I. P. Ivanova et al. / Journal of Biophysical Chemistr y 3 (2012) 88-1 00 99
is less than 1 for all concentrations of ascorbic acid,
which means that the ascorbic acid itself is an antioxi-
Prooxidant properties were observed in [14,15] at the
concentration of ascorbic acid of approximately 103
mol/l, which is close to the concentration at which the
maximum S/S0 is reached in Figure 7 for the solution
containing oxygen. The authors of [14,15] report ascor-
bic acid at concentrations of 103 mol/l to have different
properties compared to larger concentrations. In terms of
Scheme 1 the properties of the acid do not change with
concentration, but the ratio of individual channels of re-
actions occurring in course of Asc oxidation is changed.
According to the mechanism of its oxidation, ascorbic
acid itself cannot be prooxidant, which is confirmed by
other studies . The results of the present study sug-
gest that the prooxidant properties do not appertain to the
ascorbic acid, but to products oxidized by DHA oxygen,
among which can be oxalic acid. Figure 4 shows that
oxalic acid in the presence of oxygen has prooxidant
properties. Reduction of the prooxidant activity observed
experimentally in [Asc] > 103 mol/l in the presence of
oxygen can be attributed to the expenditure of dissolved
oxygen in reaction 40, and perhaps its shortage as the
concentration of oxygen in the water is about 104 mol/l,
while [Asc] is more than 103 mol/l.
The form of the impulse also helps to draw the con-
clusion on the composition of the secondary radicals. If
the secondary radicals are of complex sedentary nature
2, which are discharged into reaction 32 (Table 6)
with the formation of singlet oxygen, a flash of light
should have the form shown in Figure 5, i.e. the delay in
appearance of light—tens of seconds, and duration of the
light pulse—a few minutes. If the secondary radicals are
highly movable (2,2), the flash of light will be
short, less than 30 seconds. Such flash is observed in
oxalic acid and at all concentrations of ascorbic acid.
Such radicals as 2 might yet form during the de-
composition of DHA. However, their formation is ex-
tended in time, is not bound by the time of introducing
the components of the Fenton reagent, and cannot be
detected by the applied technique of chemiluminescence.
Thus, the analysis of the properties of antioxidants in
Scheme 1 allows revealing the features of the tested sub-
1) If 30 seconds have been selected as radiation regis-
tration time, the preferred concentration of Fenton solu-
tion reagent is [Fe2+] = 103 mol/l, [H2O2] = 104 mol/l.
2) The radiation background produced by cosmic rays
and other sources induce luminescence which is regis-
tered by luminometer and is to be taken into account.
3) The main radiant product in the solution of Fenton
is a dimer of singlet oxygen. The glow of the solution is
stopped when the ferric iron that is formed absorbs al-
most all radicals 2
. The same mechanism of lumi-
nescence quenching operates for luminol in a neutral
4) The luminescence of organic substances relates to
. No luminescence appears in a non-oxy-
gen solution. In the presence of oxygen with an increas-
ing concentration of RH the reaction RH + 2
+ ROOH begins to dominate and the emission stops.
5) Depending on the mechanism of the process, che-
miluminescence is grouped in different time intervals
after the injection of all the substances. In the period
from 0 to 30 seconds the glow is caused by reactive
oxygen species. During the time period from 30 seconds
to several miutes it is due to the emission of free-radical
reactions occurring in the sample.
6) Observed for a single substance oxidant and prooxi-
dant properties are caused by the same reaction mecha-
nism. At high concentration the intermediate radicals, the
reaction products are absorbed by the initial substance
and antioxidant properties are observed. At low concen-
trations of introduced substances intermediate radicals
are preserved and prooxidant properties are observed.
7) Prooxidant properties of ascorbic acid observed in
some cases are associated with the DHA oxidation prod-
ucts with oxygen. In the absence of oxygen the prooxi-
dant effect does not occur.
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