A novel schiff base ligand and its copper complex: Synthesis, characterization, X-ray crystal structure and biological evaluation

Article abstract of DOI:10.1016/j.molstruc.2020.129395

A novel Schiff base ligand of (4-nitroaniline-2-hydroxybenzaldehyde) (a) and its complex [Cu(4-nitroaniline-2-hydroxybenzaldehyde)₂] (b) were synthesized and characterized by FT-IR, UV–Vis spectroscopy, elemental analysis and X-ray crystallography.The antibacterial activities have been studied in-vitro against two gram-negative (E. coliandPseudomonas aeruginosa) and two gram positive (Staphylococcus aureus and Enterococcus faecalis)standard bacterial strains. The ligand and its metal chelate were screened for their antimicrobial activities using the disk diffusion method against the selected bacteria. The complex (b) was a stronger antibacterial agent than the free ligand as well as its corresponding metal salt, and it was more active against the gram-positive than gram-negative bacteria.

Full text of DOI:10.1016/j.molstruc.2020.129395

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A novel Schiff base ligand and its copper complex: Synthesis,
characterization, X-ray crystal structure and biological evaluation
Tahere Kondori , NiloufarAkbarzadeh T , Mahboubeh Fazli ,
ˇ
´
Behrouz Mir , Michal Dusek , Vaclav Eigner
PII:
S0022-2860(20)31712-9
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.129395
MOLSTR 129395
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30 September 2020
1 October 2020
Please cite this article as: Tahere Kondori , NiloufarAkbarzadeh T , Mahboubeh Fazli , Behrouz Mir ,
ˇ
´
Michal Dusek , Vaclav Eigner , A novel Schiff base ligand and its copper complex: Synthesis, charac-
terization, X-ray crystal structure and biological evaluation, Journal of Molecular Structure (2020), doi:
https://doi.org/10.1016/j.molstruc.2020.129395
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Highlights
The novel Schiff base ligand of(a) crystallized in monoclinic system, in centrosymmetric
space group P21/c and complex (b) crystallized in triclinic system, in centrosymmetric space
group P-1.
The antibacterial activities have been studied in-vitro against two gram-negative
(Escherichiacoli and Pseudomonas aeruginosa) and two gram positive (Staphylococcus
aureus and Enterococcus faecalis) standard bacterial strains.The complex (b) was a stronger
antibacterial agent than the free ligand (a).

A novel Schiff base ligand and its copper complex: Synthesis, characterization, X-ray
crystal structure and biological evaluation
Tahere Kondori1, NiloufarAkbarzadeh-T*1, Mahboubeh Fazli1, Behrouz Mir1, , Michal
Dušek2 and Václav Eigner2
1Department of Chemistry, University of Sistan and Baluchestan, P.O. Box 98135-674, Zahedan,
Iran*e-mail: n.akbarzadeh@chem.usb.ac.ir
bInstituteof Physics of the Czech Academy of Sciences, Prague, Czech Republic.

Abstract
A novel Schiff base ligand of (4-nitroaniline-2-hydroxybenzaldehyde) (a) and its complex
[Cu(4-nitroaniline-2-hydroxybenzaldehyde)2] (b) were synthesized and characterized by FT-
IR, UV–Vis spectroscopy, elemental analysis and X-ray crystallography.The antibacterial
activities have been studied in-vitro against two gram-negative (Escherichia
coliandPseudomonas aeruginosa) and two gram positive (Staphylococcus aureus and
Enterococcus faecalis)standard bacterial strains. The ligand and its metal chelate were
screened for their antimicrobial activities using the disc diffusion method against the selected
bacteria. The complex (b) was a stronger antibacterial agent than the free ligand as well as its
corresponding metal salt, and it was more active against the gram-positive than gram-
negative bacteria.
Keywords:Schiff base ligand, X-ray crystallography, Antibacterial activity, Gram Negative,
Disc Diffusion

1. Introduction
The Schiff base ligands have many advantages among ligands in the coordination
chemistry[1–4]. Schiff base ligands that have the azomethin group in their structure are a
product of the direct condensation reaction between different aldehydes or ketones with the
primary amines [5,6]. These ligands with nitrogen and oxygen donor atoms are good
chelating agents for both transition and non-transition metals and form stable compounds
with most transition metals [7–9]. Metal compounds containing N and O donor ligands are
important due to different applications [10]. Schiff bases are important for their biological
properties [11,12], namely antibacterial and antifungal [13–15]. The metal compounds of
Schiff bases have potential application as drugs [16–19]. They can easily attach to metal ions
and form stable complexes. These complexes are very popular due to their variety of
applications in different fields [20]. Many drugs related to analogues of pyridine, bipyridine
and their derivatives display remarkable antibacterial and antitumor properties [21,22].
Complexes with bipyridine ligands have shown various applications like catalysts and
organic light-emitting diodes. They have also been studied in the field of electron transfer in
transition metal complexes and biological systems, artificial photosynthesis, and
electrochemistry [23–27]. Schiff base ligands are good chelating agents for metal ions to
coordination and form stable complexes [9]. Schiff base metal complexes have been
investigated for various applications in a variety of fields including biological,
pharmaceutical and analytical[20]. Inorganic complexes containing copper metal ion are of
great interest due to their flexibility in the preparation of various complexes with a
coordination number of 4-6[28]. In the last decade, copper complexes derived from Schiff
base binary ligands have been taken into consideration because of diversities in their
structural chemistry[29]. Copper metal-containing complexes have antibacterial
properties[30–34].
Here, we report the synthesis, characterization and crystal structure of a novel Schiff base
ligand of (4-nitroaniline-2-hydroxybenzaldehyde) (a) and its complex [Cu (4-nitroaniline-2-
hydroxybenzaldehyde) ] (b). Antibacterialproperties of compounds (a) and (b) were
2
evaluated. The antimicrobial effects of the complex (b) against twogram-positive
(Staphylococcus aureus; S. aureusATCC 25923 and Enterococcus faecalis; Enter_faeca
ATCC 29212) and twogram-negative (Escherichia coli; E. coli ATCC 25922 and
Pseudomonas aeruginosa; P. aeruginosa ATCC 27853) bacteria strains were examined by
standard methods and compared with free ligand (a), metal salt Cu(CH3COO)2. H2O, and
standard antibacterial agents Gentamycin and Amikacin.
2. Experimental Section
2.1. Materials and physical assessments:
Chemicals were purchased from Sigma Aldrich and applied during the experiments without
further purification. Electronic absorption spectra in H2O solution were recorded at room
temperature on a Cary Bio 300 spectrometer with a Heraeus CHN Rapid analyzer. FT-IR
Spectrum of compounds was obtained as 1% dispersions by Shimadzu-470 spectrometer with
KBr pellet (4000-400 cm-1). X-ray diffraction (XRD) measurements were carried out by
Philips X’pert diffractometer with graphite monochromatic Cu Kα (λ= 0.15418 nm)
radiation.

2.2.Synthesis of ligand (a)
Ligand (a) was synthesized by addition of methanol solution of paranitroaniline (0.069 g, 0.5
mmol) to a constantly stirring methanol solution of 2- hydroxybenzaldehyde(0.061g, 0.5
mmol) at room temperature. The resulting solution was refluxed for 6 hours at 313-323 K.
The mixture maintained at room temperature for slow crystallization by slow
evaporation(m.p ≈ 425 K). Elemental analysis calculated for ligand (a) (MW= 242.2):C,
64.46; H, 4.16; N, 11.56; O, 19.82. Found: C, 64.40; H, 4.12; N, 11.49; O, 19.85. FT-IR
(KBr, cm- 1, w = weak, m = medium, s = strong): 3405m, 1765w, 1623s, 1432w, 1182m,
1169m, 576m. The molecule of ligand (a) in its crystal structure is shown in Fig. 2.
2.3.Synthesis of complex (b)
Complex (b) was synthesized by addition of acetonitrile solution of ligand (a) (0.09 g, 0.4
mmol) to a constantly stirring methanol solution of Cu(CH3COO)2. H2O(0.04g, 0.2mmol) at
temperature of 313-323 K. The resulting solution was refluxed for 6 hours at 313-323 K. The
mixture maintained at room temperature for slow crystallization by slow evaporation
(m.p>573 K). Elemental analysis calculated for (b) (MW= 628.10): C: 57.37, H: 3.85, N:
13.38, O: 15.28. Found: C: 57.28, H: 3.87, N: 13.35, O: 15,21 FT-IR (KBr, cm- 1, w = weak,
m = medium, s = strong): 3443m, 1827w, 1703s, 1467w, 1206m, 1198m, 924m, 513s. The
molecule of (b) in its crystal structure is shown in Fig. 3.
2.4.Single crystal X-ray crystallography
The suitable single crystals of ligand (a) and complex (b) were selected for measurement on
Rigaku OD Supernova diffractometer, equipped with AtlasS2 CCD detector, using Cu-Kα
radiation (λ = 1.54184 Å) from micro-focused sealed X-ray tube. The measurements were
performed at 95 K. The data reduction, absorption correction and scaling were done using
CrysAlis PRO [35]. The structures were solved by charge flipping in Superflip[36] and
refined by full matrix least squares on F2 in Jana2006 [37]. The residual electron density
maps were visualized using MCE [38]. In both structures all hydrogen atoms were discernible
in difference Fourier maps and could be refined to reasonable geometry. According to
common practice H atoms bonded to C were kept in ideal positions with C–H = 0.96 Å while
positions of H atom bonded to O were refined with restrained bond length. In both cases
Uiso(H) was set to 1.2Ueq(C,O). All non-hydrogen atoms were refined using harmonic
refinement. For further details on crystal data, data collection and refinement see Table 1.
Please insert Table.1 here
2.5.UV–Vis investigation ligand (a) and complex (b)
Electronic spectrum of ligand (a) and complex (b) in H2O solution includes various
absorption peeks in the UV-Vis region (Fig. 1). The Schiff base ligand (a) shows absorption
band in 313 nm attributed to π →π* transition of azomethine and benzene ring of Schiff base
ligand. A weaker and less energetic peak in 367 nm is assigned to n→π* transition the lone
pair electrons of azomethine nitrogen. n→π* and π →π* transitions were shifted to another
wavelength on (b) formation. It arises due to the coordination of the lone pair of electrons
with the metal ion. The complex (b) exhibits absorption band in 415 nm, assigned to a ligand-
metal charge transfer (LMCT) and a band at around 448 nm correspond to d→d transitions in
the complex and has been observed in similar complexes[39,40].
Please insert Fig.1 here

3. Antibacterial tests
3.1.In-vitro antimicrobial experiment
.
The antibacterial activities of the complex (b), ligand (a) and metal salt Cu(CH3COO)2 H2O
were obtained against two standard gram-positive strains of Staphylococcus aureus (S.aureus
ATCC 25923) and Enterococcus faecalis (Enter_faeca ATCC 29212)and two standard gram-
negative strains of Escherichia coli (E. coli ATCC 25922) and Pseudomonas aeruginosa (P.
aeruginosa ATCC 27853). In this research, the broth macro-dilution test was performed to
measure the minimum bactericidal concentration (MBC) and the minimum inhibitory
concentration (MIC) of the synthesized complex[41]. Also, the disk diffusion method was
carried out for obtaining the diameter of expanded inhibition zone (IZ) near the cavity. The
MIC method is the minimum amount of an antibacterial complex that prevents the
considerable growth of bacteria at 310 K after 24-hour incubation condition. The stock
solution of complex (b) was obtained by DMSO and distilled water. Finally, we measured the
diameter of each inhibition zone (IZ) exhibiting no bacterial growth. The MIC, MBC and
inhibition zone experiments were carried out three times, and the results were obtained in an
average. In these tests, Gentamycin and Amikacinwere used as standard confirmed
antibacterialagents.
4. Results and discussion
The ligand (a) crystallized in monoclinic system, in centrosymmetric space group P21/c. The
asymmetric unit consists of one molecule of the ligand (see Fig.2). Since the only good
hydrogen bond donor is used on intramolecular O–H···N hydrogen bond, the structure
packing is dominated by weaker noncovalent interactions. The most prominent is formation
of dimer held π···π interactions (see Fig.3). The dimers are held together by C–H···O and C–
H···π weak hydrogen bonds, forming a well-developed network. For further information on
noncovalent interactions see Table 2.
Please insert Fig.2 here
Please insert Fig.3 here
Please insert Table.2 here
The complex (b) crystallized in triclinic system, in centrosymmetric space group P-1. The
asymmetric unit consists of half of the complex and one molecule of acetonitrile (see Fig.4).
In the absence of strong hydrogen bond donors, weaker noncovalent interactions are the
driving force of crystal packing. The weak hydrogen bond C–H···O assisted by π···π
stacking interactions form an infinite chain in direction of <0 1 0> (see Fig.5).The chains are
further connected into the network by C–H···O bonds, while C–H···N and C–H···O
hydrogen bonds hold acetonitrile in its position (see Fig.6). For further information on
noncovalent interactions see Table 3.
Please insert Fig.4 here
Please insert Fig.5 here
Please insert Fig.6 here
Please insert Table.3 here
The Cu(II) cation is coordinated by two nitrogen and two oxygen atoms from two ligand
molecules, resulting in slightly distorted square planar geometry. This distortion is a result of
weaker coordination to nitrogen with Cu–N bond length 2.0244 (16) Å compared to Cu–O
bond length 1.8805 (10) Å. The formation of six membered chelate rings has a significant
effect on the ligand molecule, affecting bond lengths, angles as well as the conformation.The

most affected bond C1–O1 is shortened by 0.051 Å and the most affected angle C7–N1–C8 is
decreased by 6.04 °. The pronounced change of ligand conformation is observed, as a free
ligand (a) is almost planar, with angle between phenyl planes at 3.10 (5) °, in complex (b) it
becomes heavily twisted, with the corresponding angle of 49.43 (7) °. This is probably caused
by disruption of delocalized electron system in observed in ligand (a) by formation of Cu–N
bond. For further information on selected bond lengths, angles and interplanar angles see
Table 4.
Please insert Table.4 here
5. Antimicrobial Activity Investigations
The MBC, MIC and IZ parameters of the Cu(II) complex, the metal salt Cu(CH3COO)2. H2O
and its ligand (4-nitroaniline-2-hydroxybenzaldehyde) were screened against two gram-
positive standard strainsStaphylococcusaureus (S.aureus ATCC 25923) and Enterococcus
faecalis (Enter_faeca ATCC 29212) and also two gram-negative standard strains Escherichia
coli (E. coli ATCC 25922) and Pseudomonas aeruginosa (P. aeruginosa ATCC 27853). In
these experiments, Gentamycin and Amikacin were used as standard antibacterial agents.
A comparative study of the ligand and the complex indicates that the Cu compound exhibit
more profound antibacterial activity than free ligand. Such increase activity of the metal
complexes can be explained on the basis of Overtone’s concept [41–44] and Tweedy’s
chelation theory [42]. There is a decrease in the polarity of the Cu atom significantly after
chelation, because of the partial sharing of its positive charge with the donor groups and also
due to π-electron delocalization on the whole chelate ring. The Cu complex interaction is
preferred with the lipids and polysaccharides which are the important constituents of the
cell wall and membranes. Furthermore, decreasing of polarity led to increasing the lipophilic
character of the chelates and an interaction between the Cu complex and the lipid is favoured
which may lead to the subsequent breakdown of the permeability barrier of the cell resulting
in obstruction with the normal cell processes [45]. Due to liposolubility, this is an important
factor that controls antimicrobial activity. Comparison of the antimicrobial properties of the
titled compound with similar compounds [46-48]indicates that (b) is a more potent
antibacterial agent.
Please insert Table.5 here
Please insert Table.6 here
6. Conclusion
In this article, we reported the synthesis of a novel Schiff base ligand of (4-nitroaniline-2-
hydroxybenzaldehyde) (a) and [Cu(4-nitroaniline-2-hydroxybenzaldehyde)2] (b). The
obtained compounds were characterized by FT-IR, UV–Vis spectroscopy, elemental analysis
and X-ray crystallography. The ligand (a) crystallized in monoclinic system, in
centrosymmetric space group P21/c. The asymmetric unit consists of one molecule of the
ligand. The complex (b) crystallized in triclinic system, in centrosymmetric space group P-1.
The asymmetric unit consists of half of the complex and one molecule of acetonitrile. The
Cu(II) cation is coordinated by two nitrogen and two oxygen atoms from two ligand
molecules, resulting in slightly distorted square planar geometry.

The MBC, MIC and IZ parameters of the Cu(II) complex, the metal salt Cu(CH3COO)2. H2O
and its ligand (a) were screened against two gram-positive standard strains and also two
gram-negative standard strains. The results show that antibacterial property of the complex
(b) is considerably better than the one of its free ligand (a). Also, complex (b) is a strong
antibacterial agent, and it was more active against the gram-positive than gram-negative
bacteria.

Acknowledgement
The authors sincerely thank the University of Sistan and Baluchestan for providing financial
support for this work. The crystallographic part was supported by the project 18-10504S of
the Czech Science Foundation using instruments of the ASTRA laboratory established within
the Operation program Prague Competitiveness - project CZ.2.16/3.1.00/24510.

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Figure captions
Fig.1. The electronic absorption spectrum of 4-nitroaniline-2-hydroxybenzaldehyde free
ligand (a) and the [Cu(4-nitroaniline-2-hydroxybenzaldehyde)2] (b) (3.5×10–5 M) in H2O.
Fig.2. The numbered scheme of ligand (a), ADPs drawn at 50% probability.
Fig.3. The dimer in structure (a), intramolecular hydrogen bonds in black, π···π interactions
in magenta.
Fig.4. Numbered scheme of complex (b).The ADPs drawn at 50% probability level.
Symmetry code: (i) 1-x, 1-y, 1-z.
Fig.5. Noncovalent interactions forming chain in direction <0 1 0>. Hydrogen bonds depicted
in black, π···π stacking interactions in pink. Hydrogen atoms not involved in hydrogen bonds
were omitted for clarity.
Fig.6. Hydrogen bonds connecting chains and solvent in complex (b). C–H···O depicted in
magenta, C–H···N depicted in green. Hydrogen atoms not involved in hydrogen bonding
were omitted for clarity.

Table 1
Crystallographic information on structures (a) and (b).
(a)
(b)
Empirical formula
Formula weight (g mol-1)
Crystal size (mm3)
T (K)
C13H10N2O3
242.2
0.26 x 0.13 x 0.06
95
C26H18CuN4O6·2(C2H3N)
628.1
0.06 x 0.05 x 0.03
95
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/c
12.4277 (5)
5.7006 (2)
15.3573 (7)
90
97.549 (3)
90
1078.56 (8)
4
Triclinic
P-1
7.0864 (4)
8.6614 (4)
11.8517 (7)
72.114 (5)
83.376 (5)
79.599 (4)
679.51 (7)
1
α (°)
β (°)
γ (°)
V (Å3)
Z
ρcalc (g cm-3)
μ (mm-1)
1.492
0.90
1.535
1.62
F(000) (e)
θmin, θmax (°)
Refl. Col., Indep., Rint
Ref. Param., Restr.
Goodness of fit
R1 [I>3σ(I)]
wR2 [I>3σ(I)]
R1 [all]
504
323
3.6, 74.0
3748, 2108, 0.014
166, 1
1.64
0.0337
3.9, 74.0
4561, 2656, 0.021
196, 0
1.19
0.0268
0.0971
0.0389
0.0724
0.0336
wR2 [all]
0.1016
-0.19, 0.21
2019596
0.0768
-0.36, 0.27
2019598
Δρmin, Δρmax (e Å-3)
CCDC number

Table 2
Selected noncovalent interactions in structure (a).
D–H···A
D–H (Å)
0.821(10)
0.96
0.96
0.96
H···A (Å)
1.917(13)
2.72
2.61
2.56
D···A (Å)
2.6436(13)
3.4790(17)
3.2723(14)
3.3353(15)
3.4656(16)
3.5667(16)
D–H···A (°)
147.0(14)
136
127
138
O1–H1o1···N1
C3–H1c3···C2i
C4–H1c4···O3ii
C5–H1c5···O2iii
C12–H1c12···O3iv
C13–H1c13···C5i
CgX···CgY
0.96
0.96
2.67
2.80
141
137
CgX···CgY (Å) CgX···PlY (Å) CgY···PlX (Å)
3.5595 (18) 3.2528 (12) 3.3259 (12)
Cg1···Cg2v
Symmetry codes: (i) 1-x, 0.5+y, 0.5-z; (ii) -1+x, 1.5-y, -0.5+z; (iii) -1+x, 0.5-y, -0.5+z; (iv) 2-
x, 2-y, 1-z; (v) 1-x, 1-y, 1-z.
Cg1 is a centroid of C1, C2, C3, C4, C5 and C6 atoms. Cg2 is a centroid of C8, C9, C10,
C11, C12 and C13 atoms. PlY and PlX are mean planes of corresponding atoms.

Table 3
Selected noncovalent interactions in structure (b).
D–H···A
D–H (Å)
0.96
0.96
0.96
0.96
H···A (Å)
2.67
2.71
2.58
2.35
D···A (Å)
3.531(2)
3.561(2)
3.394(2)
3.242(2)
3.594(2)
3.4929(19)
D–H···A (°)
150
149
143
155
C4–H1c4···O2i
C6–H1c6···N1sii
C7–H1c7···O3iii
C12–H1c12···O1iv
C13–H1c13···N1s
C2s–H3c2s···O2i
CgX···CgY
0.96
0.96
2.67
2.69
163
141
CgX···CgY (Å) CgX···PlY (Å) CgY···PlX (Å)
4.410(2) 3.1899(14) 3.1899(14)
Cg1···Cg1v
Symmetry codes: (i) x, 1+y, -1+z; (ii) x, 1+y, z; (iii) -x, -y, 1-z; (iv) x, -1+y, z; (v) 1-x, -y, 1-z.
Cg1 is a centroid of C8, C9, C10, C11, C12 and C13 atoms. PlY and PlX are mean planes of
corresponding atoms.

Table 4
Selected bond lengths, angles and interplanar angles in structures (a) and (b).
(a)
(b)
(a)
(b)
X–Y
X–Y (Å)
X–Y (Å)
X–Y–Z
X–Y–Z (°)
X–Y–Z (°)
O1–C1
N1–C7
N1–C8
C1–C2
C2–C7
Cu1–O1
Cu1–O1i
Cu1–N1
Cu1–N1i
1.3502 (13)
1.2893 (14)
1.4097 (14)
1.4144 (16)
1.4462 (15)
1.299 (2)
1.298 (2)
1.4315 (19)
1.425 (3)
1.4330 (19)
1.8805 (10)
1.8805 (10)
2.0244 (16)
2.0244 (16)
C7–N1–C8
O1–C1–C2
C1–C2–C7
N1–C7–C2
O1–Cu1–O1i
O1–Cu1–N1
O1–Cu1–N1i
O1i–Cu1–N1
O1i–Cu1–N1i
N1–Cu1–N1i
Pl1*–Pl2**
121.12 (10) 115.08 (15)
121.62 (9) 123.76 (13)
122.29 (10) 122.19 (15)
121.86 (10) 127.36 (17)
180
91.69 (5)
88.31 (5)
88.31 (5)
91.69 (5)
180
3.10 (5)
49.43 (7)
Symmetry code: (i) 1-x, 1-y, 1-z.
* Pl1 represents mean plane through atoms C1, C2, C3, C4, C5 and C6.
** Pl2 represents mean plane through atoms C8, C9, C10, C11, C12 and C13.

Table 5
The data of MIC and MBC of ligand (a) and Cu(II) complex (b), Cu(CH3COO)2. H2O, 4-
nitroaniline-2-hydroxybenzaldehyde against investigated bacterial strains.
MIC/ MBC (mg/mL)
Ligand (a)
Cu(CH3COO)2. H2O
Complex (b)
0.0125/0.0250
0.0125/0.0250
0.0062/0.0125
0.0030/0.0060
EscherichiaColi
0.0125/0.0250
0.0250/0.0500
0.0125/0.0250
0.0125/0.0250
0.0250/0.0250
Pseudomonas aeruginosa
Staphylococcusaureus
Enterococcus faecalis
unavailable for the study
0.0250/0.0500
unavailable for the study

Table 6
The data of IZ (mm) for Cu(CH3COO)2 salt, free ligand (a) (4-nitroaniline-2-
hydroxybenzaldehyde) and Cu(II) complex (b) against investigated bacterial strains.
Inhibition zone (mm)
Cu(CH3COO)2. H2O
Ligand(a) Complex (b) Gentamycin
Amikacin
18
10
11
22
24
26
28
19.2
-
EscherichiaColi
Pseudomonas aeruginosa
Staphylococcusaureus
Enterococcus faecalis
unavailable for the study
16
20.1
-
`13
14
-
-
18.8
19.2
unavailable for the study

Fig.1.

Fig.2.

Fig.3.

Fig.4.

Fig.5.

Fig.6.

Declaration of Competing Interest
The authors have no declaration of interest.