Synthesis and characterization of N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan derivatives with antioxidant and antifungal activities

Article abstract of DOI:10.3390/md18030163

Chitosan is an active biopolymer, and the combination of it with other active groups can be a valuable method to improve the potential application of the resultant derivatives in food, cosmetics, packaging materials, and other industries. In this paper, a series of N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan derivatives were synthesized. The combination of chitosan with ureidopyridinium group and quaternary ammonium group made it achieve developed water solubility and biological properties. The structures of chitosan and chitosan derivatives were confirmed by FTIR,¹H NMR spectra, and elemental analysis. The prepared chitosan derivatives were evaluated for antioxidant property by 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging ability, hydroxyl radical scavenging ability, and superoxide radical scavenging ability. The results revealed that the synthesized chitosan derivatives exhibited improved antioxidant activity compared with chitosan. The chitosan derivatives were also investigated for antifungal activity against Phomopsis asparagus as well as Botrytis cinerea, and they showed a significant inhibitory effect on the selected phytopathogen. Meanwhile, CCK-8 assay was used to test the cytotoxicity of chitosan derivatives, and the results showed that most derivatives had low toxicity. These data suggested to develop analogs of chitosan derivatives containing ureidopyridinium group and quaternary ammonium group, which will provide a new kind of promising biomaterials having decreased cytotoxicity as well as excellent antioxidant and antimicrobial activity.

Full text of DOI:10.3390/md18030163

marine drugs
Article
Synthesis and Characterization of
N,N,N-trimethyl-O-(ureidopyridinium)acetyl
Chitosan Derivatives with Antioxidant and
Antifungal Activities
Jingjing Zhang 1,2,3, Wenqiang Tan 1,2, Qing Li 1,2 , Fang Dong 1,2 and Zhanyong Guo 1,2,3,
*
1
Key Laboratory of Coastal Biology and Bioresource Utilization, Yantai Institute of Coastal Zone Research,
Chinese Academy of Sciences, Yantai 264003, China; jingjingzhang@yic.ac.cn (J.Z.); wqtan@yic.ac.cn (W.T.);
qli@yic.ac.cn (Q.L.); fdong@yic.ac.cn (F.D.)
Center for Ocean Mega-Science, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China
University of Chinese Academy of Sciences, Beijing 100049, China
2
3
*
Correspondence: zhanyongguo@hotmail.com; Tel.: +86-535-2109171; Fax: +86-535-2109000
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 20 February 2020; Accepted: 14 March 2020; Published: 16 March 2020
Abstract: Chitosan is an active biopolymer, and the combination of it with other active groups
can be a valuable method to improve the potential application of the resultant derivatives
in food, cosmetics, packaging materials, and other industries. In this paper, a series of
N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan derivatives were synthesized. The combination
of chitosan with ureidopyridinium group and quaternary ammonium group made it achieve developed
water solubility and biological properties. The structures of chitosan and chitosan derivatives were
confirmed by FTIR, 1H NMR spectra, and elemental analysis. The prepared chitosan derivatives
were evaluated for antioxidant property by 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging
ability, hydroxyl radical scavenging ability, and superoxide radical scavenging ability. The results
revealed that the synthesized chitosan derivatives exhibited improved antioxidant activity compared
with chitosan. The chitosan derivatives were also investigated for antifungal activity against Phomopsis
asparagus as well as Botrytis cinerea, and they showed a significant inhibitory effect on the selected
phytopathogen. Meanwhile, CCK-8 assay was used to test the cytotoxicity of chitosan derivatives,
and the results showed that most derivatives had low toxicity. These data suggested to develop
analogs of chitosan derivatives containing ureidopyridinium group and quaternary ammonium
group, which will provide a new kind of promising biomaterials having decreased cytotoxicity as
well as excellent antioxidant and antimicrobial activity.
Keywords: antifungal activity; antioxidant activity; chitosan derivatives; ureidopyridinium group;
quaternary ammonium group
1. Introduction
With the popularization of the concept of environmental protection, searching for environmentally
friendly biomaterials has become a new direction of scientific development. Natural, abundant
polysaccharides are attractive biomaterials for various applications such as food wrapper, tissue
engineering scaffolds, drug delivery vehicles, flocculating agents, and cosmetics, due to their
non-toxicity, biocompatibility, and biodegradability properties [1]. In this regard, chitosan, as a
natural polysaccharide, has a particularly extensive prospect in application of biomaterials [2,3].
Chitosan is the only cationic aminopolysaccharide in nature, and it is prepared by the
N-deacetylation of chitin, a polymer that can be extracted from the exoskeleton of insects and
Mar. Drugs 2020, 18, 163; doi:10.3390/md18030163
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crustaceans [
in biotechnology, pharmaceutics, papermaking, wastewater, cosmetology, food science, agriculture,
and textiles fields [ 13]. Presently used methods for the modification of chitosan mainly include
4–7]. Due to the unique biological activity, chitosan has attracted sufficient attention
8
–
quaternization, alkylation, acetylation, carboxylation, phosphorylation, and grafting [14]. Among the
derived products, quaternary ammonium chitosan salt not only maintains the unique characteristics of
chitosan but also possesses better solubility and biological activities, so it plays an important role in the
development of new functional chitosan derivatives [15,16]. N,N,N-trimethyl chitosan is well-known
as a kind of quaternary ammonium derivative of chitosan, which has attracted an increasing number
of researchers to study its properties and explore different applications. For example, we had designed
several N,N,N-trimethyl chitosan derivatives with better antioxidant and antifungal properties through
exchanging anions [17,18]. Martins et al. had synthesized N,N,N-trimethyl chitosan at two different
degrees of quaternization and prepared polyelectrolyte complexes of N,N,N-trimethyl chitosan/alginate
at pHs 2, 7, and 10 by mixing the aqueous solutions of unlike polymers. Results of controlled release of
curcumin indicated that the polyelectrolyte complexes with better solubility and biological activity
could be used in controlling the release of curcumin [19]. Besides, Britto et al. had evaluated the
biocompatibility of chitosan and chitosan quaternary salt coatings as edible coatings for sliced apple.
The beneficial use of water-soluble quaternary salt of chitosan was clearly seen by its low browning
effect on cut apple surface, representing a potential alternative for commercial hazardous anti-browning
compounds [20].
In the structure of N,N,N-trimethyl chitosan, there are still free hydroxyl groups, which can be
modified by active compounds. Borchard et al. presented the synthesis of O-carboxymethyl-N-trimethyl
chitosan (CMTMC) from the N,N,N-trimethyl chitosan obtained by applying an optimized synthesis
pathway. Further in vitro results indicated that CMTMC was devoid of toxicity on human dermal
fibroblasts with no negative influence on cell growth and morphology [21]. Wei et al. had synthesized
several series of N,N,N-trimethyl chitosan derivatives bearing imidazole groups and Schiff bases.
They reported that these chitosan derivatives held up fairly well in antifungal and antioxidant
activities [22,23]. In addition, N,N,N-trimethyl chitosan derivatives modified with 1,2,3-triazolium
groups also had excellent antifungal ability against Colletotrichum lagenarium, Fusarium oxysporum
f. sp. niveum, and Fusarium oxysporum f. sp. cucumebrium Owen as described by Li et al. [24].
Therefore, taking N,N,N-trimethyl chitosan as intermediate materials and grafting active groups onto
free hydroxyl groups is a feasible idea to further improve its biological activity.
Urea group is reported to exhibit a wide range of bioactivities and is of considerable interest in
applications of many fields [25,26]. Our research group has systematically studied the preparation
of functional chitosan derivatives containing urea group. We previously synthesized several urea
groups bearing nitrogen-containing heterocycles and grafted them onto N,N,N-trimethyl chitosan to
obtain several urea-functionalized chitosan derivatives. The obtained chitosan derivatives exhibited
excellent antifungal and antioxidant activities [27]. In addition to nitrogen-containing heterocycles,
benzene and halogenated benzene are also known to possess various pharmacological applications
such as antioxidant, antibacterial, antifungal, antitumor, and analgesic activity [28]. In this paper,
several urea groups containing benzene and halogenated benzene were synthesized and grafted onto
N,N,N-trimethyl chitosan to synthesized a new type of N,N,N-trimethyl-O-(ureidopyridinium)acetyl
chitosan derivatives. 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging ability, hydroxyl-radical
scavenging ability, and superoxide-radical scavenging ability of chitosan derivatives were evaluated
in vitro. Meanwhile, two phytopathogenic fungi, including Phomopsis asparagus (P. asparagus) and
Botrytis cinerea (B. cinerea) were selected to test the antifungal activity of the chitosan derivatives.
Moreover, the cytotoxicity of the synthesized compounds was investigated in vitro on L929 cells
and most tested compounds showed decreased cytotoxicity. This synthetic approach provides a
new direction for the preparation of chitosan-based biomaterials, such as hydrogels, biofilms, and
nanoparticles, and further broadens the application of chitosan in medicine, food, cosmetics, and other

Mar. Drugs 2020, 18, 163
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fields. Therefore, exploring analogs of chitosan derivatives containing ureidopyridinium group and
quaternary ammonium group is of great significance.
2. Results and Discussion
2.1. Chemical Synthesis and Characterization
Herein, N,N,N-trimethyl chitosan was treated as a semi-finished material to prepare derivatives
including ureidopyridinium group and quaternary ammonium group, and the detailed synthetic
strategy of N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan derivatives was shown in Scheme 1.
Firstly, several urea groups bearing pyridine and benzene ring were synthesized. Then, the intermediate,
CTCS, was synthesized via N,N,N-trimethyl chitosan. Finally, the obtained urea groups were grafted
onto CTCS based on the reaction mechanism of that chitosan derivatives with chloride acetyl group
could be attacked by pyridine to give N-alkypyridinium salts and four final chitosan derivatives
bearing urea groups were achieved [22,23,28]. The structures of all synthesized derivatives were
confirmed by FTIR (Figure 1) and 1H NMR (Figure 2). Their yields and degrees of substitution were
analyzed and the results were shown in Table 1.
O
OCCH2Cl
OH
CH I, NaI,
O
NaOH
O
3
O
ClCH2COCl
O
HO
HO
n
NH2
n
I
N
CH3
H3C
CH3
CS
CTCS
O
C
NaN3
H
N
H
RNH2
N R
COCl
N C O
N
N
N
DMSO CTCS
Br
O
C
H
H
a: R=
c: R=
;
;
b: R=
O
N
R
N
Cl
OCCH2
N
O
O
Cl
F
HO
n
I
N
d: R=
CH3
H3C
CH3
Scheme 1. Synthesis routes for chitosan derivatives.
2.1.1. FTIR Spectra
Figure 1 showed the FTIR spectra of chitosan and chitosan derivatives. For chitosan, the
characteristic peaks appeared at 3421 cm−1 (angular deformation of O-H and N-H), 2919 and 2881 cm−1
(-CH stretching), 1654 cm−1 (C=O stretching of amide band I), as well as 1596 cm−1 (amide band
II) [29,30]. As for CTCS, the spectrum showed the introduction of the quaternary ammonium salt group
and chloride acetyl group. Specifically, the peak of 1470 cm−1 showed the stretching vibration of C-H
from the trimethylammonium group. Meanwhile, the peaks at 1745 and 785 cm−1 were observed for
C=O stretching and C-Cl bending from chloride acetyl group, respectively [27]. After the introduction
of urea groups, the peaks of the C=O at 1745 cm−1 and C-Cl at 790 cm−1 were weaker for the partial

Mar. Drugs 2020, 18, 163
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substitution of new active groups [23
,
28]. Additionally, the spectra of chitosan derivatives a, b, c,
and d presented new peaks at 1693, ~1560, ~1520, and ~750 cm−1 compared with CTCS. Among of
these peaks, the existence of pyridine and benzene ring was confirmed by the peaks at ~1560, ~1520,
and ~750 cm−1
was also evidenced by the peak at 1698 cm−1
[
23
,27]. Meanwhile, the characteristic absorption of the structure of -NH-CO-NH-
[
27]. In addition, the characteristic peak of -N+(CH3)3
was also found at 1470 cm−1, indicating that the quaternary ammonium group was still present in
these final products. Hence, the above data preliminarily confirmed the successfully synthesized of
N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan derivatives.
CS
3421
1596
1654
2881
2919
785
CTCS
1470
1745
a
b
758
750
1507
1552
1693
1525
1589
1589
c
758
748
1534
1507
d
1560
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber(cm-1)
Figure 1. FTIR spectra of chitosan and chitosan derivatives.
1
2.1.2. H NMR Spectra
Figure 2 showed the 1H NMR spectra of chitosan and chitosan derivatives. Chemical shifts at
3.6–3.9, and 3.0 ppm were attributed to [H1], [H3]–[H6], and [H2] of chitosan [31]. For the
δ
5.5,
δ
δ
spectrum of CTCS, a new chemical shift (
δ
3.1 ppm), which could be assigned to the proton signal
of -N+(CH3)3, was appeared [22
,
23]. Meanwhile, another new characteristic peak at 4.4 ppm could
δ
1
be observed easily from the H NMR spectra of CTCS, which was attributed to proton signal of
-COCH2Cl [27]. After urea groups had been grafted onto CTCS, changes in the 1H NMR spectra of
derivatives a, b, c, and d were visible. In the 1H NMR spectra of derivatives a, b, c, and d, the chemical
shift of -COCH2Cl at
appeared in the low field. Specifically, the peaks at
ring [28]. The absorption signals that appeared at the range of
protons on the pyridine ring. Moreover, the peaks at 9.0–9.5 ppm were characteristic absorptions of
-NH-CO-NH- on urea groups [27]. Besides, the characteristic peak of -N+(CH3)3 at
3.1 ppm still existed,
δ
4.4 ppm was weaker and new absorption signals of pyridine and benzene ring
6.7–7.5 ppm were related to the protons of benzene
7.5–9.0 ppm were assigned to the
δ
δ
δ
δ
indicating the present of quaternary ammonium group in these final products. Therefore, the 1H NMR
data sufficiently indicated the successful synthesis of N,N,N-trimethyl-O-(ureidopyridinium)acetyl
chitosan derivatives.

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Figure 2. 1H NMR spectra of chitosan and chitosan derivatives.
2.1.3. Elemental Analysis
The degrees of substitution (DS) were calculated by elemental analysis. The results of DS and the
yields of chitosan derivatives were shown in Table 1. The degree of substitution of intermediate CTCS
was the highest. Meanwhile, among all the N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan
derivatives, derivative b presented the highest DS and derivatives c showed the lowest DS, which
were 0.42 and 0.34, respectively.

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Table 1. Yields and the degrees of substitution of chitosan derivatives.
Degrees of
Substitution
Compounds Yields (%)
Elemental Analyses (%)
Deacetylation
C
N
H
C/N
CS
CTCS
a
b
c
43.42
35.90
43.31
39.88
39.81
42.28
7.98
4.50
7.88
7.35
7.00
7.62
6.30
6.27
5.77
5.34
6.47
5.83
5.44
7.98
5.50
5.43
5.69
5.55
0.83
85.9
72.2
69.6
70.3
69.4
0.60
0.40
0.42
0.34
0.38
d
2.2. Antioxidant Activity
Free radicals, which are produced in the body as byproducts of normal metabolism and other
environmental factors, can damage proteins, nucleic acids, as well as carbohydrates and are believed
to contribute to premature aging and dementia [32
to reduce cellular damage and prevent different diseases. Because of their beneficial effects on human
health, there is widespread interest in antioxidants in both pharmaceutical and food science [35 36].
–34]. Antioxidants can scavenge these free radicals
,
We tested the antioxidant activity of the synthetic chitosan derivatives (Figures 3–5) and very satisfactory
results had been achieved.
110
100
90
Vc
CS
CTCS
80
70
a
b
c
60
d
50
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
concentration (mg/mL)
Figure 3. DPPH-radical scavenging activity of chitosan and chitosan derivatives.
80
Vc
CS
CTCS
70
a
b
60
c
d
50
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
concentration (mg/mL)
Figure 4. Hydroxyl-radical scavenging activity of chitosan and chitosan derivatives.

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100
90
80
70
60
50
40
30
20
Vc
CS
CTCS
a
b
c
d
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
concentration (mg/mL)
Figure 5. Superoxide-radical scavenging activity of chitosan and chitosan derivatives.
DPPH molecule is a very stable free radical at room temperature, and it has absorption band at
517 nm with deep purple color. Electron and hydrogen donors can react directly with DPPH radicals
to convert them to a more stable DPPH or DPPH2 molecule with pale yellow color. Hence, the change
of color and the decrease of absorbance can indicate the ability of a compound to act as a free radical
scavenger or a free radical hydrogen donor as well as to evaluate its antioxidant property [37,38].
Scavenging property of chitosan, chitosan derivatives, and Vc on DPPH radicals was measured and
the results were shown in Figure 3. The data of DPPH-radical scavenging rate were given in Table 2.
At all test concentrations, Vc always maintained the highest scavenging activity, and the scavenging
effects were around 100%, while the scavenging capacity of chitosan was always poor. Compared with
chitosan, all chitosan derivatives showed better DPPH radical scavenging property due to the presence
of trimethyl group and iodide ion, which were both active substance that was beneficial to improve
antioxidant activity. Moreover, chitosan derivatives bearing ureidopyridinium group had the better
DPPH scavenging ability compared with the intermediate CTCS. Moreover, the scavenging ability
of products d, c, and b, which contain halogens in the structure of urea group, was improved more
obviously. It indicated that derivatives with stronger electron-withdrawing capacity would possess
higher DPPH scavenging ability because these electrophilic groups could act as stabilizer for DPPH
radicals to inhibit the chain reaction of oxidation process, which was confirmed by the experimental
results. In Figure 3, when the concentration was 0.8 mg/mL, the scavenging activity of product b was
relatively prominent. Perhaps it was because derivative b had the highest degree of substitution. That
is, derivative b had the most active ingredients and therefore exhibited the higher scavenging capacity
at high concentrations.
Table 2. DPPH-radical scavenging activity of chitosan and chitosan derivatives (0.01 < * p < 0.05, 0.001
< ** p < 0.01, *** p < 0.001 vs. CS at the same concentration).
DPPH-Radical Scavenging Rate (%)
Sample
0.1 mg/mL
0.2 mg/mL
0.4 mg/mL
0.8 mg/mL
1.6 mg/mL
CS
CTCS
a
b
c
2.61 ± 1.53
6.68 ± 1.23 **
6.51 ± 0.89 *
5.07 ± 1.32
11.48 ± 2.34 *
11.89 ± 2.15 *
18.24 ± 1.25 ***
19.35 ± 1.35 ***
20.15 ± 2.54 *
6.94 ± 0.46
20.68 ± 2.01 *
21.91 ± 2.25 *
32.17 ± 2.36 **
42.35 ± 3.25 **
32.15 ± 3.64 *
7.66 ± 2.87
7.92 ± 1.32
34.69 ± 3.05 ***
37.62 ± 2.98 ***
80.46 ± 3.67 ***
68.73 ± 2.58 ***
55.94 ± 4.21 ***
53.09 ± 3.69 **
58.71 ± 1.99 ***
79.32 ± 3.54 ***
85.14 ± 1.99 ***
77.77 ± 3.54 ***
9.69 ± 1.21 ***
12.35 ± 0.87 **
15.54 ± 1.21 ***
d

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Hydroxyl radical is produced from H2O2 by Fenton reaction. In detail, hydrogen peroxide can
be catalyzed to dissociate into hydroxyl radicals (•OH) and hydroxide ions (OH−) while ferrous ions
(Fe2+) are oxidized to ferric ions (Fe3+). Safranine T can react with •OH to form colored substance,
which has absorbance at 520 nm. When hydroxyl radical scavenger is added into solution, •OH is
scavenged, and colored compounds significantly decrease, resulting in a decrease of the UV absorption
peak. Hydroxyl radical is the most active radical among all harmful reactive oxygen species, which
can react with biological macromolecules in living cell and hereby cause tissue damage as well as
cell death. Therefore, it makes sense to remove hydroxyl radicals to protect the organism from
damage [39,40]. Figure 4 illustrated the results of the scavenging ability evaluation of chitosan, chitosan
derivatives, and Vc on hydroxyl radicals. The data of hydroxyl-radical scavenging rate were given in
Table 3. At all tested concentrations, chitosan derivatives a, b, c, and d exhibited excellent scavenging
activity with a concentration-dependent manner. The scavenging activities of hydroxyl radical by
chitosan and derivatives were in the following order at 1.6 mg/mL: d > c > b > a > CTCS > chitosan.
This order indicated that the introduction of urea groups had a noticeable effect on improving the
scavenging ability. Besides, previous conclusion, which had indicated that the antioxidant activity of
polysaccharides was related to electron-absorbing ability of the substituted atoms, was confirmed by
the hydroxyl radical scavenging rule of d > c > b > a (-F > -Cl > -Br > -H). For example, the hydroxyl
radical scavenging indices of chitosan, CTCS, derivative a, derivative b, derivative c, and derivative d
were 21.45%, 49.75%, 66.37%, 69.88%, 74.21%, and 78.70% at 1.6 mg/mL.
Table 3. Hydroxyl-radical scavenging activity of chitosan and chitosan derivatives (0.01 < * p < 0.05,
0.001 < ** p < 0.01, *** p < 0.001 vs. CS at the same concentration).
Hydroxyl-Radical Scavenging Rate (%)
Sample
0.1 mg/mL
0.2 mg/mL
0.4 mg/mL
0.8 mg/mL
1.6 mg/mL
CS
CTCS
a
b
c
0
0
2.01 ± 2.33
20.63 ± 0.42 *
44.06 ± 0.46 **
53.58 ± 1.53 ***
60.90 ± 0.55 **
60.88 ± 0.96 ***
6.48 ± 1.32
21.45 ± 0.54
4.83 ± 0.58 *
20.28 ± 0.49 ***
16.81 ± 2.41 *
22.46 ± 0.40 ***
28.45 ± 1.39 **
11.65 ± 1.32 *
33.26 ± 1.33 ***
36.94 ± 0.54 ***
28.79 ± 1.29 **
47.59 ± 0.36 ***
33.28 ± 1.36 ***
57.55 ± 0.87 ***
62.89 ± 0.78 ***
68.39 ± 0.32 ***
67.55 ± 1.66 ***
49.75 ± 1.41 ***
66.37 ± 0.32 ***
69.88 ± 2.33 ***
74.21 ± 1.51 ***
78.72 ± 0.65 ***
d
Under a weak alkaline condition, nicotinamide adenine dinucleotide reduced-phenazine
methosulfate (PMS-NADH) system is prone to produce superoxide anion free radicals through
autoxidation. The superoxide anion radical can further reduce nitro blue tetrazolium (NBT) to purple
formazan, which has a maximum absorption band at 560 nm. When superoxide radical scavenger is
added, a part of superoxide radical will be consumed, resulting in a decrease of the absorption band at
560 nm. Based on the mechanism, this method can be applied to assay superoxide-radical scavenging
activity of chitosan derivatives [41,42]. The scavenging ability of chitosan, chitosan derivatives,
and Vc on superoxide radicals was determined and the results were shown in Figure 5. The data of
superoxide-radical scavenging rate were given in Table 4. As shown in Figure 5, within the experimental
concentration range, Vc had always maintained a good scavenging ability, and the scavenging rates
were above 90%. The scavenging effects of all chitosan derivatives were ascended rapidly with the
increase of the solution concentration. Of all chitosan derivatives, the antioxidant activity of chitosan
derivatives containing urea groups was significantly stronger than that of intermediate CTCS, which
further confirmed the important role of urea groups. However, the scavenging effects of derivatives
a, b, c, and d were not significantly different. For example, at 0.8 mg/mL, the superoxide radical
scavenging indices of derivative a, derivative b, derivative c, and derivative d were 76.69%, 78.57%,
80.52%, and 82.60%, respectively.

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Table 4. Superoxide-radical scavenging activity of chitosan and chitosan derivatives (0.01 < * p < 0.05,
0.001 < ** p < 0.01, *** p < 0.001 vs. CS at the same concentration).
Superoxide-Radical Scavenging Rate (%)
Sample
0.1 mg/mL
0.2 mg/mL
0.4 mg/mL
0.8 mg/mL
1.6 mg/mL
CS
CTCS
a
b
c
23.31 ± 0.41
27.01 ± 1.39
25.22 ± 0.54
32.48 ± 3.36
28.49 ± 1.64
39.74 ± 2.96 *
60.91 ± 1.98 ***
64.03 ± 2.62 ***
63.51 ± 2.71 ***
69.48 ± 0.42 ***
34.17 ± 0.78
50.16 ± 1.66 **
76.49 ± 1.59 ***
78.57 ± 0.78 ***
80.52 ± 2.69 ***
82.60 ± 2.36 **
34.72 ± 1.33
70.14 ± 0.65 ***
82.73 ± 0.70 ***
90.52 ± 1.33 ***
92.61 ± 1.29 ***
93.93 ± 1.41 ***
44.42 ± 1.41 **
48.05 ± 0.40 *
51.69 ± 0.43 **
53.77 ± 1.58 ***
54.29 ± 1.36 ***
57.14 ± 2.54 **
57.79 ± 1.56 ***
61.82 ± 1.32 *
d
2.3. Antifungal Activity
Plant mycosis is a kind of mycotoxin that has harmful effects on food safety and human health.
The antifungal agents currently used have the limitations of few available fungicides and the severe
toxicity of some fungicides. Meanwhile, the increasing resistance of pathogen fungi forces the
development of new antifungals [43]. Chitosan and chitosan derivatives have been recognized as a
potential antifungal substance. Herein, the antifungal activity of the synthetic chitosan derivatives
against P. asparagus (Figure 6) and B. cinerea (Figure 7) was carried out.
90
0.1 mg/mL
0.5 mg/mL
1.0 mg/mL
**
80
70
60
50
40
30
20
10
0
***
***
**
***
***
***
***
***
**
***
*
**
CS
CTCS
a
b
c
d
Figure 6. The antifungal activity of chitosan and chitosan derivatives against P. asparagus (0.01 < * p <
0.05, 0.001 < ** p < 0.01, *** p < 0.001 vs. CS at the same concentration).
As seen in Figure 6, the antifungal activity of chitosan and chitosan derivatives against P. asparagus
was positively correlated with their concentration. The antifungal ability of the four final chitosan
derivatives was significantly higher than that of chitosan and CTCS. Hence, it illustrated that the
enhancement of antifungal activity of chitosan derivatives was mainly attributed to the existence of
urea group. In addition, the different structure of urea group also led to a difference in the inhibitory
rate. The antifungal activity of N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan derivatives
showed that d > c > b > a, which was in accord with the order of the electronegativity of urea group
(-F > -Cl > -Br > -H). For example, at 1.0 mg/mL, the inhibition rates of derivative a, derivative b,
derivative c, and derivative d against P. asparagus were 64.02%, 69.05%, 76.98%, and 80.65%. One of
the more plausible explanations was that the electrophilic groups could attract and bind to negatively
charged substances on cell wall of microorganisms. This established interaction changed the membrane
permeability, hindered the metabolism of cell, and led to the death of microorganisms. Therefore,
chitosan derivatives with strong electron-absorbing ability had better antifungal activity.

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100
90
80
70
60
50
40
30
20
10
0
0.1 mg/mL
0.5 mg/mL
1.0 mg/mL
***
***
***
***
***
***
***
***
CS
CTCS
a
b
c
d
Figure 7. The antifungal activity of chitosan and chitosan derivatives against B. cinerea (*** p < 0.001 vs.
CS at the same concentration).
Figure 7 showed the antifungal activity of chitosan and its derivatives against B. cinerea. All samples
had inhibition on B. cinerea and the inhibitory indices increased with increasing concentration. However,
it was still chitosan and CTCS that showed the weakest antifungal effect, which further emphasized the
important role of urea group in improving the antifungal activity of chitosan derivatives. Additionally,
the antifungal order discussed above, which was d > c > b > a, still appropriated for the inhibitory
effect of chitosan derivatives on B. cinerea. The difference was that the ability of the final derivatives in
inhibiting B. cinerea was stronger than that of P. asparagus. For example, when the concentration was
0.5 mg/mL, the inhibition rates of derivative a, derivative b, derivative c, and derivative d against B.
cinerea were 55.63%, 66.05%, 67.43%, and 64.29%, while their inhibition rates against P. asparagus were
28.27%, 33.27%, 46.45%, and 41.08%.
2.4. Cytotoxicity Analysis
The in vitro CCK-8 assay was used to analyze the cytotoxicity of chitosan and chitosan derivatives
on L929 cells after incubation for 24 h. Figure 8 showed the cell viability of all samples. As seen
in Figure 8, the cell survival rates of chitosan were always around 100%. The cell survival rates of
chitosan derivatives decreased with test concentration indicating that the cell viability was negatively
correlated with their concentration. In addition, derivative CTCS possessed the strongest cytotoxicity.
For example, at 1000, 500, and 100
11.35%, and 46.95%, respectively. As to derivatives a, b, c, and d, they exerted no significant cytotoxic
effect at concentrations of 1 to 100 g/mL. When the concentrations were 1000 and 500 g/mL, these
derivatives exhibited low toxicity. When the concentration was 1000 g/mL, the L929 cells incubated
with derivatives a, b, c, and d had survival rates of 69.03%, 70.99%, 64.82%, and 65.20%.
µg/mL, the corresponding cell viabilities of CTCS were 10.64%,
µ
µ
µ

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120
1000 g/mL
500 g/mL
100 g/mL
10 g/mL
1 g/mL
0 g/mL
100
80
60
40
20
0
*
*
*
**
***
*
*
**
***
***
***
***
**
***
***
***
***
***
CS
CTCS
a
b
c
d
Figure 8. The cytotoxicity of chitosan and chitosan derivatives on L929 cells (0.01 < * p < 0.05, 0.001 < **
p < 0.01, *** p < 0.001 vs. CS at the same concentration).
3. Materials and Methods
3.1. Materials
Through the exploration of preliminary experiments, it was found that using pristine
chitosan with molecular weight of 200 kDa and degree of deacetylation of 85% could obtain
N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan derivatives with high yield and degree of
substitution. Chitosan (MW 200 kDa, the degree of deacetylation 85%) was purchased from Qingdao
Baicheng Biochemical Corp. (China) and used without purification. The materials, including
iodomethane, chloroacetyl chloride, nicotinoyl chloride hydrochloride, aniline, 2-fluoroaniline,
2-chloroaniline, and 2-bromoaniline, used in this research were purchased from the Sigma-Aldrich
Chemical Corp. N-Methyl pyrrolidone (NMP), sodium azide, dimethyl sulfoxide (DMSO),
N,N-dimethylformamide (DMF), ethanol, acetone, and methylbenzene were provided by Sinopharm
Chemical Reagent Co., Ltd. (Shanghai, China). All the chemical solvents and reagents were obtained
from commercial sources and used as received.
3.2. Synthesis of Chitosan Derivatives
3.2.1. Synthesis of Pyridylurea Groups
Nicotinoyl chloride hydrochloride (20 mmol) was dispersed equably in 15 mL of acetone firstly.
The mixture was then added dropwise to the aqueous solution of sodium azide and the reaction
mixture was stirred for 3 h at 0 ◦C. When the reaction was finished, the mixture had been stratified, and
the lower layer was removed. The remaining solution was poured into methylbenzene and stirred for
2–3 h at 60 ◦C. Then, the mixture was cooled and pink crystals were precipitated out. The precipitate
was filtered and pyridine-3-isocyanate was obtained. Pyridine-3-isocyanate was subsequently reacted
◦
with aniline in 20 mL of methylbenzene for 24 h at 60 C. Then N-phenyl-N0-pyridylurea was
formed and was further purified by crystallization from the solvent that the ratio of water and
ethanol was 1:1. N-(2-fluorobenzene)-N0-pyridylurea, N-(2-chlorobenzene)-N0-pyridylurea, and
N-(2-bromobenzene)-N0-pyridylurea were prepared by the same method.

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3.2.2. Synthesis of Chitosan Derivative CTCS
Chitosan (2 mmol) was dispersed in 20 mL of N-methyl-2-pyrrolidone (NMP) and stirred for 1 h
at 25 ◦C. Then, NaI (0.9 g), CH3I (3 mL), and 15% NaOH aqueous solution (3 mL) were added and the
mixture was refluxed at 60 ◦C for 2 h. Then, the solution was precipitated by excess ethanol, and the
precipitate was filtrated to obtain N,N,N-trimethyl chitosan. The N,N,N-trimethyl chitosan was then
dissolved in 30 mL of N,N-dimethylformamide (DMF) with chloracetyl chloride (1.5 mL) and stirred
at 30 ◦C for 12 h. Next, the mixture was precipitated and filtered with ethanol. Finally, the product
N,N,N-trimethyl-O-chloroacetyl quaternary ammonium salt chitosan (CTCS) was washed with excess
ethanol and achieved after vacuum freeze-drying for 24 h.
3.2.3. Synthesis of Chitosan Derivatives a, b, c, and d
The final chitosan derivative BCTCS was synthesized as follows: CTCS (1 mmol) and
N-phenyl-N0-pyridylurea (3 mmol) were dissolved in 20 mL of dimethyl sulfoxide (DMSO) and
◦
stirred for 24 h at 60 C. After reaction, the solution was precipitated with acetone. The obtained
precipitate was then filtered and washed with acetone. The byproducts could be extracted in this way.
Finally, the corresponding chitosan derivative a was obtained after freeze-drying. Chitosan derivatives
b, c, and d were prepared by the same method.
3.3. Analytical Methods
3.3.1. Fourier Transform Infrared (FTIR) Spectroscopy
The FTIR spectra of chitosan and chitosan derivatives were recorded on a Jasco-4100 FTIR
spectrometer (Tokyo, Japan, provided by JASCO China Co. Ltd., Shanghai, China) at a resolution of
4.0 cm−1 in the 4000–400 cm−1 region. The tested samples were mixed with KBr disks and scanned 32
times at 25 ◦C for observation.
3.3.2. Nuclear Magnetic Resonance (NMR) Spectroscopy
The 1H NMR spectra of chitosan and chitosan derivatives were measured via a Bruker Avance III
500 NMR Spectrometer (500 MHz, Fällanden, Switzerland, provided by Bruker Tech. and Serv. Co.,
Ltd., Beijing, China) at 25 ◦C. All the tested samples were dissolved in D2O for analysis.
3.3.3. Elemental Analysis
Elemental analysis was performed by Vario Micro Elemental Analyzer (Elementar, Hanau,
Germany). The degrees of substitution (DS) of all tested chitosan derivatives can be calculated on the
basis of the carbon–nitrogen ratios, and the calculated equations were as follows:
n1 × MC − MN × WC/N
DS1 =
n2 × MC
MN × WC/N + n2 × MC × DS1 − n1 × MC
DS2 =
n3 × M
MN × WC/N + n2 × MC × DS1 −Cn1 × MC − n3 × MC × DS2
DS3 =
n4 × MC
n0 × MN × WC/N + n2 × MC × DS1 − n1 × MC − n3 × MC × DS2 − n4 × MC × DS3
1
DS4 =
n5 × MC − n02 × MN × WC/N
where DS1, DS2, DS3, and DS4 represent the deacetylation degree of chitosan, the degrees of
N,N,N-trimethyl chitosan, N,N,N-trimethyl-O-chloroacetyl quaternary ammonium salt chitosan, and
N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan derivatives; MN and MC are the molar mass
of nitrogen and carbon, MN = 14, MC = 12; n1, n2, n3, n4, and n5 are the number of carbon of chitin,

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acetamido group, trimethyl, chloroacetyl group, and urea group, n1 = 8, n2 = 2, n3 = 3, n4 = 2, n5 = 12;
n01 and n02 are the number of nitrogen of trimethyl and urea group, n01 = 1, n02 = 3; WC/N represents the
mass ratio between carbon and nitrogen in chitosan derivatives.
3.4. Antioxidant Assay
3.4.1. DPPH-Radical Scavenging Activity Assay
The DPPH scavenging ability of chitosan and its derivatives was carried out by the following
method [17]: 1 mL of testing samples◦with different concentrations and 2 mL of DPPH ethanol solution
(180 µmol/L) were incubated at 25 C for 20 min. The control groups containing 1 mL of testing
samples with different concentrations and 2 mL of ethanol were also incubated at 25 ◦C for 20 min.
The absorbance of the remained DPPH radical was measured at 517 nm against a blank. Three
replicates for each sample were carried out and the scavenging effect was obtained according to the
following equation:
"
#
Asample 517 nm − Acontrol 517 nm
Scavenging effect (%) = 1 −
× 100
Ablank 517 nm
where Asample 517 nm is the absorbance of the samples at 517 nm, Acontrol 517 nm is the absorbance of the
control (ethanol replaced DPPH) at 517 nm, and Ablank 517 nm is the absorbance of the blank (distilled
water replaced samples) at 517 nm.
3.4.2. Hydroxyl-Radical Scavenging Activity Assay
The hydroxyl scavenging ability of chitosan and its derivatives was carried out by the following
method [22]: the solution including testing samples with different concentrations (10 mg/mL, 0.045,
0.09, 0.18, 0.36, and 0.72 mL), safranine T (0.23 µ µM), and H2O2 (60 µM) in
M), EDTA-Fe2+ (220
potassium phosphate buffer (150 mM, pH 7.4) was incubated at 37 ◦C for 30 min. The absorbance of
the remained hydroxyl radical was measured at 520 nm. Three replicates for each sample were carried
out, and the scavenging effect was obtained according to the following equation:
Asample 520 nm − Ablank 520 nm
Scavenging effect (%) =
× 100
Acontrol 520 nm − Ablank 520 nm
where Asample 520 nm is the absorbance of the samples at 520 nm; Acontrol 520 nm is the absorbance of the
control (potassium phosphate buffer replaced H2O2) at 520 nm, and Ablank 520 nm is the absorbance of
the blank (distilled water replaced samples) at 520 nm.
3.4.3. Superoxide-Radical Scavenging Activity Assay
The superoxide scavenging ability of chitosan and its derivatives was carried out by the following
method [23]: The reaction solution including test samples with different concentrations (5 mg/mL, 0.06,
0.12, 0.24, 0.48, and 0.96 mL), nicotinamide adenine dinucleotide reduced (NADH, 338
µM), phenazine
methosulfate (PMS, 30 M), and nitro blue tetrazolium (NBT, 72
µ
µM) in Tris-HCl buffer (16 mM, pH 8.0)
was incubated at 25 ◦C for 5 min. The absorbance of the remained superoxide radical was measured at
560 nm against blank. Three replicates for each sample were carried out, and the scavenging effect was
obtained according to the following equation:
!
Asample 560 nm − Acontrol 560 nm
Scavenging effect (%) = 1 −
× 100
Ablank 560 nm
where Asample 560 nm is the absorbance of the samples at 560 nm; Acontrol 560 nm is the absorbance of the
control (distilled water replaced NADH) at 560 nm, and Ablank 560 nm is the absorbance of the blank
(distilled water replaced samples) at 560 nm.

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3.5. Antifungal Assay
The antifungal ability was carried out by the method of hyphal measurement [27]. Briefly, the
stock solutions of chitosan and derivatives were prepared with a concentration of 6 mg/mL. Then,
each sample solution was added to sterilized potato dextrose agar (PDA) medium to obtain final
concentrations of 0.1, 0.5, and 1.0 mg/mL. The culture media containing samples were poured into
Petri dishes (7 cm). After solidification, 5.0 mm diameter of fungi mycelium was transferred to the test
plate and incubated at 27 ◦C for 2–3 days. When the mycelia of fungi reached the edges of the control
plate (without the presence of samples), the inhibition indices of all samples were calculated as follows:
!
Da
Db
Antifungal index (%) = 1 −
× 100
where Da is the diameter of the growth zone in the test plates, and Db is the diameter of the growth
zone in the control plate.
3.6. Cytotoxicity Assay
The cytotoxicity of chitosan and synthesized chitosan derivatives at different concentrations (1.0,
10.0, 100.0, 500.0, and 1000.0
µ
g/mL) on L929 cells was tested by CCK-8 assay. After culturing in RP◦MI
medium (including 1% mixture of penicillin and streptomycin and 10% fetal calf serum) at 37 C,
the L929 cells were transferred to 96-well flat-bottom culture plates at a density of 1.0 × 105 cells and
◦
incubated at 37 C (5% CO2) for 24 h. Then, the samples with different final concentrations were
introduced to cells, separately. Next, the cells were cultured for another 24 h. Afterward, CCK-8
solution (10 µ
L) was added in each well and incubated for 4 h at 37 ◦C. The absorbance at 450 nm was
recorded using a microplate reader. Cell viability was measured by the following formula:
Asample − Ablank
Cell viability (%) =
× 100
Anegative − Ablank
where Asample is the absorbance of the samples (including cells, sample solution, and CCK-8 solution);
blank is the absorbance of the blank (including CCK-8 solution and RPMI medium), and Anegative is
A
the absorbance of the negative (including CCK-8 solution and cells).
3.7. Statistical Analysis
All the experiments were performed in triplicate, and the data were expressed as mean
standard deviation (SD, n = 3). Significant difference analysis was determined using Scheffe’s multiple
range test. A level of p < 0.05 was considered statistically significant.
±
the
4. Conclusions
In the present work, the effect of the structural characteristics of chitosan derivatives bearing urea
groups on the antioxidant and antifungal properties was studied. A number of chitosan derivatives
were synthesized: intermediate CTCS as well as final products a, b, c, and d. FTIR technique was
1
applied for the confirmation of chitosan derivatives. H NMR characterization also provided proof
of this successful synthesis. Elemental analysis confirmed the degrees of substitution of synthesized
derivatives. The in vitro DPPH-radical scavenging activity, hydroxyl-radical scavenging activity,
and superoxide-radical scavenging activity were carried out to evaluate the antioxidant activity
of chitosan and chitosan derivatives. Their antifungal activity against P. asparagus and B. cinerea
was also investigated. The results indicated that the four final chitosan derivatives bearing urea
groups exhibited higher biological property than pristine chitosan and intermediate CTCS. This
conclusion showed the key role of urea group in enhancing the antioxidant and antifungal activities
of chitosan. Furthermore, the bioactivity could be influenced by electron-withdrawing capacity

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and derivatives with higher electron-withdrawing capacity showed the better antioxidant as well
as antifungal activities. Additionally, the cytotoxicity of chitosan and its derivatives was studied
by CCK-8 assay. The results revealed that the safe concentration of the sample was less than 1000
µg/mL under the feasibility of cell experiment. Taken together, we believed that the obtained
N,N,N-trimethyl-O-(ureidopyridinium)acetyl chitosan derivatives were a series of novel compounds
with low toxicity and high antioxidant and antifungal activities and provided a chitosan derivative
platform for application of biomaterials. For instance, the presence of charges on ureidopyridinium
group and quaternary ammonium group enabled formulation of nanoparticles or gels as carriers,
providing targeted delivery modes for various biomedical applications. Moreover, if these bioactive
derivatives were made into biofilms, they would also attract attention in the fields of food and cosmetics.
Therefore, special emphasis should be given on developing N,N,N-trimethyl chitosan derivatives
bearing active groups.
Author Contributions: Z.G. conceived and designed the experiment. J.Z. performed the experiments. J.Z. and
Z.G. analyzed the data. J.Z., W.T., Q.L., F.D., and Z.G. contributed reagents/materials/analysis tools. J.Z. and Z.G.
wrote the paper. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by National Key R&D Program of China (2019YFD0900705), the Natural Science
Foundation of Shandong Province of China (ZR2019BD064 and ZR2017BD015), Youth Innovation Promotion
Association CAS (2020219), the National Natural Science Foundation of China (41576156), and the Natural Science
Foundation of Shandong Province Science and Technology Development Plan (2019GHY112010).
Conflicts of Interest: The authors have declared no conflicts of interest.
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