Titania-based photocatalytic degradation of two nucleotide bases, cytosine and uracil

Article abstract of DOI:10.1016/j.apcata.2014.07.043

The photocatalytic degradation of two components of DNA and RNA, cytosine (C₄H₅N₃O) and uracil (C₄H ₄N₂O₂) differing only by the presence of an amine or a carbonyl group was investigated in the presence of UV-irradiated TiO₂ aqueous suspensions. The adsorption in the dark and under UV-A conditions, the photolysis, the kinetics of degradation, the fate of nitrogen and the identification of some intermediate products were investigated. The impact of pyrimidine cycles on the coverage of TiO₂ under UV-A, the effect of NH₂ substituent on the oxidation products and mineralization and the importance of carbonyl and amine groups on the fate of nitrogen atoms were evaluated. Electronic density was used to propose a possible chemical pathway. The comparison of the disappearance and mineralization rates in the photocatalytic process was discussed.

Full text of DOI:10.1016/j.apcata.2014.07.043

Applied Catalysis A: General 485 (2014) 207–213
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Titania-based photocatalytic degradation of two nucleotide bases,
cytosine and uracil
L. Elsellamia,b, K. Sahela,c, F. Dappozze , S. Horikoshi , A. Houas , C. Guillard
a
d
b
a,∗
a
IRCELYON, CNRS UMR 5256/Université Lyon1, 2 av. Albert Einstein, Villeurbanne Cedex, 69626, France
Unité de recherché Catalyse et Matériaux pour l’Environnement et les Procédés URCMEP (UR11ES85) Campus Universitaire Hatem BETTAHAR-Cité
Erriadh, 6072 Gabes, Tunisia
b
c
Laboratoire Physico-chimie des Matériaux, Catalyse et Environnement, Département de Chimie, Faculté des Sciences, Université des Sciences et de la
Technologie d’Oran (USTO) , Oran, Algeria
d
Sophia University, Faculty of Science and Technology, Department of Materials and Life Sciences, 7-1 Kioicho, Chiyodaku, Tokyo, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The photocatalytic degradation of two components of DNA and RNA, cytosine (C H N O) and uracil
4
5
3
Received 23 March 2014
Received in revised form 22 July 2014
Accepted 30 July 2014
(C4H4N2O2) differing only by the presence of an amine or a carbonyl group was investigated in the pres-
ence of UV-irradiated TiO2 aqueous suspensions. The adsorption in the dark and under UV-A conditions,
the photolysis, the kinetics of degradation, the fate of nitrogen and the identification of some interme-
diate products were investigated. The impact of pyrimidine cycles on the coverage of TiO2 under UV-A,
the effect of NH2 substituent on the oxidation products and mineralization and the importance of car-
bonyl and amine groups on the fate of nitrogen atoms were evaluated. Electronic density was used to
propose a possible chemical pathway. The comparison of the disappearance and mineralization rates in
the photocatalytic process was discussed.
Available online 7 August 2014
Keywords:
Cytosine
Uracil
TiO2
Photocatalysis
Reaction mechanism
© 2014 Elsevier B.V. All rights reserved.
1
. Introduction
Heterogeneous photocatalysis is an advanced oxidation pro-
The pyrimidine bases, issued from the decomposition of nucleic
acids, are present in natural waters and sediments [17]. All these
compounds may be considered as water pollutants which can be
eliminated by photocatalysis [12,14,16].
Jaussaud et al. [18] and Dhananjeyan et al. [11] have studied
the influence of different parameters such as concentration of pol-
cess (AOT) allowing efficient production of hydroxyl radicals under
appropriate conditions depending of the photocatalyst used. These
radicals have an oxidizing power much greater than traditional oxi-
dants. They are capable of partially or completely mineralizing most
of organic compounds [1–4]. The most widely used catalyst is TiO2
because of its relative high photocatalytic activity, its stability, its
availability and its relative low cost [5–7]. Under UV-light, titanium
dioxide is an extremely powerful oxidant able to break the car-
bon chains. The pollutants are mineralized into water and carbon
dioxide.
Pyrimidine compounds are largely found in biomolecules and
agrochemicals [8]. Several studies deal with the photocatalytic
degradation of pyrimidine compounds [9–16]. They mainly focused
on the degradation mechanism of DNA bases (uracil, thymine, and
cytosine) and on ionic effects. Such studies are highly relevant to
water and cancer treatments.
lutant, concentration of catalyst, pH, CdCl , oxygen concentration,
2
influence of metallic ions on the photodegradation of pyrimidine
bases in the presence of titanium dioxide. Horikoshi et al. [16]
have studied the photomineralization pathways of pyrimidine and
purine bases by UVA/UVB illuminated TiO and the respective rates
2
of formation of NH4+ and NO3 ions. Recently, Li at al [15] have
compared the photocatalytical and photo-electrochemical degra-
dation mechanism of these nucleotide bases. Singh et al. [19] gave
a detailed kinetic study of uracil and 5-bromouracil. They deduced
−
that TiO can efficiently catalyze the photomineralization of uracil
2
and 5-bromouracil. They also found that photocatalyst Degussa P25
showed the highest photocatalytic activity and they suggested that
the addition of electron acceptors such as hydrogen peroxide and
potassium bromate can enhance the decomposition.
Our objective is a better understanding of the photocatalytic
mechanism of the elimination of uracil and cytosine, which are the
simplest molecules constituting the microorganisms (DNA, RNA,
proteins, etc.).
∗
Corresponding author at: 2 avenue Albert Einstein, F-69626 Villeurbanne,
France. Tel.: +33 472445316; fax: +33 472445399.
E-mail address: chantal.guillard@ircelyon.univ-lyon1.fr (C. Guillard).
http://dx.doi.org/10.1016/j.apcata.2014.07.043
0
926-860X/© 2014 Elsevier B.V. All rights reserved.

2
08
L. Elsellami et al. / Applied Catalysis A: General 485 (2014) 207–213
3
8
8NH2
O
Cytosine
Uracil
1
2,5
2
1
5
5
4
2
3
NH
2
3
N
4
1
,5
1
N
H
6
O
N
6
O
7
H
7
Uracil
Cytosine
0
,5
0
Fig. 1. Structural formulas of uracil and cytosine.
0
100
200
300
Ceq (µmol/L)
400
500
600
2
. Experimental
.1. Reagents and chemicals
Pyrimidine bases uracil (Ura: C H N O ) and cytosine (Cyt:
Fig. 2. Amounts of cytosine and of uracil adsorbed per gram of TiO2 as a function of
2
−1
the equilibrium concentration Ceq on TiO2 Degussa P25 (1.25 g L ).
4
4
2
2
injection volume was 100 ␮L and the mobile phase was H SO
2
4
C H5N O) (99% purity) were purchased from SIGMA-Aldrich and
4
3
−3
−1
(
5 × 10 mol L ).
used as received. Their formulae are given in Fig. 1. Water used
for preparation of samples was ultra-pure water, filtered through
a milli-Q PLUS 185 water system. The photocatalyst was titanium
dioxide Degussa P-25 (particle size, 20–30 nm; crystal structure,
The formation of nitrate ions was monitored using ionic
chromatography with a Dionex DX-120 pump and conductivity
detector, and an IonPac AS14A (250mm × 4 mm) column. The flow
−
1
rate was 1 mL min and the mobile phase was an alkaline buffer
(
2
−1
8
0% anatase and 20% rutile; surface area, 50 m g ).
− −1
NaHCO (1.0 mmol L ) + Na CO (8.0 mmol L )).
3 2 3
1
The formation of ammonium ions was also followed using ionic
chromatography with a Dionex DX-120 pump and conductivity
2.2. Reactor and light source
detector. The column was a CS 12A (250 mm × 4 mm). The flow
The aqueous suspensions were irradiated in a 100 mL open
−1
rate was 1 mL min and the mobile phase was H SO solutions
2
4
cylindrical reactor whose base contained an optical window with a
−1
containing 610 ␮L L of pure sulfuric acid.
2
surface area of about 12.6 cm . The output of a Philips HPK 125 W
For all analyses the error bars are about 5%.
high mercury lamp was filtered through a circulating water cell
Computer simulations with MOPAC allowed to calculate the
(
thickness = 2.2 cm), avoiding the solution warming by IR, and a
•
frontier electron density used to determine the positions of OH
2
Corning 0.52 mW/cm filter to remove radiation with wavelength
below 340 nm. The radiant flux was measured using a VLX-3 W
radiometer with a detector CX-365 (355–375 nm).
radical attack in uracil and cytosine.
3. Results and discussion
2.3. Photocatalytic experiments
3.1. Adsorption
A volume of 20 mL of uracil and cytosine solution with dif-
In order to ensure that the adsorption process reached equilib-
rium, different concentrations of uracil and cytosine were stirred
in the dark in the presence of TiO2 and analysed as a function of
time. In both cases, the uracil and cytosine adsorptions reached
equilibrium after about 60 min.
−
1
ferent concentrations containing a concentration of 1.25 g L
of
TiO , sufficient to absorb all photons entering the photoreactor was
2
used [20]. The degradation was carried out at room temperature
◦
(
T = 25 C) and at natural pH (pH 5). The suspension was first stirred
Fig. 2(a) represents the amounts (␮mol g−1) of cytosine and
uracil adsorbed per gram of TiO2 as a function of the cytosine
and uracil equilibrium concentration (Ceq). In dark conditions,
the amounts of cytosine and uracil adsorbed on the TiO2 surface
(Qeq) increase with the equilibrium concentration until reaching a
plateau. The amounts of cytosine and of uracil adsorbed are iden-
tical, probably because of their similar formulae. The maximum
coverage of cytosine and uracil is about 0.03 molecule nm . This
represents about 0.6% of the maximum coverage in OH surface
groups equal to 5 OH/nm2 [21]. This value is similar to values
found for molecules containing aromatic cycles such as tryptophan,
phenylalanine [22,23].
in the dark until equilibrium adsorption was achieved. Then, the
solution was irradiated at ꢀ > 340 nm and a radiant flux equal to
2
3
.5 mW/cm . Samples taken at different times of irradiation were
filtered through 0.45 ␮m Waters filters to remove TiO2 particles
before analyses.
2.4. Methods of analysis
−
2
The degradation of uracil and cytosine were followed by HPLC
with a Varian System equipped with a Varian Prostar 230 isocratic
pump and a Varian Prostar 330 Diode Area Detector adjusted at
2
4
9
54 nm. A Hypersil BDS C18 reverse phase column (125 mm long,
mm diameter) was used. The mobile phase was constituted by
0% of ultra-pure water containing 62 ␮L H PO at pH 3 and 10% of
As for the majority of organic compounds [22–27], the adsorp-
tion isotherms of cytosine and uracil can be modelled using the
Langmuir approach:
3
4
−
1
methanol. The flow rate was 0.8 mL min
.
Mineralization of pyrimidine bases was monitored by deter-
mination of Total Organic Carbon (TOC) concentrations by direct
injection of the filtered samples using TOC-VCSH Shimadzu and
ASI-V Shimadzu sampler.
The carboxylic acids formed were analyzed by LC using a
Varian Prostar 230 pump, a Varian Prostar 325 UV detector
Qeq = KadsQmaxCeq(1 + KadsCeq)
Qeq is the adsorbed quantity of pollutant on the photo-
−
1
catalyst at the equilibrium (␮mol g ), Kads is the adsorption
−
1
constant (L ␮mol ), Qmax is the maximum amount to be adsorbed
(
−
1
␮mol g ), and Ceq is the concentration of the compound at the
−
1
adsorption equilibrium (␮mol L ). The values of the Langmuir
parameters for cytosine and uracil are presented in Table 1.
(
(
detection at 210 nm), and a Transgenomic Icsep Coregel 87H
300 mm × 4.6 mm) column. The flow rate was 0.7 mL min . The
−
1

L. Elsellami et al. / Applied Catalysis A: General 485 (2014) 207–213
209
Table 1
Langmuir parameters for cytosine and uracil adsorption.
1
Kads (L ␮mol 1
−
)
Qmax (␮mol g
−1
)
−2
Qmax (molecule nm )
Compounds
0.8
Cytosine
Uracil
0.024
0.018
2.5
2.5
0.03
0.03
0
.6
Uracil (UV)
Cytosine (UV)
Uracil (dark)
Cytosine (dark)
1
1
1
1
6
4
2
0
8
6
4
2
0
0.4
0
.2
0
0
100
200
300
400
500
600
Cytosine
Uracil
C (µmol/L)
Fig. 4. Reactant coverages in the dark and under UV as a function of uracil and
cytosine equilibrium concentrations (C).
Table 3
0
100
200
300
C (µmol/L)
400
500
600
700
Radical frontier density of cytosine and of uracil.
Atoms
Cytosine radical frontier density
Uracil radical frontier density
Fig. 3. Variations of the initial rates of disappearance of uracil and of cyto-
sine as a function of the concentration in solution (C). The lines are fits to the
Langmuir–Hinshelwood model.
C1
C2
C3
C4
N5
N6
O7
N8
O8
0.303
0.457
0.380
0.080
0.120
0.301
0.087
0.270
/
0.211
0.578
0.473
0.104
0.069
0.297
0.096
/
Table 2
Langmuir–Hinshelwood constants obtained under UV conditions.
k (␮mol L 1 min
−
−1
)
−1
Kads(UV) (L ␮mol )
0.165
Uracil
Cytosine
11
16
0.022
0.008
3
.2. Photodegradation of uracil and cytosine
At similar coverage under UV, cytosine is more quickly degraded
than uracil (Fig. 6). This behaviour can be explained considering the
We first checked that the direct photolysis was negligible.
The initial rates of disappearance of uracil and cytosine are
presence of donor NH group activating the attack of OH radical.
2
reported as a function of the concentrations of uracil and cytosine
in solution (C) (Fig. 3). Until a concentration of about 100-
−1
3.3. Organic carbon mineralization
1
20 ␮mol L , the initial rate is proportional to the uracil and
cytosine concentration in solution (r = kKads(UV)C) indicating that
the degradation of uracil and cytosine follows first-order kinetics
at low concentration [28].
Mineralization of organic carbon was followed by measuring
the TOC evolution during the photocatalytic degradation of uracil
and cytosine as shown in Fig. 7. The comparison of mineralization
curves shows that at same concentration, uracil is about twice as
rapidly mineralized than cytosine. More than 21 h and 41 h were
necessary to respectively mineralized more than 99% of organic
carbon.
On the basis of the evolution of initial TOC, the presence of the
amine group seems to decrease the rate of the decarboxylation and
therefore the opening of the cycle. The twice higher decarboxyl-
ation rate of uracil could be explained by a more rapid opening
of cycle. Actually, considering the radical frontier density (Table 3)
before opening the cycle, several hydroxylations of the cycle should
be observed, mainly at carbon C2, which has the higher electronic
density and also at C3, on nitrogen N6 and finally at C1. In the case
of uracil, the hydroxylation on C1 should allow the opening of the
cycle whereas in the case of cytosine another step is necessary.
Fig. 8 indicates that after more than 95% of disappearance of
cytosine and of uracil, only 15% and 30% of TOC have been miner-
alized from uracil and cytosine respectively.
Subsequently, the initial rate became independent of uracil
and cytosine concentration. This phenomenon is expected
from Langmuir–Hinshelwood type kinetics [29,30] where
r = kKads(UV)C/(1 + Kads(UV)C), since the product of the adsorption
constant, Kads(UV), by the concentration, cannot be neglected with
respect to one in the denominator of r at higher concentrations.
Kads(UV) and k can be determined by the least squares method
−
1
−1
where k is the rate constant (␮mol L min ) and K
is the
ads(UV)
−
1
adsorption constant under UV conditions (L ␮mol ). The values of
Kads(UV) obtained under UV conditions and the values of rate con-
stant k are listed in Table 2. From the value of Kads(UV), the coverage
(
ꢁ = Kads(UV)C/(1 + Kads(UV)C) under UV was determined.
The coverages of uracil and cytosine in the dark and under UV
are given in Fig. 4.
The coverages of uracil under UV and in the dark are simi-
lar, whereas in the case of cytosine an important difference was
observed. This behaviour can be explained considering the electri-
cal charge of the molecule. Actually, since the pK of uracil is 9.3,
at natural pH (pH 5), uracil is always neutral, whereas at this pH,
cytosine (pK 4.2 and 9.3) is positively charged (Fig. 5). It means that
under UV, TiO2 surface is modified and becomes charged, modi-
fying the adsorption of non-neutral molecules. At same coverage,
a higher amount of cytosine was found present on TiO2 surface,
leading to a higher disappearance rate.
The mineralization of uracil is faster compared to that of cyto-
sine whereas the opposite was observed for the initial degradation
rates. The initial disappearance rates of parent molecules are not
sufficient to determine the efficiency of photocatalytic process.
Actually, degradation rates of one parent molecule do not neces-
sarily correlate with total organic carbon removal.

2
10
L. Elsellami et al. / Applied Catalysis A: General 485 (2014) 207–213
Fig. 5. Ionization states of cytosine and of uracil as a function of pH.
1
1
1
4
2
0
8
6
4
2
0
Cytosine
Uracil
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
coverage rate (%)
Fig. 6.
modifies the proportion of NH +/NO3 ions released into the solu-
−
3.4. Nitrogen mineralization
4
tion (Fig. 9a and b).
The formation of NO3 and NH + from cytosine and uracil show
−
In the case of cytosine, NH + ions are formed initially,
4
4
that the presence of primary amine function on the ring completely
whereas the formation of nitrate ions occurs only after the total
1
1
1
4
2
0
8
6
4
2
0
1
4
12
10
8
6
0
10
20
30
t (min)
Cytosine
Uracil
0
250
500
750
1000
1250
1500
1750
2000
2250
2500
t(min)
Fig. 7. Kinetics of the total organic carbon (TOC) disappearance during uracil and cytosine degradation with identical initial concentrations
−
1
(
[uracil]0 = [cytosine]0 = 245 ␮mol L ).

L. Elsellami et al. / Applied Catalysis A: General 485 (2014) 207–213
211
35
30
25
20
15
10
5
0
40
Uracil
Cytosine
3
5
Uracil
Cytosine
30
2
2
1
5
0
5
10
5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
20
40
60
80
100
120
%
conversion
%
disappearance
Fig. 10. Formation of ammonium as a function of cytosine and of uracil conversions
in %).
Fig. 8. Percentages of uracil and cytosine mineralization as a function of the per-
centage of their disappearance.
(
of the % of cytosine disappearance indicates that NH + ions are not
formed from the beginning of degradation and that the hydroxyl-
4
800
700
600
500
400
300
200
100
0
(
a)
ation of C1 in cytosine, contributing to the breaking of C-NH bond,
2
is not a primary step in agreement with electronic density.
In the case of photocatalytic degradation of uracil, the formation
of nitrate and of ammonium ions occurs only after total disappear-
−
1
ance of uracil ([uracil = 245 ␮mol L ]). After an irradiation time of
2
0 h about 98% of nitrogen mineralization was observed. The more
−
important formation of NO3 ions is in agreement with the work of
Horikoshi et al. [16] and of Pelizzetti et al. [34]. Actually, the latter
authors considered that the presence of a carbonyl group near the
amine group (case of urea or of formamide) favours the formation
of nitrate ions.
0
500
1000
t (min)
1500
2000
2500
3.5. Organic compounds
In order to better understand the different steps of the
degradation of pyrimidine compounds, the formation of organic
intermediate compounds are followed in the photocatalytic degra-
dation of uracil and of cytosine.
Fig. 11a and b presents a chromatogram obtained after 30 min of
degradation of both molecules. First of all, we noticed that uracil is
not an intermediate compound of cytosine degradation. This results
is in agreement with the formation of NH4+ and the electronic den-
6
5
4
3
2
1
00
00
00
00
00
00
0
(
b)
sity indicating that C-NH oxidation is not a primary step. The first
2
step should be hydroxylation of carbon C2.
Secondly, we observed that five identical compounds (A, B, C,
D and F) are found in the degradation of both molecules. However,
only three are identified: oxalic acid (peak A), tartronic acid (HOOC-
CH(OH)-COOH: peak B) and mesoxalic acid (HOOC-CO-COOH: peak
C).
In the degradation of cytosine, lactic acid (CH -CH(OH)-COOH:
3
0
200
400
600
t (min)
800
1000
1200
1400
peak G) is detected but not in the degradation of uracil.
The formations of peak A, B and C observed in the degradation
of both molecules are given in Fig. 12.
Fig. 9. Formation of ammonium and nitrate ions as a function of irradiation time
Oxalic acid appears from the beginning and increases until a
maximum corresponding to the time necessary to the complete
disappearance of cytosine and of uracil in solution. The five other
acids are not primary products. The organic carbon coming from
these different acids has been compared to the experimental TOC
found in solution (Fig. 7). It is interesting to notice that after the total
disappearance of uracil and of cytosine, about 80% of the organic
matter comprises from acid compounds.
All the other organic compounds detected were not identified.
However, considering the electronic density given in Table 3 and
the presence of peak D and F similar for both molecules but not
identified, we propose that these two compounds could correspond
to 2-hydroxy uracil and 2,3-dihydroxyuracil.
during the degradation of (a) cytosine and (b) uracil.
disappearance of cytosine (about 2 h) indicating that nitrate ions
are not primary oxidation products. After an irradiation time of
4
0 h, more than 82% of nitrogen initially present in cytosine was
converted into ammonium and nitrate ions. The more important
formation of NH4+ ions is in agreement with the work of Hidaka
and co-workers on different amino acids [31,32] and with our pre-
vious work showing that the amine group is mainly transformed
into ammonium at the start of reaction [24,33]. Actually, the oxida-
tion degree of nitrogen in amine groups and in ammonium is equal
+
to −3. Fig. 10 represents the formation of NH4 ions as a function

2
12
L. Elsellami et al. / Applied Catalysis A: General 485 (2014) 207–213
2
1
1
0
5
0
5
0
200
1
1
1
1
80
60
40
20
(
a)
mesoxalic acid
tartronic acid
oxalic acid
100
80
60
40
20
0
0
200
400
600
800
1000
t(min)
1
5
0
5
0
20
1
8
(
b)
16
14
12
10
80
60
40
20
0
1
mesoxalic acid
lactic acid
tartronic acid
oxalic acid
0
200
400
600
800
1000
t(min)
Fig. 12. Formation of oxalic, tartronic and mesoxalic acids during the photocatalytic
degradation of uracil (a) and cytosine (b)
Fig. 11. Chromatogram of intermediate compounds formed after 30 min of degra-
dation of (a) cytosine and (b) uracil.
O
O
OH
OH
(a)
OH
HO
HO
NH
HO
NH
N
N
OH°
OH°
HOOC
COOH
tartronic acid
N
H
O
N
H
O
N
OH HO
N
OH
F
D
OH
O
HOOC
COOH
HOOC
COOH
HOOC
COOH
(
b)
Fig. 13. Tentative chemical pathways for uracil (a) and cytosine (b) photocatalytic degradation.

L. Elsellami et al. / Applied Catalysis A: General 485 (2014) 207–213
213
A suggesting relating to one step of degradation of uracil and
cytosine is presented in Fig. 13.
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that of uracil was not modified. This behaviour has been explained
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[
of TiO surface under irradiation whereas neutral form of uracil is
2
always present.
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(
At the same coverage, a higher amount of cytosine has been
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[
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2
a lower mineralization rate of cytosine was observed indicating that
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(
[
[
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−
[
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[
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−
NO3 ions has been observed for uracil having the carbonyl group.
+
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[
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[
carbon atom bearing the NH group. Actually, uracil is not a degra-
2
+
dation product of cytosine and NH4 ions are not formed from
[
[
[
[
[
[
the beginning of degradation. The first attack of OH radicals was
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atom bearing the NH2 substituent leads to the formation of five
identical intermediates in cytosine and uracil degradation, which
are oxalic, tartronic, mesoxalic acids and mono and di-hydroxy
uracil.
This type of study is important to understand the potential appli-
cations of photocatalysis in the “bioworld” such as disinfection,
sterilization and degradation of biomolecules such as DNA and RNA
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1
[
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[
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