀ

1384
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 8, August, 2006
Perevozkina et al.
Table 2. Kinetic characteristics of phenoxyl radicals derived from phenols of various structures*
k9eff•10
3
W •10
–5
W •10
–5
k10eff•10
2
Antiꢀ
oxidant
d
r
/L mol–1 s–1
/mol L s–1
–1
/mol L s–1
–1
/L mol s–1
–1
I
II
III
IV
V
II
III
8.0
IV
V
II
III IV
V
Hydroxyꢀ
benzene
αꢀTP
SOꢀ4
Dibunol
0.75
3.0
2.7
2.0
1.4
1.6
3.6
10.5
9.5
0.21 0.48 0.63 0.57
2.08
0.90
2.78
8.3
3.6
5.0
2.9
2.5
2.2
0.6
1.1
5.6
1.0
1.5
6.7
2.5
1.8
1.5
5.5
2.3
1.7
7.0
6.2
6.0
6.5
4.5
4.3
0.16 0.34 0.44 0.31
0.11 0.14 0.39 0.28
0.08 0.09 0.31 0.22
16.0 12.5 10.0
*
(
CAOx = CPL = 8•10–3 mol L–1, T = 20 °C; I — no substrate; oxidation substrate: II — methyl oleate (one πꢀbond), III — linolic acid
two πꢀbonds), IV — arachidonic acid (four πꢀbonds), V — phospholipids containing polyunsaturated fatty acids; W and W are
d
r
consumption and recording rates, respectively.
constant k9eff for SOꢀ4 is due, most likely, to the great
bulk of the paraꢀsubstituent, which creates steric hinꢀ
drance to the formation of dimers upon disproportionꢀ
ation (see reaction (9)), as was also demonstrated for
other groups of compounds.6
It was found that the degree of hindrance has a subꢀ
stantial influence on the rate of AOx consumption. Inꢀ
deed, unhindered phenol present in the same concentraꢀ
tion is consumed 5 times more slowly than sterically hinꢀ
dered dibunol (see Table 2).
The variation of the consumption rate and the AOx
regeneration rate in the presence of substrates with differꢀ
ent unsaturation degrees has been studied. The substrates
used included methyl oleate and linolic and arachidonic
acids containing one, two, and four double bonds, reꢀ
spectively, and PL containing polyunsaturated fatty acids.
The data given in Table 2 show that during oxidation
AOx are regenerated through the reaction of phenoxyl
radicals with unsaturated compounds. The radicals deꢀ
rived from slightly hindered αꢀTP are reduced much more
easily than phenoxyl radicals of sterically hindered AOx,
dibunol and SOꢀ4. It can be seen that the rate of regenꢀ
eration of AOx and the reactivity of phenoxyl radicals in
reaction (10) decrease as the OH group becomes more
shielded and increase with an increase in the number of
double bonds in the oxidation substrate (see Table 2). The
possibility of regeneration of the active form of AOx durꢀ
ing oxidation may also be favorable for efficient inhiꢀ
bition.
The phenoxyl radicals formed in the steadyꢀstate phoꢀ
tolysis of solutions of phenols can not only undergo disꢀ
proportionation in the presence of unsaturated comꢀ
pounds but can also be consumed in the reactions with
the substrate. Reaction (10) results in the recovery of
the phenolic form of AOx. It was shown that photolysis
of solutions of AOx containing additionally an oxidaꢀ
tion substrate shows a different consumption kinetics of
the inhibitor. The kinetic curves for the consumption
of SOꢀ4 with addition of methyl oleate, linolic acid,
arachidonic acid, or phospholipids (PL) are shown in
Fig. 4, b. It can be seen that the slope of the kinetic curves
and the consumption rate of AOx in the presence of
an unsaturated compound decrease. The decrease in the
consumption rate of the inhibitor is attributable only to
the reduction of phenoxyl radicals to phenol. The amount
of phenol (∆[InH]) reduced in reaction (10) over a cerꢀ
tain period of time is determined by the difference beꢀ
tween the phenol concentrations in the presence ([InH]S)
and in the absence ([InH]) of the substrate related to the
same irradiation time as ∆[InH] = [InH]S – [InH].
The AOx consumption rates in the presence and in the
absence of the substrate and the rate of phenoxyl reducꢀ
tion, ∆[InH]/∆t, were found from the kinetic curves (see
Fig. 4, b).
However, this occurs only in the case of low activity of
•
•
alkyl and peroxyl radicals (R and RO ) in chain propaꢀ
2
gation reactions
In• + RH
InH + R•
RO2•
ROOH + R•.
If the resulting radicals (R and RO2•) are more reacꢀ
tive in reaction (2) than phenoxyl radicals, and the inꢀ
hibitor reactivity in reaction (7) is relatively low, then the
oxidation will be accelerated due to reaction (10). Thus,
the inhibitory action of AOx may be related not only to
their reactivity toward peroxyl radicals but also to the
reactivity of phenoxyl radicals toward the oxidation subꢀ
strate. Synergism will be observed if the radicals formed in
•
the presence of the synergist are less reactive than RO2•
.
The k10eff values were determined from the equation
Synthetic AOx are widely used to stabilize systems
containing natural inhibitors, for example, αꢀTP. Thereꢀ
fore, we studied the combined action of SOꢀ4 and αꢀTP
and determined their optimal concentrations, ensuring
the highest inhibitory effect.
eff
•
∆
[InH]/∆t = k10 [RH][In ]. The results are given in
Table 2. The tocopheroxyl radicals are more reactive than
the phenoxyl radicals derived from sterically hindered
AOx, namely, dibunol and SOꢀ4.

Inhibitory effect of phenol SOꢀ4
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 8, August, 2006
1385
∆
τind/Στadd•100%
Table 3. Synergistic effect vs. concentration of the antioxidants*
CAOx•10–4 τind
τadd Στi ∆τ (∆τ/Στ )•100%
4
3
2
1
0
0
0
0
0
i
a
mol L–1
/
min
The αꢀTP + SOꢀ4 mixture,
–4
–1
СSOꢀ4 = const = 1•10 mol L , τind = 130 min
0.25
2.50
60
160
350
450
600
800
230
510
970
1200
1210
980
190
290
480
580
730
930
40
220
490
620
480
50
17.4
76
102
107
66
5.4
4
–1
2
4
6
CSOꢀ4•10 /mol L
–
–
10
20
5
7
1
1
.00
.50
0.00
5.00
∆τind/Στadd•100%
The αꢀTP + SOꢀ4 mixture,
8
6
4
2
0
0
0
0
0
–4
–1
СαꢀTP = const = 2.5•10 mol L , τind = 160 min
b
0
1
2
.10
.00
.00
75
130
240
500
760
900
280
510
710
870
710
690
235
290
400
660
920
45
220
310
210
210
370
19
76
78
32
–23
–35
4.00
.00
8.00
6
4
–1
1060
2
4
6
CSOꢀ4•10 /mol L
–
–
20
40
The mexidol + SOꢀ4 mixture,
СSOꢀ4 = const = 1•10– mol L , τind = 130 min
4
–1
1
2
5
7
8
1
1
2
2
3
3
.0
.0
.0
.0
60
110
220
280
320
400
650
800
950
1100
1300
190
240
350
410
450
530
780
930
270
360
540
680
710
80
120
190
270
260
310
450
530
600
680
780
42.1
50.0
54.3
65.9
57.8
58.5
57.7
57.0
55.6
55.3
54.6
Fig. 5. Synergistic effect in αꢀTP + SOꢀ4 (a) and mexidol +
SOꢀ4 (b) blends vs. concentration of SOꢀ4; W = 4.2•10
L mol
–
8
+
i
–
1
–1
s , T = 60 °C. Concentrations of αꢀTP (a) and
.0
–
4
–4
–1
mexidol (b) are 2.5•10 and 1•10 mol L , respectively.
0.0
6.0
0.0
5.0
0.0
5.0
840
1230
1460
1680
1910
2210
The efficiency of the binary mixture was studied in
comparison with the expected additive effect based on
individual AOx. The dependence of the synergistic effect
on the concentration of each component was studied. The
1080
1230
1430
The mexidol (A) + SOꢀ4 mixture,
–
4
optimal concentration range of SOꢀ4 is (1.5—2.0)•10
–4
–1
СA = const = 1•10 mol L , τind = 60 min
mol L–1. These concentrations of SOꢀ4 ensure the highꢀ
est synergistic effect (65—68%). The synergism increases
with an increase in the αꢀTP concentration in the mixture
0.5
1.0
80
130
240
500
580
800
1100
140
190
300
560
640
860
1160
160
270
430
740
720
790
980
20
80
130
180
80
14.2
42.1
43.3
24.3
11.1
2
4
5
7
1
.0
.0
.0
.0
(
Fig. 5, a, Table 3).
The mechanism of combined action of components of
the binary mixture is apparently as follows. The rate conꢀ
stant k of αꢀTPꢀphenol (R OH) is high, and the antiꢀ
radical activity of sterically hindered SOꢀ4 (R OH) is
much lower. The oxidation of αꢀTP gives reactive tocoꢀ
pheroxyl radicals,2
–70
–180
–8.9
–18.4
0.0
1
7
2
* Conditions: methyl oleate as the oxidation substrate, W =
i
4.2•10–8 mol L
–1
s
–1
, T = 60 °C.
2,24,25
while oxidation of hindered
2
1,26,27
phenols yields unreactive phenoxyl radicals.
The
The phenoxyl radicals R O• have low reactivity and they
hardly participate in the subsequent chain process.
A study of the efficiency of combined action of binary
mixtures comprising SOꢀ4 and mexidol has shown that
2
first oxidation stages consume, first of all, the more reacꢀ
tive inhibitor, and the resulting tocopheroxyl radicals rapꢀ
idly exchange hydrogen atoms with the hindered phenol
by the reaction
–
4
the concentration range of choice for SOꢀ4 is (1—5)•10
–
1
1
•
2
1
2
•
mol L
(see Fig. 5, b), and that for mexidol is
2—20)•10–4 mol L (see Table 3). These concentraꢀ
R O + R OH
R OH + R O .
–1
(
tions of SOꢀ4 and mexidol ensure the highest synergistic
effect (50%). These data show that the hindered phenols
form highly efficient synergistic mixtures with hydroxyꢀ
pyridine derivatives.
The reduced species of more reactive AOx can again
terminate the oxidation chains:
1
2
2
1
•
R OH + R O
R OOH + R O .
2

1
386
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 8, August, 2006
Perevozkina et al.
Apparently, the synergistic effect for mexidol and SOꢀ4
∆τind/Στadd•100%
a
mixtures is enhanced owing to the ability of these AOx to
destroy hydroperoxides without giving rise to free radiꢀ
cals, excluding the additional consumption route of AOx
in the reaction with alkoxyl radicals.
The maximum efficiency of inhibition provided by the
mixtures can be attained with neat mexidol in a concenꢀ
tration 2.5 as high as that in the synergistic blend.
Here we studied the action of PL blended with the
oxidation inhibitor SOꢀ4. The action of the mixture was
compared with the effect of the reference AOx (dibunol
3
0
1
2
25
2
1
1
0
5
0
5
3
and αꢀTP), which were shown previously to exhibit the
0
3
–1
2
4
6
CPL•10 /mol L
synergistic effect with PL.1
9,20
The combined action of
AOx with PL was compared with the antioxidant effect
from the individual components that form the mixture. It
is known that PL do not inhibit the oxidation but can act
as synergistic agents for AOx.
∆τind/Στadd•100%
b
3
0
1
25
The kinetic parameters of oxygen absorption by meꢀ
thyl oleate in the presence of AOx and their mixtures with
PL are given in Table 4. A comparison of the induction
periods shows that a mixture of PL and AOx is more
efficient than the single AOx. In addition, an increase in
the induction times (∆τ) and a decrease in the highest
2
2
1
1
0
5
0
5
3
max
oxidation rate (∆WO2 ) were observed.
The following relations indicative of a synergistic acꢀ
0
3
–1–1
2
4
6
CAOx•10 /mol L
tion hold for AOx and PL mixtures: τ < τ (τ and τ are
i
Σ
i
Σ
the induction periods for methyl oleate oxidation in the
presence of AOx and an AOx + PL mixture, respectively);
Fig. 6. a. Induction times in the synergistic compositions of
various AOx with PL vs. concentration of PL: (1) α ꢀTP,
max 1
max 2
max 1
max 2
–8
–1 –1
(
∆WO2
)
> (∆WO2
)
((∆WO2
)
and (∆WO2
)
(2) SOꢀ4, (3) dibunol; Wi = 4.2•10 mol L
s , CAOx =
–
4
–1
2
•10 mol L , T = 60 °C. b. Synergistic effect in the combined
are the oxidation rates of methyl oleate in the presence of
AOx and an AOx + PL mixture, respectively).
action of PL with various AOx vs. their concentration: (1) αꢀTP,
–
8
–1 –1
–3
(
2) SOꢀ4, (3) dibunol; W = 4.2•10 mol L
s , CPL = 5•10
i
The dependence of the synergistic effect on the PL
concentration in the mixture was studied. It can be seen
in Fig. 6, a that the curves for all of these AOx are similar.
–
1
mol L , T = 60 °C.
In the (0—3)•10– mol L range, the action of the mixꢀ
3
–1
Fig. 6, b. The efficiency of synergism decreases in the
following sequence: αꢀTP > SOꢀ4 > dibunol. Thus, in the
presence of the same PL, the magnitude of the synergistic
effect is determined by the chemical structure of the inꢀ
hibitor. Note that the effect is most pronounced for unꢀ
hindered phenols and least pronounced for sterically subꢀ
stituted AOx.
ture increases in direct proportion to the concentration,
–
3
–1
the next range, (3—9)•10 mol L , is matched by an
efficiency plateau, and a further increase in the PL conꢀ
centration induces a decrease in the synergistic effect.
The dependences of the synergistic effect of the AOx and
PL blends on the AOx concentration are shown in
The mechanism of the synergistic effects in the comꢀ
9
Table 4. Kinetic characteristics of methyl oleate oxidation in the
bined action of αꢀTP and PL was studied previously. It
presence of AOx and PL* mixtures
was shown by the direct method that the polyunsaturated
fatty acids incorporated in PL promote the reduction of
the reactive phenolic form of αꢀTP. This decreases parꢀ
ticipation of αꢀTP in side reaction (10) leading to addiꢀ
tional initiation of the process. Amino alcohols (ethanoꢀ
lamine, choline) present in the PL structure can destroy
hydroperoxides by a nonradical way. This excludes one
more consumption route of αꢀTP in the reaction with
hydroxyl and alkoxyl radicals formed upon homolytic deꢀ
composition of hydroperoxides.
max 1
max 2
Antiꢀ
τind τΣ
∆τ (∆τ/Στ ) (∆W
)
(∆WО2
)
i
О2
oxidant
•100%
7
10 mol L s–1
–1
min
αꢀTP
SOꢀ4
160 200 40
240 300 60
25.0
20.9
9.5
6.50
12.10
6.30
6.00
4.35
5.80
Dibunol 190 210 20
The concentrations of AOx and PL were 2•10 and 5•10–4
–
4
*
mol L , respectively, W = 4.2•10 mol L s–1, T = 60 °C;
–
1
–8
–1
i
max 1
max 2
(
∆WO2 ) and (∆WO2 ) are the oxidation rates in the presꢀ
Thus, the synergistic compositions show real prosꢀ
pects for wide use as agents for preservation of the properꢀ
ence of an AOx or its mixture with a PL, respectively.

Inhibitory effect of phenol SOꢀ4
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 8, August, 2006
1387
ties and extension of shelf lives of biologically active lipids,
foodstuffs, medicines, and cosmetics.
11. G. I. Klebanov, O. B. Lyubitskii, and O. V. Vasil´eva, Voprosy
Med. Khim. [Problems of Med. Chem.], 2001, 3, 288 (in
Russian).
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 02ꢀ04ꢀ49452)
and the Foundation "Integration of Higher Education
with the Fundamental Science" (Project I 0566/1653).
12. V. L. Antonovskii anf S. L. Khursan, Fizicheskaya khimiya
organicheskikh peroksidov [Physical Chemistry of Organic Perꢀ
oxides], Akademkniga, Moscow, 2003, 460 pp. (in Russian).
1
3. G. V. Karpukhina and N. M. Emanuel´, Dokl. Akad. Nauk
SSSR, 1984, 276, 1163 [Dokl. Chem., 1984 (Engl. Transl.)].
4. J. Pospishil, Polym. Degr. Stab., 1991, 34, 85.
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Received February 15, 2005;
in revised form June 16, 2006
1