EFFECT OF ASCORBIC ACID AND ITS DERIVATIVES
81
in the yield of acetaldehyde and a decrease in the
yields of hydrogen peroxide are observed, which indiꢀ
cate the ability of the test compound to prevent the
formation of hydroxyalkyl peroxyl radicals (reaction
OH
O
OH
OH
O
OH
O
(
17)
O
O
ᮎ
,
ᮎ•
+ H O
2
•
2
+ OH + HO
(
3)) by the oxidation of ꢀHER via reaction (12). At
α
an equimolar additive : ethanol ratio, the effects are
caused by the IDMAA scavenging of the radical prodꢀ
ucts of water radiolysis. Under these conditions, the
yields of both products of radiationꢀinduced transforꢀ
mations of ethanol in aqueous solutions decrease. The
yield of IDMAA decomposition is two times lower
O
O
O
OH
OH
OH
O
OH
O
O
O
ᮎ•
ᮎ
+ O2
•
.
+
O2
than that for AA. In this case, the rate constants for the
reactions of these compounds with ОН and •
•
Н
are
O
O
O
O
(
18) approximately equal [13, 14]. Consequently, differꢀ
ences in the radiation conversion of the compounds
can be attributed to the inability of IDMAA to particꢀ
Using standard electrochemical potentials of radical
intermediates, it is possible to assess the feasibility of
reactions (17) and (18) during radiolysis of oxygenated
aqueous ethanol solutions at pH 7. The semidehyꢀ
droascorbate radical anion displays weak oxidizing
ipate in the reactions similar to (17) and (18).
CONCLUSIONS
and reducing properties, the corresponding electroꢀ
During
and its aqueous solutions at pH 7, ascorbic acid and
,6ꢀOꢀisopropylideneꢀ2,3ꢀOꢀdimethylascorbic acid
γꢀirradiation of oxygenꢀsaturated ethanol
+
chemical potentials are
0.228 V [2] and
Е
°
(SDHA, H /AA) =
5
+
Е°
(DHA/SDHA) = +0.160 V [11].
can lower the yields of the main products of radiolysis
by reducing peroxyl radicals and oxidizing the
Therefore, relatively stable SDHA radical anions in
aqueous solutions at pH 7 tend to undergo disproporꢀ
tionation to AA and DHA with a reaction rate conꢀ
α
ꢀhydroxyethyl radical, respectively. 2ꢀOꢀglucopyraꢀ
5
–1 –1
nosylascorbic acid displays a low reactivity towards
peroxyl radicals. In addition to the ability to inhibit
radiationꢀinduced transformations of oxygenated ethꢀ
anol and its aqueous solutions at pH 7, AA can enter
into reactions leading to the formation of an addiꢀ
tional amount of oxygenꢀcentered radicals. One of
such processes is the oneꢀelectron reduction of oxygen
and hydrogen peroxide by the SDHA radical anion.
The ambivalent properties of ascorbic acid in reactions
involving oxygenꢀcentered radicals should be taken
into account when considering its role in homolytic
processes in biosystems.
stant of
2
×
10 L mol s [12]. Molecular oxygen
present in aqueous solutions at initial concentrations
of about
is accumulated at a concentration of 10 mol/L at an
absorbed dose of 0.5 kGy, have the following standard
–4
2 10 mol/L and hydrogen peroxide, which
×
–
4
i −
oneꢀelectron reduction potentials:
Е
•
°
(
O O2
=
)
2
+
+
0.160 V and
Е°
(H O , H /H O, HO ) = +0.320 V
2
2
2
[
1]. Comparison of these standard electrochemical
potentials for the SDHA radical anion, О2, and Н О2
2
leads to the conclusion that reactions (17) and (18) are
energetically feasible.
Thus, the prooxidant effect of AA often observed in
biochemical studies may be associated with the ability
of the SDHA radical anion to act as a oneꢀelectron
reductant of molecular oxygen or hydrogen peroxide,
resulting in the production of oxygenꢀcentered radiꢀ
cals.
REFERENCES
1
. Halliwell, B. and Gutteridge, J.M.C., Free radicals in
Biology and Medicine, Oxford: Oxford University Press,
999.
. Buettner, G.R. and Jurkiewicz, B.A., Radiat. Res.
996, vol. 145, p. 532.
1
2
3
,
Low radiationꢀchemical degradation yields of
GAA and the absence of its influence on the yields of
the molecular products of radiolysis of oxygenated
ethanol solutions show its low reactivity toward radical
species formed in these conditions. The yield of aceꢀ
taldehyde in the presence of GAA significantly
decreases because of higher rate reaction constants
1
. Davies, M.B., Austin, J., and Patridge, D.A., Vitamin
C: Its Chemistry and Biochemistry, Cambridge: The
Royal Chemical Society, 1991.
4
. Childs, A., Jacobs, C., Kaminski, T., Halliwell, B., and
Leeuwenburgh, C., Free Rad. Boil. Med, 2001, vol. 31,
no. 6, p. 745.
•
•
with ОН and
Н only in the case of radiolysis of its
5
. Shadyro, O.I., Sosnovskaya, A.A., Edimecheva, I.P.,
Ostrovskaya, N.I., Kazem, K.M., Hryntsevich., I.B.,
and Alekseev A.V, Bioorg. Med. Chem. Lett., 2007,
vol. 17, no. 22, p. 6383.
–3
ethanol solutions at a concentration of 10 mol/L.
The absence of a decrease in the yield of hydrogen perꢀ
oxide in the presence of the additive may indicate its
additional formation due to the reaction of type (11).
6
. Shadyro, O.I., Sosnovskaya, A.A., Edimecheva, I.P.,
Ostrovskaya, N.I., and Kazem, K.M., High Energy
Chem., 2008, vol. 42, no. 2, p. 83.
Upon radiolysis of oxygenated aqueous ethanol
solutions in the presence of IDMAA, a slight increase
HIGH ENERGY CHEMISTRY
Vol. 49
No. 2
2015