Emitters of chemiluminescence occurring during autoxidation of substituted hydroquinones in water

Article abstract of DOI:10.1134/S1070363217070052

A mathematical processing method for determination of spectral parameters of chemiluminescence emitters during the autoxidation of phenolic compounds in aqueous-alkaline media has been developed. The presence of a single luminescence emitter (the corresponding p-benzoquinone in the triplet state) has been demonstrated in the hydroquinone–oxygen–water system. The emitters spectra have been obtained.

Full text of DOI:10.1134/S1070363217070052

ISSN 1070-3632, Russian Journal of General Chemistry, 2017, Vol. 87, No. 7, pp. 1476–1480. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © E.A. Kalinichenko, A.V. Kalinichenko, I.D. Odaryuk, L.V. Kanibolotskaya, A.N. Shendrik, 2017, published in Zhurnal Obshchei
Khimii, 2017, Vol. 87, No. 7, pp. 1082–1087.
Emitters of Chemiluminescence Occurring
During Autoxidation of Substituted Hydroquinones in Water
E. A. Kalinichenko*, A. V. Kalinichenko, I. D. Odaryuk,
L. V. Kanibolotskaya, and A. N. Shendrik
Donetsk National University, ul. 600-Letiya 21, Vinnitsa, 21021 Ukraine
*e-mail: e.a.kalinichenko@gmail.com
Received November 3, 2016
Abstract―A mathematical processing method for determination of spectral parameters of chemiluminescence
emitters during the autoxidation of phenolic compounds in aqueous-alkaline media has been developed. The
presence of a single luminescence emitter (the corresponding p-benzoquinone in the triplet state) has been
demonstrated in the hydroquinone–oxygen–water system. The emitters spectra have been obtained.
Keywords: chemiluminescence, emitter, hydroquinones, autoxidation
DOI: 10.1134/S1070363217070052
The liquid-phase oxidation of phenolic compounds
(including biologically important polyphenols and
quinones: pyrocatechol, adrenalin, p-benzoquinone,
pyrogallol, and gallic acid) with molecular oxygen in
an aqueous-alkaline medium is accompanied by a
super-weak chemiluminescence [1–6]. The appearance
of the chemiluminescence emitter (p-benzoquinone) in
the reaction between peroxyl and phenoxyl radicals
has been detected during the inhibition of the liquid-
phase oxidation of alkylbenzenes [7]. Being a primary
product of the hydroquinone autoxidation in an
alkaline medium (along with hydrogen peroxide),
p-benzoquinone acts as the chemiluminescence emitter
in this process as well [8]. Analysis of the published
data has revealed that the spectral parameters of
chemiluminescence emitters formed upon oxidation of
polyphenols have been scarcely (if at all) studied.
shortening of the induction period after addition of
p-benzoquinone (p-Q) at the initial stage of hydro-
quinone oxidation [6].
The formation of the radicals in the aqueous-alkaline
hydroquinone–oxygen system can be explained by
electron transfer from anionic forms of hydroquinones
to oxygen or the corresponding benzoquinone [1, 3, 9, 10].
k1a
HQ– + O2 → Q·– + O2·– + H+,
(1a)
(1b)
(2)
k1b
Q2– + O2 → Q·– + O2·–,
k2
HQ– + Q → 2Q·– + H+.
Here, HQ‾, Q2‾, and Q·– are anionic and semiquinone
forms of hydroquinone.
The formed semiquinone radicals Q·– are in the acid-
base equilibrium with the protonated form HQ·. The
dissociation constant of semiquinone radical and its
methyl derivatives can be expressed as рKа = 4.1 +
0.25n, with n being the number of methyl groups [11].
The single-electron transfer to oxygen gives rise to
superoxide radical, which is in the acid-base equilibrium
with рKа 4.7 [11] or 4.8 [12]. In a weakly alkaline
solution, hydroquinone and its derivatives partially
undergo the first stage of dissociation, since pKa1 for
these compounds are as follows: 8.90 for ClQH2 [13],
9.91 for p-QH2 [14], and 10.05 for МеQH2 (at the
ionic strength 0.65 mol/L) [13]. The participation of
This work aimed to investigate the spectral param-
eters of emitters of chemiluminescence occurring during
autoxidation of hydroquinones in aqueous-alkaline media.
Oxidation of hydroquinone (p-QH2), 2-chlorohydro-
quinone (ClQH2), and 2-methylhydroquinone (МеQH2)
solutions with molecular oxygen was accompanied by
chemiluminescence exhibiting a single maximum after
the induction period (Fig. 1). This is due to the
accumulation of intermediate products accelerating the
process. The corresponding benzoquinone is likely to
be such intermediate. That fact was confirmed by the
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EMITTERS OF CHEMILUMINESCENCE OCCURRING DURING AUTOXIDATION
superoxide anion-radical was confirmed by the effect
1477
I, mV
of addition of superoxide dismutase (EC 1.15.1.1), and
p-Q and H2O2 are the only stable primary products of
autoxidation of hydroquinone. Accumulation of these
products at the first stage of the reaction was symbate [8].
The observed chemiluminescence could be due to
the relaxation of electron-excited form of benzo-
quinones (Q*) being formed via the recombination of
the semiquinone radical with superoxide anion-radical
[6] or hydroxyl radical [8].
Q·– + O2·– + 2H+ → Q* + H2O2,
Q·– + HO· + H+ → Q* + H2O,
(3a)
(3b)
t, min
k4
Fig. 1. Kinetic curves of chemiluminescence in the reaction
of autoxidation of p-QH2 (1), ClQH2 (2), and МеQH2 (3).
c0 = 5 mmol/L, pH = 8.0, T = 308 K.
Q* → Q + hν.
(4)
Quinones are formed via the laccase-catalyzed
oxidations of phenols as well. The enzymatic process
also includes the stage of electron transfer from the
hydroquinones to oxygen yielding semiquinone radicals
[15–17]. In this case, oxygen is reduced directly into
water rather than into hydrogen peroxide in the case of
the autoxidation.
of the hydroquinones and accumulation of p-benzo-
quinones were monitored by photometry at 290 and
245 nm, respectively (Fig. 3).
UV spectra of benzoquinones obtained via enzy-
matic oxidation of hydroquinones were identical to the
spectra of the reaction mixtures sampled during autoxida-
tion of those phenols. Hence, it could be assumed that
the excited state of quinone formed via the autoxi-
dation of the hydroquinones was the luminescence
emitter.
The products of the enzymatic oxidation were
identified by sampling the reaction mixture and
comparison of its retention time and spectra with pure
hydroquinones and quinones. Using the p-QH2/p-Q
pair as an example, we demonstrated the coincidence
of the retention time and absorbance spectra of the
individual compounds (Fig. 2). The accumulation of
the corresponding p-benzoquinones (p-Q, МеQ, and
ClQ) in the products of cooperative enzymatic
oxidation of three hydroquinones with laccase was
detected by means of chromatography. Consumption
The use of diffraction slits with a photomultiplier
was not efficient for the investigation of spectral
parameters of the emitter in the studied reaction due to
low signal to noise ratio (i.e. superweak chemilumine-
scence). To elucidate the nature of the emitter in the
reaction of the hydroquinones autoxidation, we
D
D
D
D
λ, nm
λ, nm
Fig. 2. UV absorbance spectra of the reaction mixture in the maximum of the chromatographic peak at tR 3.66 (1) and 5.18 min (3),
of 1.92×10–4 mol/L of hydroquinone solution (2), and of 4.7×10–5 mol/L p-benzoquinone solution (4).
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1478
KALINICHENKO et al.
teristics (Fig. 4b) installed between the reactor and the
U, mV
U, mV
photomultiplier. The similar shape of the chemilumine-
scence curves suggested the presence of a single
luminescence emitter in the system, and the ratio
between the emission intensities with the color filter
(ILF) and without it (I) could be used as the parameter
of the intensity in the chemiluminescence spectrum
(L).
Emission spectra of the chemiluminescence during
autoxidation of hydroquinones were fitted with
Gaussian function (Fig. 5). Their maxima for p-QH2,
MeQH2, and ClQH2 were found in the green range of
the spectrum (see the table). Since the average energy
of the excited state of the emitter was strongly
correlated with the T1 → S0 transition energy of
p-benzoquinone (222 kJ/mol) [9], we suggested that
the corresponding p-benzoquinone molecule in the
electron-excited triplet state was the luminescence
emitter during autoxidation of the hydroquinones.
t, min
Fig. 3. Curves of simultaneous consumption of ClQH2 (1),
p-QH2 (2), and МеQH2 (3) and of accumulation of their
enzymatic oxidation products МеQ (4), p-Q (5), and ClQ
(6). с0 = 2 mmol/L, laccase activity 0.27 units.
measured the integral intensities of the emission over
the narrow spectral ranges limited by the color filters.
The obtained data were processed as follows.
Sensitivity of photocathode was conveniently ex-
pressed as the quantum yield Kλ (the ratio of the
number of emitted electrons to that of the quants
absorbed at each wavelength). Quantum yield of a
FEU-38 photomultiplier as a function of the
wavelength is found in Ref. [18]. Since the spectral
response of FEU-38 within the range of 400–800 nm,
the latter was used for the spectral data analysis. To
account for the sensitivity of the photomultiplier for
the used color filters, each of the light transmission
values of the discrete set TLF,λ1, TLF,λ2, …, TLF,λn
(recorded with 1 nm step) should be multiplied by the
corresponding Kλi value.
In summary, the corresponding p-benzoquinone in
the triplet state is the only luminescence emitter during
autoxidation of hydroquinones in aqueous media. The
developed method of the spectral data processing
allows determination of the position of the emitter
chemiluminescence band for the systems without any
chemiluminescence promotor, and it can be used for
spectral study of other systems exhibiting superweak
emission.
EXPERIMENTAL
Hydroquinone (99%, Aldrich), 2-chlorohydro-
quinone (85%, Aldrich), 2-methylhydroquinone (98%,
Aldrich), and p-benzoquinone (“chemical pure”) were
purified by vacuum sublimation.
Certain wavelengths λi corresponding to higher
transmission values Tλi should have higher statistical
weight, and it was reasonable to use the weight-
average value (λ*) for the λ1, λ2,..., λn set, determined
with Eq. (5).
Kinetic studies of autoxidation of 5 mmol/L
solutions of the hydroquinones with molecular oxygen
were performed at 308 K and atmospheric pressure in
0.1 mol/L phosphate buffer (рН 8.0) prepared from
Na2НРО4·2H2O (≥99%, Fluka) and NaН2РО4·H2O
(≥98%, Sigma Aldrich) used as received [19]. The
solutions were prepared just before the experiment
using deionized water with resistance of at least
18.2 MΩ cm.
Σni=1λiKλiTλi
λ* = ————— .
(5)
Σni=1KλiTλi
Here, Tλi and Kλi are the light transmittance and the
photomultiplier quantum yield at wavelength λi.
Chemiluminescence intensity was measured using a
FEU-38 photomultiplier. The signal was amplified
with an elecrometric amplifier and postprocessed using
an L-305 аnalog-to-digital converter (LCARD, Russia).
The data collection and processing were performed
We obtained the kinetic curves of chemilumine-
scence during autoxidation of hydroquinones (Fig. 4a)
using the color filters with different spectral charac-
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EMITTERS OF CHEMILUMINESCENCE OCCURRING DURING AUTOXIDATION
(a) (b)
1479
I, mV
T, %
λ, nm
t, min
Fig. 4. (a) Kinetic curves of luminescence intensity during autoxidation of hydroquinone solution (с0 = 5 mmol/L, pH = 8.0,
T = 308 K) without color filter (1) and with ZhS-18 (2), OS-13 (3), KS-10 (4), KS-11 (5), and KS-17 (6) color filters; (b) light
absorbance spectra of the color filters.
(a)
(b)
(c)
L
L
L
E, kJ/mol
E, kJ/mol
E, kJ/mol
Fig. 5. Emission bands of chemiluminescence during autoxidation of p-QH2 (a), ClQH2 (b), and MeQH2 (c).
using PowerGraph v.2.1 software. The kinetic regime
of the reaction was ensured by the choice of the
stirring rate and air feed. Kinetic curves of chemi-
luminescence were corrected for the measured average
dark current (I = Iexp – I0).
35°C through a mixture based on citrate buffer system
(рН 4.6) prepared from as-received sodium citrate
dihydrate (≥99%, Merck) and citric acid monohydrate
(≥99.9998%, Fluka) [19]. 0.2 mL of a solution of the
substrate in acetonitrile and 4.8 mL of a laccase
solution in the buffer system (EC 1.10.3.2, Trametеs
To determine the position of the chemilumine-
scence maximum, a set of 100 standard color filters
(according to GOST 9411-91 for optical colored
inorganic glass) was used, the filter was installed
between the reactor and the photomultiplier. Spectral
characteristics of the color filters were measured using
an SF-2000 spectrophotometer (Russia) over the 200–
1100 nm wavelength range.
Energy parameters of the T1 → S0 transition for p-benzo-
quinones (phosphate buffer, рН = 8.0, T = 308 K,
confidence interval 3σ)
Substrate
p-QH2
Еmax, kJ/mol
218±6
λmax, nm
548±14
547±8
MeQH2
ClQH2
219±3
Enzymatic oxidation of hydroquinones was
performed by bubbling air at atmospheric pressure and
220±4
545±10
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 87 No. 7 2017

1480
KALINICHENKO et al.
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Chtomatographic separation was performed using a
NUCLEOSIL® 100-5 С18HD column (Macherey-
Nagel) with length 250 mm and inner diameter 4.0 mm
equipped with the corresponding precolumn (eluent
flow rate 0.8 cm3/min, column thermostat temperature
40°С, injected volume 10 mm3; linear gradient elution:
0–8 min from 85 to 50%, 8–10 min from 50 to 40% of
the mobile aqueous phase). To prepare the mobile
aqueous phase, 1.50 g of sodium octanesulfonate
monohydrate (≥97.0%, Aldrich) and 2.50 g of KH2PO4
(≥99.5%, Merck) were dissolved in 960 cm3 of water.
pH of the solution was adjusted to 2.5 with
orthophosphoric acid (≥85.0%, Merck) monitoring the
pH with a HANNA HI9813 pH-meter, and then 40 cm3
of acetonitrile (≥99.9%, CHROMASOLV®, for HPLC)
was added. The prepared solution was thoroughly stirred
and filtered through a Supelco Nylon66 Membranes
0.45 µm membrane filter. The buffer was freshly
prepared before each experimental series. Acetonitrile
was used as the mobile organic phase. The absorbance
spectra were recorded at 190–700 nm using an
SPD-M20A diode matrix detector of an LC-20AD
Prominence high-performance liquid chromatograph
(Shimadzu, Japan). Retention time for (tR) hydro- and
benzoquinones p-QH2, MeQH2, p-Q, ClQH2, MeQ,
and ClQ were 3.66, 4.62, 5.18, 5.71, 7.33, and
7.93 min, respectively.
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UV absorbance spectra of solutions of pure hydro-
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ACKNOWLEDGMENTS
17. Morozova, O.V., Shumakovich, G.P., Shleev, S.V., and
Yaropolov, A.I., Appl. Biochem. Microbiol., 2007,
vol. 43, no. 5, p. 523. doi 10.1134/S0003683807050055
Authors acknowledge the assistance in the study
from the management of Luks company (Donetsk) and
the head of laboratory N.N. Skripka.
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