Benzoquinoline derivatives: A straightforward and efficient route to antibacterial and antifungal agents

Article abstract of DOI:10.3390/ph14040335

We report here the design, synthesis, experimental and in silico evaluation of the antibacterial and antifungal activity of some new benzo[f]quinoline derivatives. Two classes of benzo[f]quinolinium derivatives-(benzo[f]quinolinium salts (BQS) and pyrrolobenzo[f]quinolinium cycloadducts (PBQC)-were designed and obtained in two steps via a direct and facile procedure: quaternization followed by a cycloaddition reaction. The synthesized compounds were characterized by elemental and spectral analysis (FT-IR, 1H-NMR, 13C-NMR). The antimicrobial assay reveals that the BQS salts have an excellent quasi-nonselective antifungal activity against the fungus Candida albicans (some of them higher that the control drug nystatin) and very good antibacterial activity against the Gram positive bacterium Staphylococcus aureus. The PBQC compounds are inactive. Analysis of the biological data reveals interesting SAR correlations in the benzo[f]quinolinium series of compounds. The in silico studies furnished important data concerning the pharmacodynamics, pharmacokinetics and ADMET parameters of the BQS salts. Studies of the interaction of each BQS salt 3a-o with ATP synthase in the formed complex, reveal that salts 3j, 3i, and 3n have the best fit in a complex with ATP synthase. Study of the interaction of each BQS salt 3a-o with TOPO II in the formed complex reveals that salts 3j and 3n have the best-fit in complex with TOPO II. The in silico ADMET studies reveal that the BQS salts have excellent drug-like properties, including a low toxicity profile. Overall, the experimental and in silico studies indicate that compounds 3e and 3f (from the aliphatic series), respectively, and 3i, 3j and 3n (from the aromatic series), are promising leading drug candidates.

Full text of DOI:10.3390/ph14040335

pharmaceuticals
Article
Benzoquinoline Derivatives: A Straightforward and Efficient
Route to Antibacterial and Antifungal Agents
Vasilichia Antoci 1, Liliana Oniciuc 1, Dorina Amariucai-Mantu 1, Costel Moldoveanu 1 , Violeta Mangalagiu 2
,
Andreea Madalina Amarandei 3, Claudiu N. Lungu 4,
*
, Simona Dunca 3,*, Ionel I. Mangalagiu 1,2
and Gheorghita Zbancioc 1,
*
1
Faculty of Chemistry, Alexandru Ioan Cuza University of Iasi, 11 Carol 1st Bvd, 700506 Iasi, Romania;
vasilichia.antoci@uaic.ro (V.A.); lili_oniciuc@yahoo.com (L.O.); dorina.mantu@uaic.ro (D.A.-M.);
costel.moldoveanu@uaic.ro (C.M.); ionelm@uaic.ro (I.I.M.)
Institute of Interdisciplinary Research—CERNESIM Centre, Alexandru Ioan Cuza University of Iasi,
11 Carol I, 700506 Iasi, Romania; violeta.mangalagiu@uaic.ro
Faculty of Biology, Alexandru Ioan Cuza University of Iasi, 11 Carol 1st Bvd, 700506 Iasi, Romania;
aamarandei16@yahoo.com
Department of Surgery, Emergency Clinical Hospital, 810325 Braila, Romania
Correspondence: lunguclaudiu5555@gmail.com (C.N.L.); sdunca@uaic.ro (S.D.);
gheorghita.zbancioc@uaic.ro (G.Z.)
2
3
4
*
Abstract: We report here the design, synthesis, experimental and in silico evaluation of the an-
tibacterial and antifungal activity of some new benzo[f]quinoline derivatives. Two classes of
benzo[f]quinolinium derivatives—(benzo[f]quinolinium salts (BQS) and pyrrolobenzo[f]quinolinium
cycloadducts (PBQC)—were designed and obtained in two steps via a direct and facile procedure:
quaternization followed by a cycloaddition reaction. The synthesized compounds were characterized
by elemental and spectral analysis (FT-IR, 1H-NMR, 13C-NMR). The antimicrobial assay reveals that
the BQS salts have an excellent quasi-nonselective antifungal activity against the fungus Candida
albicans (some of them higher that the control drug nystatin) and very good antibacterial activity
against the Gram positive bacterium Staphylococcus aureus. The PBQC compounds are inactive.
Analysis of the biological data reveals interesting SAR correlations in the benzo[f]quinolinium series
of compounds. The in silico studies furnished important data concerning the pharmacodynamics,
pharmacokinetics and ADMET parameters of the BQS salts. Studies of the interaction of each BQS
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Antoci, V.; Oniciuc, L.;
Amariucai-Mantu, D.; Moldoveanu,
C.; Mangalagiu, V.; Amarandei, A.M.;
Lungu, C.N.; Dunca, S.; Mangalagiu,
I.I.; Zbancioc, G. Benzoquinoline
Derivatives: A Straightforward and
Efficient Route to Antibacterial and
Antifungal Agents. Pharmaceuticals
2021, 14, 335. https://doi.org/
10.3390/ph14040335
salt 3a–o with ATP synthase in the formed complex, reveal that salts 3j, 3i, and 3n have the best fit
in a complex with ATP synthase. Study of the interaction of each BQS salt 3a-o with TOPO II in
the formed complex reveals that salts 3j and 3n have the best-fit in complex with TOPO II. The in
silico ADMET studies reveal that the BQS salts have excellent drug-like properties, including a low
toxicity profile. Overall, the experimental and in silico studies indicate that compounds 3e and 3f
(from the aliphatic series), respectively, and 3i, 3j and 3n (from the aromatic series), are promising
leading drug candidates.
Academic Editor: Mary Meegan
Received: 11 March 2021
Accepted: 2 April 2021
Published: 6 April 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Keywords: antibacterial; antifungal; molecular docking; mechanism of action; ADMET; SAR/QSAR;
benzo[f]quinoline; salts; cycloadducts
1. Introduction
According to the WHO, infectious diseases, especially those caused by bacterial (Gram
positive and Gram negative) and fungal microorganisms, have become a serious threat
to the global health system, being responsible for 22% of all deaths and 27% of disability-
Copyright:
© 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
adjusted life years worldwide [1]. Antibiotics play a key central role in antimicrobial
therapy, and are one of the most effective and successful weapons against different mi-
croorganisms. However, the overuse and misuse of antibiotics have led to widespread
drug resistance (DR), multi-drug resistance (MDR) and extensive-drug-resistance (EDR),
creating an urgent need for new antimicrobial agents [1–4].
Pharmaceuticals 2021, 14, 335. https://doi.org/10.3390/ph14040335
https://www.mdpi.com/journal/pharmaceuticals

Pharmaceuticals 2021, 14, 335
2 of 21
Quinoline and its benzo-fused derivative, benzoquinoline, are crucial scaffolds in
medicinal chemistry and they have been reported to possess a large variety of biologi-
cal activities which include antiplasmodial and antimalarial, antitubercular, antibacterial,
antifungal, anti-HIV, anticancer, antinociceptive and anti-inflammatory, antipsychotic, anal-
gesic, anti-Alzheimer’s, antihypertensive properties, etc. [5–17]. Moreover, the quinoline
pharmacophore has proved to be one of the most effective motifs used in antibacterial and
antifungal therapy, and many of the existing drugs on the market incorporate a quino-
line scaffold e.g., ciprofloxacin (antibacterial), bedaquiline (antitubercular), etc. [18
,19],
(Scheme 1).
Taking into consideration the above considerations and our expertise in the field of an-
tibacterial and antifungal agents [13 15 20 25], we report here the design, synthesis, molecular
,
, –
docking, antibacterial and antifungal evaluation of some new benzo[f]quinoline derivatives.
2. Results and Discussion
2.1. Desingn and Chemistry
Taking into consideration the abovementioned data, we decided to combine the phar-
macophoric antimicrobial capabilities (Scheme 1) of quinoline and its benzo derivatives [26
]
and also to convert them in salts, in view of the fact that salts usually have better antimicro-
bial activity and also better pharmacokinetic properties; we also had in mind increasing the
number of fused cycles from three in benzo[f]quinoline to four and to see their influence
in regards to the antimicrobial activity. In equal measure we were interested in seeing
the influence on antimicrobial activity of the substituents on the quaternized nitrogen,
taking into account two series an aliphatic and aromatic one. As a result, two series
of benzo[f]quinoline derivatives was designed and synthesized: BQS salts and PBQC
cycloadducts (Scheme 1).
Scheme 1. Design in the class of benzoquinolines derivatives with antimicrobial activity.
In order to synthesize our fused quinoline derivatives we used a direct and facile
two step procedure: quaternization of nitrogen heterocycles followed by a cycloaddition
reaction. Using an adaptation of the setup procedure from the literature [27
first step, we performed a quaternization reaction of benzo[f]quinoline ( ) with variously
activated -halocarbonyl compounds 2a (such as: 1-bromo-alkyl-2-one, 2-iodoacetamide,
(un)substituted phenacyl bromides), when the corresponding benzo[f]quinolinium qua-
–29], in the
1
α
–o

Pharmaceuticals 2021, 14, 335
3 of 21
ternary salts BQS 3a
–
o
were obtained (Scheme 2). The next step consists in a Hüisgen
[3 + 2] dipolar cycloaddition of the benzo[f]quinolinium ylides (generated in situ from the
corresponding benzo[f]quinolinium quaternary salts BQS 3a , in alkaline medium) to
–
o
alkyne dipolarophiles (non-symmetrically or symmetrically substituted Z-alkynes, methyl
propiolate and dimethyl acetylenedicarboxylate, DMAD), when the corresponding fused
pyrrolobenzo[f]quinolinium cycloadducts PBQC 4, were obtained (Scheme 2). Initially, the
cycloaddition reactions were performed in the case of salts 3e (aliphatic) and 3g (aromatic),
and the obtained cycloadducts PBQC (4e1, 4e2, 4g1, and 4g2) was subject to antimicrobial
assay. Because the antimicrobial activity of the obtained cycloadducts PBQC 4e and 4g
,
was negligible, we did not continue our studies in this direction.
Scheme 2. Reaction pathways to obtain benzo[f]quinoline salts 3a–o and cycloadducts 4e and 4g.
The structures of compounds were proved by elemental and spectral analysis (FT-IR,
1H-NMR, 13C-NMR, and two-dimensional experiments 2D-COSY, HMQC, HMBC). The
main data furnished by FT-IR and NMR spectral analysis are listed in Table 1. In the FT-IR
spectra of BQS salts 3a
–
o, the most important signals are those one of carbonyl groups.
In the aromatic BQS salts 3g
–
o, the signals corresponding to carbonyl ketone group are
situated between 1717 cm−1 (R = NO2, 3l) and 1673 cm−1 (R = OCH3, 3i), in accordance
with the electronic effects of substituents on the 4-position of a phenyl ring (electron-
withdrawing for NO2 and electron-donating for OCH3). In the aliphatic series (−B1QS salts
3a–f), the signals corresponding to carbonyl group are situated between 1626 cm (amide
CO, 3a) and 1743 cm−1 (carboxymethyl CO, 3b), in accordance with the structures of the
compounds (amide and aliphatic ester). In the 1H-NMR spectra the most important signals
are those of the H-2, H-4, and H-11 (methylene (–CH2, –) hydrogen) atoms. As it can
be seen in Table 1 the H-11 methylene hydrogen atoms appear at an unusual very high
chemical shift, around 6.50 ppm in the aliphatic series and around 7.00 ppm in the aromatic
series. This is in accordance with the powerful electron-withdrawing effect exerted by the
positive α–endocyclic nitrogen atom of the benzo[f]quinoline ring. The aromatic/aliphatic
difference of about 0.50 ppm is because of the supplementary electron-withdrawing effect
exerted by the aromatic phenyl ring. The H-2 and H-4 protons from the pyridine ring
appear at high chemical shift, around 10.25 ppm H-4 (positive γ–endocyclic nitrogen) and
around 9.50 ppm H-2 (positive α–endocyclic nitrogen).

Pharmaceuticals 2021, 14, 335
4 of 21
Table 1. Main spectral data of BQS salts 3a–o.
13C-NMR
−1
Salt
IR, cm
H-2
H-4
H-11 (CH2)
3a
3b
3c
3d
3e
3f
1626 (νCO
1743 (νCO
1741 (νCO
1736 (νCO
1723 (νCO
1708 (νCO
1682 (νCO
)
)
)
)
)
)
)
9.47
9.62
9.60
9.39
9.40
9.46
9.45
10.18
10.23
10.25
10.20
10.21
10.24
10.20
5.91
6.33
6.30
6.43
6.42
6.68
7.05
166.0 (C-12: CO), 148.5 (C-2), 142.1 (C-4), 59.3 (C-11: CH2)
166.6 (C-12: CO), 148.3 (C-2), 142.9 (C-4), 57.9 (C-11: CH2)
166.1 (C-12: CO), 148.4 (C-2), 142.8 (C-4), 57.9 (C-11: CH2)
199.7 (C-12: CO), 147.8 (C-2), 144.0 (C-4), 66.0 (C-11: CH2)
202.3 (C-12: CO), 147.8 (C-2), 142.2 (C-4), 65.3 (C-11: CH2)
207.0 (C-12: CO), 148.2 (C-2), 142.3 (C-4), 63.0 (C-11: CH2)
190.6 (C-12: CO), 148.2 (C-2), 142.5 (C-4), 63.9 (C-11: CH2)
3g
3h
3i
3j
3k
3l
3m
3n
3o
1677 (νCO
1673 (νCO
1689 (νCO
1706 (νCO
1717 (νCO
1696 (νCO
1698 (νCO
1694 (νCO
)
)
)
)
)
)
)
)
9.55
9.52
9.56
9.51
9.52
9.52
9.47
9.51
10.25
10.25
10.28
10.28
10.29
10.27
10.28
10.27
7.12
7.08
7.18
7.15
7.18
7.11
7.08
7.11
190.2 (C-12: CO), 148.2 (C-2), 142.4 (C-4), 63.8 (C-11: CH2)
188.9 (C-12: CO), 148.2 (C-2), 142.4 (C-4), 63.5 (C-11: CH2)
190.3 (C-12: CO), 148.2 (C-2), 142.5 (C-4), 63.9 (C-11: CH2)
190.3 (C-12: CO), 148.2 (C-2), 142.6 (C-4), 64.1 (C-11: CH2)
190.1 (C-12: CO), 148.2 (C-2), 142.7 (C-4), 64.1 (C-11: CH2)
190.1 (C-12: CO), 148.2 (C-2), 142.5 (C-4), 63.8 (C-11: CH2)
189.9 (C-12: CO), 148.2 (C-2), 142.6 (C-4), 63.8 (C-11: CH2)
189.5 (C-12: CO), 148.2 (C-2), 142.5 (C-4), 63.8 (C-11: CH2)
The 13C-NMR spectra of BQS salts 3a
dance with the FT-IR and 1H-NMR data presented above. The carbon from the ketone CO
group (C-12) appears around 190 ppm in the aryl-alkyl BQS salts 3g ; around 166 ppm in
the amide and carboxylic BQS salts 3a , and around 205 ppm in the in the alkyl-alkyl BQS
salts 3d–f. The C-11 methylene carbon atoms appear at high chemical shifts, between 66.0–
59.3 ppm, in accordance with the electron-withdrawing effect of the positive –endocyclic
–o also confirm the structurse and are in accor-
–
o
–
c
α
nitrogen atom from benzo[f]quinoline ring and with the structure of the ketone carbonyl
group (alkyl or aryl or amide/aliphatic ester). The C-2 and C-4 atoms appear around
148 ppm and 143 ppm, respectively, in accordance with their position in the pyridine ring
(α- or γ
- endocyclic carbons). The remaining signals in the FT-IR, 1H-NMR and 13C-NMR
spectra are also in accordance with the proposed structures.
2.2. Antimicrobial Assay
The sensitivity of the microorganisms to the substances under investigation, salts
BQS and cycloadducts PBQC, was assessed based on the diameter of the inhibition zone,
using the Kirby-Bauer agar disk diffusion method, adopted by the Clinical & Laboratory
Standards Institute (CLSI M07-A11, 2018) [30]. The method uses Mueller Hinton nutrient
agar medium for antibacterial tests and Sabouraud nutrient agar medium for antifungal
tests. The disk diffusion test provides a number of advantages, such as simplicity of
the method, low cost, large number of organisms and antimicrobial agents that can be
tested, and ease of interpreting the resulting data [30,31]. The in vitro antibacterial activity
was evaluated against the Gram-positive bacterium Staphylococcus aureus ATCC 25923
and the Gram-negative Escherichia coli ATCC 25922. The in vitro antifungal activity was
evaluated against the fungus Candida albicans ATCC 10231. Penicillin (10 IU), carbenicillin
(100
µg/mL) and nystatin (500,000 IU) were used as positive control (C+) for Staphylococcus
aureus, Escherichia coli and Candida albicans, respectively. As negative control (C
−
) sterile
filter paper disks (with no antimicrobial compounds) were used. The obtained results
were expressed as diameters of inhibition zones (mm). The larger the diameter of the
inhibition zones is, the more active the compounds are as antimicrobials and antifungals.
The obtained results are listed in Table 2 (and Figures S1–S3, see the Supporting Material
for details).

Pharmaceuticals 2021, 14, 335
5 of 21
Table 2. The antibacterial and antifungal activity for BQS salts 3a–o and PBQC cycloadducts 4e and 4g.
a
Diameter of Inhibition Zone (mm)
Cpd/R
S. aureus ATCC 25923
E. coli ATCC 25922
C. albicans ATCC 10231
3a/-NH2
3b/-OMe
3c/-OEt
3d/-Me
3e/-Et
0
19.5 ± 1.5
20 ± 1
16 ± 1
17 ± 1.73
20 ± 1
18 ± 1.5
20.5 ± 1.5
21.5 ± 1.73
15 ± 1.5
15 ± 2.5
15 ± 2
16 ± 1.5
21 ± 1.5
14 ± 1.5
0
0
10.5 ± 0.5
20 ± 1.5
22 ± 1.25
20 ± 1.80
30 ± 2
27.5 ± 1.5
20 ± 1.8
21.5 ± 1.8
22 ± 1
17 ± 1
15 ± 1.73
17 ± 1
19 ± 1.5
22.5 ± 1
14 ± 0.5
0
17 ± 1.73
18.5 ± 1.5
11 ± 1
12.5 ± 1.5
15.5 ± 2
13 ± 1.5
17 ± 2
3f/-t-Bu
3g/-C6H5
3h/-C6H4(Me)p
3i/-C6H4(OMe)p
3j/-C6H4(C6H5)p
3k/-C6H4(CN)p
3l/-C6H4(NO2)p
3m/-C6H4(Br)p
3n/-C6H4(Cl)p
3o/-C6H4(F)p
4e1
18 ± 1.73
13 ± 1.73
12.5 ± 1.25
14 ± 2.6
12 ± 1.5
18 ± 1.32
10.5 ± 1.5
0
4e2
4g1
0
0
0
0
8 ± 1
0
4g2
C+
0
0
0
44 ± 1
46 ± 1.33
21 ± 1
All values represented in the table are average of results of five separately conducted experiments. Bold and underline means very active,
while underline means active. a Diameter of inhibition zone (mm); X
±
SD, mean of five measurements
±
standard deviation. C+: Penicillin
10 IU for Staphylococcus aureus, carbenicillin 100 µg/mL for Escherichia coli and nystatin 500,000 IU for Candida albicans.
The data presented in Table 2 and Figures S1–S3 reveal interesting data concerning the
biological properties of salts BQS and cycloadducts PBQC. A first observation is the fact
that the tested cycloadducts PBQC did not present any antibacterial and antifungal activity
and for this reason we did not continue the assays for this category of benzo[f]quinoline
derivatives. On the other hand, the tested BQS salts demonstrated a certain antibacterial
and antifungal activity against the tested strains. Practically all the tested the BQS salts
manifest a powerful activity against the fungus Candida albicans, with eight compounds
being very active with a diameter of inhibition zone up to 20 mm (with two compounds, 3e
and 3f, being remarkably active with inhibition zone diameters in the range of 30 mm) and
higher that the control drug nystatin. The remaining other BQS salts are also active against
C. albicans, having a diameter of inhibition zone between 15 mm (for 3k) and 19 mm (for
3m). Against the Gram positive Staphylococcus aureus bacterial strain, five BQS salts are
very active, having an inhibition zone diameter of up to 20 mm (3c
remaining other BQS salts are active (with an inhibition zone diameter between 15 mm (for
3j 3k 3l) and 19.5 mm (for 3b)]. Against Gram negative Escherichia coli bacteria, the BQS
, 3f, 3h, 3i, 3n), while the
,
,
salts have a moderate activity, six BQS salts being active, with a diameter of inhibition
zone between 15.5 mm (for 3f) and 18.5 mm (for 3c).
In the next step of the antimicrobial assays, the minimum inhibitory concentration
(MIC) of the ten most active BQS salts (namely 3b
–e
,
3g–
k,
3n) were determined, using
the standardized broth microdilution assay procedure [32
–
35]. The resulting MIC value
is defined as the lowest concentration of the antimicrobial BQS salts under investigation,
which prevents visible growth of the tested microorganism. The obtained results are listed
in Table 3.

Pharmaceuticals 2021, 14, 335
6 of 21
Table 3. The minimum inhibitory concentration (MIC) for BQS salts 3b–e, 3g–k, n (µg/mL).
MIC (µg/mL)
Strain
C+
3b
3c
3d
3e
3g
3h
3i
3j
3k
3n
S. aureus ATCC 25923
E. coli ATCC 25922
C. albicans ATCC 10231
0.5
0.25
1.56
1.56
1.56
3.12
0.39
0.78
0.78
1.56
1.56
3.12
0.78
0.78
3.12
0.195
0.195
0.139
0.195
0.195
0.195
0.00304 0.39
0.00152 0.78
1.19
1.39
0.81
0.0975
0.195
0.195
0.0575
0.78
C+: Penicillin for Staphylococcus aureus, Carbenicillin for Escherichia coli and Nystatin for Candida albicans. Bold and underline means very
active, while underline means active.
Compared with the control drug, the data from Table 3 reveal that BQS salt 3i is active
to a low concentration, having a MIC of 30.4
aureus, 15.2
10−4 µg/mL in the case of Escherichia coli and 575
of Candida albicans. Significant results were also obtained for the BQS salts 3n (with a MIC
of 975 g/mL for S. aureus and 0.195 g/mL for E. coli and C. albicans), 3h and 3g
10−4
(with a MIC in the range of 0.195 g/mL for all germs). The diameter of inhibition zone
×
10−4
µ
g/mL in the case of Staphylococcus
×
×
10−4 µg/mL in the case
×
µ
µ
µ
data presented in Table 2, MIC data in Table 3 and Figures 1–3 reveal some interesting
observations and correlations between the compound structures and their antimicrobial
activity. The BQS salts 3a–o have an excellent quasi-nonselective antifungal activity against
the fungus C. albicans, a very good antibacterial activity against the Gram positive germ S.
aureus and are less active against the Gram negative germ E. coli. The antifungal activity
is significantly more pronounced in the aliphatic series 3a
one of compounds 3g , which demonstrates a certain influence on activity of the aliphatic
substituent of carbonyl group. In the aromatic series 3g , compounds 3n [Y = -C6H4(Cl)p]
–f compared with the aromatic
–
o
–o
and 3i [Y = -C6H4(OMe)p] have the highest antifungal activity, which also indicates an
influence of the substituent (chlorine or methoxy) on the para position of the phenyl ring.
The same SAR considerations are revealed by the results obtained in the antibacterial assay
against the bacteria S. aureus and E. coli. Finally, if we compare the two series of tested
compounds, salts BQS 3 and cycloadducts PBQC 4, we may notice that only salts BQS 3
have antimicrobial activity while the cycloadducts PBQC 4 are inactive, which means that
in order to have antimicrobial activity it is better to have a mobile substituent linked on the
nitrogen atom instead of a new fused cycle.
(a)
(b)
(c)
Figure 1. PDB structures of targeted molecules represented as ribbons in gold orange (atoms
are not displayed):
(a) ATP synthase structure; (b) ATP synthase outer membrane region;
(c) Topoisomerase II.

Pharmaceuticals 2021, 14, 335
7 of 21
(a)
(b)
Figure 2. ATP synthase with ligands: (
a
) “native”, crystallographic determined structure; (
b) redocked
ligand (DNA string).
BQS salts from 3a to 3o
0
3a
3b
3c
3e
3d
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
-1
-2
-3
-4
-5
-6
Figure 3. Total energy (in eV) of the receptor-ligand in BQS 3a–o salts derivative complexes with ATP synthase.
2.3. Molecular Docking
During the last decades molecular docking became an important tool in medicinal
chemistry, furnishing relevant information concerning binding sites, binding energy and
Absorption, Distribution, Metabolism, Excretion, Toxicity (ADMET) properties. As a result,
we decided to perform a computational (in silico) study concerning these parameters for the
salts BQS 3. The chemical space was also screened for similar compounds with remarkable
bioactivity and ADMET and drug-like properties. Bioactivity is studied with the aid of a
computational build = receptor-ligand system. The complex is characterized by its energy,
hydrogen bonds, and steric constraints. The targets (binding sites) used for this class of
compounds were retrieved from the literature (mainly the work of Hunter et al. [36] and
Xavier et al. [37]) and using an online service that suggests ligand predilection for a specific
target [38] and are ATP synthase and topoisomerase II (TOPO II). Target structures were
retrieved from the PDB database: 6WLZ for ATP synthase [39] and 3KSB for TOPO II [40].
The PDB structures were energetically minimized, charges corrected, names corrected,
potential energy recomputed. Cofactors and Aa chains were kept, and ligands and water
molecules were removed. BQS 3 salts were introduced computationally as SDF. Files
that were energetically minimized and charges corrected. Binding site coordinates (Å)
were retrieved from the literature and from an algorithm based on expanded Van der

Pharmaceuticals 2021, 14, 335
8 of 21
Walls charges [41]. The maximum number of cavities which was set to be detected was
5, corresponding to the binding site molecular theory [42]. The cavity with the most
significant volume (Å3) was chosen for each target (see Supporting Materials for details).
First, the two sets of compounds were docked against the two molecular targets.
MOE 2009 software and its methodology were used in the docking procedure [43–45].
Cartesian coordinates for the two binding sites are as follows: ATP synthase: x45.94 Å,
y46.91 Å, z198.20 Å; TOPO II: x-16.33 Å, y43.11 Å, z-34.50 Å. For computational and action
mechanism reasons, only protein corona of ATP synthase was used in building the in-silico
system (Figure 1).
Docking was validated by redocking the ligands present in PDB target structures. For
this step ligands were cut from the crystallographic obtained structures converted to SDF
files, minimized and charges corrected. Ligands were then docked on to the free ligand
PDB structures. Results are depicted in Figure 2.
Figures 3 and 4, describe the interaction of each BQS salt 3a–o with ATP synthase in
the formed complex, in terms of total energies of the complexes (ET) and hydrogen bond
energy (EH).
BQS salts from 3a to 3o
0
3a
3b
3c
3e
3d
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
-0.02
-0.04
-0.06
-0.08
-0.1
-0.12
-0.14
-0.16
-0.18
-0.2
Figure 4. The hydrogen bond energy between ligand and binding pocket aminoacids (in eV) in BQS 3a–o salt derivative
complexes with ATP synthase.
Analysis of the data from Figure 3 reveals that the compounds 3j (ET =
3g (ET = 4.48 eV), 3i (ET = 4.37 eV), 3l (ET = 4.57 eV) and 3n (ET = 4.62 eV) show
the lowest energies in complex with ATP synthase. The data from Figure 4 reveals that
the compounds 3h (EH 0.1863 eV), 3g (EH 0.1325 eV), 3i (EH 0.1044 eV) and 3l
(EH 0.1393 eV) show the lowest hydrogen bond energy of the ATP synthase complex;
−
5.39 eV),
−
−
−
−
=
−
=
−
=
−
=
−
compounds 3e, 3d, 3m, and 3o don’t form hydrogen bonds.
In Figure 5 is presented the complex with ATP synthase of BQS salt 3f. In the binding
site pocket we may notice two powerful H-
from the benzo[f]quinoline moiety and the aminoacid Glu 62. Another H-
π
interactions between the two benzene rings
interaction
π
is formed between Ala 356 and the benzene from the center of the aromatic core. Also,
Ala 356 has interresidue contacts with Tyr 353 and Arg 357. A benzene carbon donates
electrons to Phe 60, which has an inter-residue contact with Pro 227. All these interactions
are stabilizing the BQS salt 3f—ATP synthase complex.

Pharmaceuticals 2021, 14, 335
9 of 21
Figure 5. Compound 3f is represented in complex with ATP synthase as sticks. Surrounding aminoacids at the binding site
are represented as ribbons colored by chain type. Also, a schematic view of the binding site pocket is represented.
In the next step, a similar protocol was followed to study the interactions of BQS salts
with TOPO II. Figures 6 and 7, describe the interaction of each BQS 3a–o salt with TOPO II
in the formed complex, in terms of total energies of the complexes (ET) and hydrogen bond
energy (EH).
BQS salts from 3a to 3o
0
-0.5
-1
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
-1.5
-2
-2.5
-3
-3.5
-4
-4.5
-5
Figure 6. BQS 3a–o salts in complex with TOPO II: energies of complexes.
Analysis of the data from Figure 6 reveal that the compounds 3j (ET =
3h (ET = 4.517 eV), 3e (ET = 4.097 eV) and 3l (ET = 4.059 eV) show the lowest en-
ergies in complex with TOPO II. The data from Figure 7 reveal that the compounds 3m
0.038 eV) and 3j (EH 0.035 eV) show the lowest hydrogen
3c 3e 3f 3g 3h 3i 3n and 3o don’t
−
4.617 eV),
−
−
−
(
EH = −0.066 eV), 3l (EH
=
−
=
−
bond energy of the TOPO II complex; compounds 3a
,
,
,
,
,
,
,
form hydrogen bonds.
From the obtained data we may notice that salts 3j and 3n have the best-fit in complex
with TOPO II (in Figure 8 the complex of salt 3j is presented). In the binding pocket of TOPO
II, a hydrogen bond is formed between the nitrogen atom of the benzo[f]quinoline ring and
aminoacid ASP-510, stabilizing the salt BQS 3j—TOPO II complex. We also noticed that
TYR 118 and His 76 interact with the aromatic rings. Also, a
His 76 (aromatic) and the positive nitrogen atom.
π bond is observed between

Pharmaceuticals 2021, 14, 335
10 of 21
BQS salts from 3a to 3o
3e 3f 3g 3h
0
3a
3b
3c
3d
3i
3j
3k
3l
3m
3n
3o
-0.01
-0.02
-0.03
-0.04
-0.05
-0.06
-0.07
Figure 7. The hydrogen bond energy (in eV) between ligand and binding pocket aminoacids in BQS 3a–o salts derivates
complexes with TOPO II.
.
Figure 8. Compound 3j is represented in complex with TOPO II as sticks. Surrounding aminoacids at the binding site are
represented as ribbons colored by chain type. Also, a schematic view of the binding pocket is represented.
Ciprofloxacin, the control molecule, when docked with ATP synthase, shows a total
energy of the complex of
with TOPO II, ciprofloxacin shows a total energy of
energy of −2.037 eV (Figure 9).
−
2.59 eV and a hydrogen bond energy of
−
8.31 eV. In a complex
−
2.56 eV and a hydrogen bonding

Pharmaceuticals 2021, 14, 335
11 of 21
(a)
(b)
Figure 9. Ciprofloxacin in complex with: (a) ATP synthase where hydrogen bond is formed between an OH group and Phe 60;
(b) TOPO II, where hydrogen bonds are formed between OH groups and Lys 38, Arg 28, Gly 169, and Ile 170, respectively.
The most active antimicrobial compounds 3i and 3n show a similar behaviour in the
ATP synthase and TOPO II binding pockets as compound 3j (Figure 10).
Lastly, regarding our computational docking study, compound 3j displayed the best
interaction energies. However, experimentally compounds 3i and 3n have the best bioactiv-
ities, while 3j has only moderate antimicrobial activity. In comparison with cyprofloxacin
where the aromatic centres seem to play no role, in the case of BQS those aromatic centres
are crucial in the interaction with the active sites. In the next step, the ADME properties of
BQS salts 3a–o were studied (Table 4).
Table 4. ADME properties computed for BQS 3a–o.
QP
Log
HERG
QP
Log
Khsa
QP
Logs
ClQP
Logs
QP
QP
QP
QP
%Ab-
Sorbtion of 5
Rule
Rule
of 3
#
Structures
Metab.
PCaco Logbb PMDCK Logkp
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
−
1.616
4.078
4.668
3.555
3.805
4.066
4.291
2.913
4.495
6.680
5.748
4.902
5.181
5.612
5.223
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
2.741
3.835
4.133
3.495
3.793
4.091
4.766
3.555
−
−
−
−
−
−
−
−
3.322
5.248
5.585
4.966
5.012
4.992
5.876
5.857
−
−
−
−
−
−
−
−
3.322
5.248
5.585
4.966
5.012
4.992
5.876
5.857
−
−
−
−
−
−
−
−
3.322 512.729 512.729 512.729 512.729 512.729
5.248 2017.43 2017.43 2017.43 2017.43 2017.43
5.585 2333.6 2333.6 2333.6 2333.6 2333.6
4.966 2001.37 2001.37 2001.37 2001.37 2001.37
5.012 2500.67 2500.67 2500.67 2500.67 2500.67
4.992 2271.19 2271.19 2271.19 2271.19 2271.19
0
0
0
0
0
0
0
0
0
1
0
0
1
1
0
0
0
0
0
0
0
0
0
0
1
1
0
1
1
0
−
−
−
−
−
−
−
−
−
−
−
−
−
−
5.876
1877
1877
1877
1877
1877
5.857 176.027 176.027 176.027 176.027 176.027
5.463 −5.52
−5.52
−5.52 2244.4 2244.4 2244.4 2244.4 2244.4
−7.42 2096.68 2096.68 2096.68 2096.68 2096.68
6.742 −7.42
−7.42
6.008
−
6.132
−
6.132
−
6.132 394.919 394.919 394.919 394.919 394.919
5.471 −5.39
−5.39
−5.39 1898.89 1898.89 1898.89 1898.89 1898.89
6.442
5.905
5.555
−
−
−
5.408
5.947
5.898
−
−
−
5.408
5.947
5.898
−
−
−
5.408 5868.17 5868.17 5868.17 5868.17 5868.17
5.947 5702.91 5702.91 5702.91 5702.91 5702.91
5.898 4182.77 4182.77 4182.77 4182.77 4182.77

Pharmaceuticals 2021, 14, 335
12 of 21
(a)
(b)
(c)
(d)
Figure 10. 3i and 3n in complex with ATP synthase and TOPO II in their binding pockets: (
a) 3i in complex with ATP
synthase arene hydrogen bonds are formed between aromatic core and Ala 366, Glu 62, Thr 228, and Gln 58 accept electrons
from aromatic and aliphatic carbons; ( 3i in complex with TOPO II, nitrogen atom interacts with Asp 508 and Glu 433,
heterocyclic aromatic carbon share electrons with Asp 508 and Glu 433; ( 3n in complex with ATP synthase, arene hydrogen
bonds are formed between an aromatic core and Tyr 25 and between heterocycle aromatic core and Ala 356; also keto group
shares electrons with Arg 357; ( 3n in complex with TOPO II, arene hydrogen bonds are formed between each heterocyclic
aromatic core and His 76, Pro 75, and Tyr 118, respectively.
b)
c)
d
)
Analysis of the data from Table 4 reveals interesting data concerning the BQS salts’
ADME properties. All salt derivatives show good aqueous solubility. This is indicated by
two descriptors: QP logS (which describes the general aqueous solubility) and ClQP logS
(which describes the conformation-independent aqueous solubility). The optimal values of
these two descriptors are in the range between
for our BQS salt 3a has good solubility. Three compounds (3j, 3m, and 3n) are exceptions
being slightly out of limit.
−
6.5 and 0.5. The data from Table 4 indicate
The QP log HERG descriptor is a computational equivalent of zebrafish model toxicity
in respect to the mode of action (MOA). This descriptor is linked with the potential K
channel-blocking effect due to the electrophilic nature of drugs. The concerned values are
below
−
5. As observed from Table 4 the majority of the synthesized compounds have
predictable, convenient values, which means that our BQS quaternary salts 3a-o have
low toxicity profiles. The reduced toxicity it is also confirmed by the Lipinski’s rule of

Pharmaceuticals 2021, 14, 335
13 of 21
five and Jorgensen’s rule of three computational results. From Table 4 we can notice that
only compounds 3j, 3m, and 3n present a violation of Lipinski’s rule of 5 and Jorgensen’s
rule of 3, respectively. According with Lipinski’s rule of five, the maximum number of
violations for a computed compound is five (these five rules are: mol_MW < 500; QP log
Po/w < 5; donor HB < 5; accpt. HB < 10) while for Jorgensen’s rule of three, the maximum
number of violations for a computed compound is three (QP log s >
−
5.7; QP PCaco
> 22 nm/s; primary metabolites < 7). As a result, all our BQS salts 3a-o have excellent
drug-like properties. However, in vitro testing should be performed in order to confirm
these theoretical results.
The QP PCaco descriptor predicts permeability at the gut drug barrier. Predictions are
for non-active transport. Values below 25 describe a low permeability while values higher
than 500 are characteristic of excellent permeability. All described compounds show poor
predicted gut barrier permeability.
QP Log BB descriptor predicts brain-blood partition coefficient for oral delivery. An
optimal interval is between descriptor values of
limit, meaning that potentially they may have poor blood-brain partition coefficients when
administered orally.
−
3.0 to 1.2. All compounds are off the
QP PMDCK is a descriptor designed to simulate MDCK cell permeability, considered
to be a good mimic of the blood-brain barrier. Values below 25 describe a poor permeability
while values higher than 500 describe an excellent permeability. The data from Table 4
indicate that all BQS salts 3a–o has excellent blood—brain barrier permeability.
QP log KP is a descriptor that predicts skin permeability, with an optimal interval
between
have good skin absorption.
−
8.0 and
−
1.0. The data from Table 4 indicate that the compounds might not
Predicted chemical reactivity (descriptor Metab.) shows very high values for our BQS
salts, suggesting a negative behavior as leads (the optimal values for Metab. descriptor are
range between 1 to −8).
Interactions of compounds with serum albumin were explored computationally using
the QP log Khsa descriptor. The optimal values for QP log Khsa descriptor are in the range
between
−
1.5 to 15. The data from Table 4 indicate that all BQS salts 3a–o have a high
potential of interacting with human serum albumin.
%Ab-sorbtion is a descriptor that predicts human oral absorption on a 0 to 100% scale.
Values below 25% describe a poor oral absorption while value higher than 80% describe
an excellent oral absorption. The data from Table 4 indicate that all BQS salts 3a–o have
values higher than 80%, which suggest excellent oral absorption.
3. Materials and Methods
3.1. General Information
Reagents and solvents were purchased from commercial sources and used without
further purification. The melting points (uncorrected) of compounds were recorded in
open capillary tubes using a MEL-TEMP Electrothermal apparatus (Barnstead Interna-
tional, Dubuque, IA, USA). The nuclear magnetic resonance spectra were recorded on an
Avance III 500 MHz spectrometer (Bruker, Vienna, Austria) operating at 500 MHz for 1H
and 125 MHz for 13C. Chemical shifts were reported in delta (
δ) units (ppm), relative to
the residual peak of solvent (ref: DMSO, H: 2.50 ppm; 13C: 39.52 ppm), and coupling
constants (J) in Hz. In the NMR spectra the multiplicity of signals was indicated using the
abbreviations s = singlet, bs = broad singlet, d = doublet, ad = apparent doublet, add =
apparent doublet of doublets, t = triplet, at = apparent triplet, td = triplet of doublets, atd =
apparent triplet of doublets, q = quartet, m = multiplet. The IR spectra were recorded using
a VERTEX 70 FTIR spectrometer (Bruker, Vienna, Austria) equipped with an ATR module.
Thin layer chromatography (TLC) was performed on commercial silica gel plates (silica gel
60 F254 plates, Merck, Darmstadt, Germany), the visualization being done using a UV lamp
1
(
λ
max
= 254 or 365 nm). The microanalysis results were in satisfactory agreement (C,
±
0.15;

Pharmaceuticals 2021, 14, 335
14 of 21
H,
±
0.10; N,
±
0.30) with the calculated values. Compound 3i was already reported in the
literature, and its spectral data were in agreement with the reported data [29].
3.2. General Procedure for the Synthesis of Benzo[f]quinoline Salts 3a–o
A solution of halogenated derivatives 2a
activity (1.1 mmol) or with aromatic skeletons (compounds 2g
of acetone was added dropwise in a solution of benzo[f]quinoline (
of acetone. The obtained mixture was heated at reflux during 16 h, and then stirred for
12 h at room temperature. The precipitated quaternary salts 3a (Scheme 3) were filtered
–
f
with aliphatic chains and increased re-
) (1.5 mmol) in 15 mL
, 1 mmol) in 15 mL
–
o
1
–o
off, washed two times with 5 mL of acetone, and then dried under vacuum. No further
purification was required.
Scheme 3. Chemical Structures of compounds 3a–o.
3.2.1. 1-(2-Amino-2-oxoethyl)benzo[f]quinolin-1-ium iodide (3a)
◦
Yellowish green powder; yield: 84%; m.p. 238–240 C; IR,
ν
3335, 3134, 3037, 2966,
max
1681, 1597, 1412, 1344 cm−1; 1H-NMR (DMSO-d6)
δ 10.18 (1H, d, J = 8.5 Hz, H-4), 9.47 (1H,
d, J = 6.0 Hz, H-2), 9.10 (1H, d, J = 8.5 Hz, H-5), 8.68 (1H, d, J = 9.5 Hz, H-9), 8.40 (1H, td,
J = 8.5 Hz, J = 6.0 Hz, H-3), 8.28 (1H, d, J = 8.0 Hz, H-8), 8.20 (1H, bs, NH), 8.14 (1H, d,
J = 9.5 Hz, H-10), 8.01 (1H, t, J = 7.5 Hz, H-6), 7.95 (1H, t, J = 7.5 Hz, H-7), 7.88 (1H, bs, NH),
5.91 (2H, s, CH2); 13C-NMR (DMSO-d6)
δ 166.0 (C = O amido), 148.5 (C-2), 142.1 (C-4), 139.6
(C-10a), 138.0 (C-9), 130.7 (C-8a), 130.1 (C-6), 130.0 (C-7), 129.4 (C-8), 127.8 (C-4a, C-4b),
124.2 (C-5), 122.6 (C-3), 115.7 (C-10), 59.3 (CH2); Anal. Calcd. for C15H13IN2O C, 49.47; H,
3.60; N, 7.69. Found C, 49.37; H, 3.70; N, 7.49.
3.2.2. 1-(2-Methoxy-2-oxoethyl)benzo[f]quinolin-1-ium bromide (3b)
Yellowish powder; yield: 70%; m.p. 149–153 ◦C; IR (KBr),
ν
3029, 2869, 1743, 1600,
max
1431, 1232, 1068 cm−1; 1H-NMR (400 MHz, DMSO-d6)
δ 10.23 (1H, d, J = 8.8 Hz, H-4), 9.62
(1H, d, J = 6.0 Hz, H-2), 9.09 (1H, d, J = 8.0 Hz, H-5), 8.67 (1H, d, J = 9.6 Hz, H-9), 8.44 (1H,
atd, J = 8.4 Hz, J = 6.0 Hz, H-3), 8.34 (1H, d, J = 10.0 Hz, H-10), 8.28 (1H, d, J = 7.6 Hz, H-8),
8.01 (2H, m, H-6, H-7), 6.33 (2H, s, CH2), 3.81 (3H, s, CH3); 13C-NMR (100 MHz, DMSO-d6)
δ
166.6 (C = O ester), 148.3 (C-2), 142.9 (C-4), 139.5 (C-10a), 138.2 (C-9), 130.7 (C-8a), 130.1
(C-6), 130.0 (C-7), 129.3 (C-8), 127.8 (C-4a), 127.7 (C-4b), 124.1 (C-5), 122.7 (C-3), 115.9 (C-10),
57.9 (CH2), 53.2 (CH3); Anal. Calcd. for C16H14BrNO2 C, 57.85; H, 4.25; N, 4.22. Found C,
57.75; H, 4.20; N, 4.12.
3.2.3. 1-(2-Ethoxy-2-oxoethyl)benzo[f]quinolin-1-ium bromide (3c)
Yellowish powder; yield: 73%; m.p. 125–127 ◦C; IR (KBr),
ν
3058, 2977, 1741, 1598,
max
1417, 1209, 1014 cm−1; 1H-NMR (DMSO-d6)
δ 10.25 (1H, d, J = 8.4 Hz, H-4), 9.60 (1H, d,
J = 5.6 Hz, H-2), 9.12 (1H, d, J = 8.0 Hz, H-5), 8.70 (1H, d, J = 9.6 Hz, H-9), 8.45 (1H, atd,
J = 8.4 Hz, J = 6.0 Hz, H-3), 8.32 (2H, m, H-8, H-10), 7.99 (2H, m, H-6, H-7), 6.30 (2H, s,
CH2), 4.26 (2H, q, J = 7.2 Hz, CH2-Et), 1.26 (3H, t, J = 7.2 Hz, CH3); 13C-NMR (DMSO-d6)
δ

Pharmaceuticals 2021, 14, 335
15 of 21
166.1 (C = O ester), 148.4 (C-2), 142.8 (C-4), 139.6 (C-10a), 138.2 (C-9), 130.7 (C-8a), 130.1
(C-6), 130.0 (C-7), 129.4 (C-8), 127.8 (C-4a), 127.7 (C-4b), 124.2 (C-5), 122.7 (C-3), 115.9 (C-10),
62.4 (CH2-Et), 57.9 (CH2), 13.9 (CH3); Anal. Calcd. for C17H16BrNO2 C, 58.97; H, 4.66; N,
4.05. Found C, 58.87; H, 4.60; N, 4.15.
3.2.4. 1-(2-Oxopropyl)benzo[f]quinolin-1-ium bromide (3d)
◦
Beige powder; yield: 67%; m.p. 159–160 C; IR,
ν
3022, 2855, 1730, 1597, 1511,
max
1411, 1341, 1173 cm−1; 1H-NMR (DMSO-d6)
δ 10.20 (1H, d, J = 8.5 Hz, H-4), 9.39 (1H, d,
J = 5.5 Hz, H-2), 9.12 (1H, d, J = 8.0 Hz, H-5), 8.64 (1H, d, J = 9.5 Hz, H-9), 8.44 (1H, dd,
J = 5.5 Hz, J = 8.5 Hz, H-3), 8.30 (2H, m, H-8, H-10), 8.03 (1H, t, J = 7.5 Hz, H-6), 7.96 (1H,
t, J = 7.5 Hz, H-7), 6.43 (2H, s, CH2), 2.50 (3H, s, CH3); 13C-NMR (DMSO-d6)
δ 199.7 (C =
O keto), 147.8 (C-2), 144.0 (C-4), 142.3 (C-10a), 139.8 (C-9), 130.8 (C-4a), 130.1 (C-7), 130.0
(C-6), 129.4 (C-8), 127.9 (C-4b), 127.8 (C-9), 124.2 (C-5), 122.7 (C-3), 116.4 (C-10), 66.0 (CH2),
27.6 (CH3); Anal. Calcd. for C16H14BrNO C, 60.78; H, 4.46; N, 4.43. Found C, 60.69; H, 4.42;
N, 4.38.
3.2.5. 1-(2-Oxobutyl)benzo[f]quinolin-1-ium bromide (3e)
Yellow powder; yield: 71%; m.p. 165–166 ◦C; IR,
ν
3052, 2964, 1723, 1599, 1413,
max
1340, 1106 cm−1; 1H-NMR (DMSO-d6)
H-2), 9.12 (1H, d, J = 8.5 Hz, H-5), 8.63 (1H, d, J = 10.0 Hz, H-9), 8.42 (1H, dd, J = 6.0 Hz
δ
10.21 (1H, d, J = 8.5 Hz, H-4), 9.40 (1H, d, J = 6.0 Hz
,
,
J = 8.5 Hz, H-3), 8.30 (2H, m, H-8, H-10), 8.02 (1H, t, J = 7.5 Hz, H-6), 7.96 (1H, t, J = 7.5 Hz,
H-7), 6.42 (2H, s, CH2), 2.93 (2H, q, J = 7.5 Hz, CH2-Et), 1.06 (3H, t, J = 7.5 Hz, CH3-Et);
13C-NMR (DMSO-d6)
δ 202.3 (C = O keto), 147.8 (C-2), 142.2 (C-4), 139.8 (C-10a), 137.9 (C-9),
130.8 (C-4a), 130.1 (C-7), 130.0 (C-6), 129.4 (C-8), 127.9 (C-4b), 127.8 (C-9), 124.2 (C-5), 122.7
(C-3), 116.3 (C-10), 65.3 (CH2), 32.8 (CH2-Et), 6.9 (CH3-Et); Anal. Calcd. for C17H16BrNO C,
61.83; H, 4.88; N, 4.24. Found C, 61.73; H, 4.81; N, 4.17.
3.2.6. 1-(3,3-Dimethyl-2-oxobutyl)benzo[f]quinolin-1-ium bromide (3f)
Yellow powder; yield: 89%; m.p. 129–130 ◦C; IR,
ν
3051, 2942, 1708, 1578, 1414,
max
1342, 1057 cm−1; 1H-NMR (DMSO-d6)
δ 10.24 (1H, d, J = 8.5 Hz, H-4), 9.46 (1H, d, J = 6.0 Hz,
H-2), 9.15 (1H, d, J = 8.5 Hz, H-5), 8.68 (1H, d, J = 9.5 Hz, H-9), 8.45 (1H, dd, J = 6.0 Hz,
J = 8.5 Hz, H-3), 8.30 (1H, d, J = 7.5 Hz, H-8), 8.08 (1H, d, J = 9.5 Hz, H-10), 8.04 (1H, t,
J = 7.5 Hz, H-6), 7.98 (1H, t, J = 7.5 Hz, H-7), 6.68 (2H, s, CH2), 1.36 (9H, s, 3
13C-NMR (DMSO-d6)
207.0 (C = O keto), 148.2 (C-2), 142.3 (C-4), 139.6 (C-10a), 138.1 (C-9),
130.8 (C-4a), 130.2 (C-7), 130.1 (C-6), 129.4 (C-8), 128.0 (C-4b), 127.9 (C-9), 124.3 (C-5), 122.8
(C-3), 115.7 (C-10), 63.0 (CH2), 43.4 (C(CH3)3-tBu), 25.7 (3 CH3-tBu); Anal. Calcd. for
×
CH3-tBu);
δ
×
C19H20BrNO C, 63.70; H, 5.63; N, 3.91. Found C, 63.59; H, 5.56; N, 3.86.
3.2.7. 1-(Phenacyl)benzo[f]quinolin-1-ium bromide (3g)
Cream-colored powder; yield: 61%; m.p. 234–236 ◦C; IR,
νmax 2989, 2898, 1682, 1577,
1343, 1222, 1065 cm−1; 1H-NMR (500 MHz, DMSO-d6)
δ
10.20 (1H, d, J = 8.5 Hz, H-4), 9.45
(1H, d, J = 5.5 Hz, H-2), 9.11 (1H, d, J = 8.5 Hz, H-5), 8.56 (1H, d, J = 10 Hz, H-9), 8.39 (1H,
dd, J = 8.5 Hz, J = 6.0 Hz, H-3), 8.23 (2H, d, J = 9.5 Hz, J = 7.5 Hz, H-8, H-10), 8.17 (2H,
d, J = 7.0 Hz, 2
J = 7.0 Hz, H-7), 7.80 (1H, d, J = 7.5 Hz, H-16), 7.67 (2H, t, J = 8.0 Hz, 2
CH2); 13C-NMR (125 MHz, DMSO-d6)
190.6 (C = O keto), 148.2 (C-2), 142.5 (C-4), 140.3
(C-10a), 138.4 (C-9), 135.0 (C-16), 133.6 (C-13), 131.0 (C-8a), 130.3 (C-7), 130.2 (C-6), 129.5
(C-8), 129.1 (2 C-15), 128.7 (2 C-14), 128.3 (C-4a), 128.1 (C-4b), 124.2 (C-5), 122.9 (C-3),
×
H-14), 8.02 (1H, atd, J = 8.0 Hz, J = 7.0 Hz, H-6), 7.95 (1H, t, J = 8.0 Hz,
×
H-15), 7.05 (2H, s,
δ
×
×
116.1 (C-10), 63.9 (CH2); Anal. Calcd. for C21H16BrNO C, 66.68; H, 4.26; N, 3.70. Found C,
66.78; H, 4.16; N, 3.65.
3.2.8. 1-(4-Methylphenacyl)benzo[f]quinolin-1-ium bromide (3h)
◦
Yellowish powder; yield: 57%; m.p. 220–224 C; IR,
ν
3027, 2946, 1677, 1600,
max
1344, 1231, 1185 cm−1; 1H-NMR (DMSO-d6)
δ 10.25 (1H, d, J = 8.5 Hz, H-4), 9.55 (1H, d,

Pharmaceuticals 2021, 14, 335
16 of 21
J = 5.5 Hz, H-2), 9.15 (1H, d, J = 8.0 Hz, H-5), 8.61 (1H, d, J = 9.5 Hz, H-9), 8.46 (1H, atd,
J = 8.5 Hz
(1H, t, J = 8.0 Hz, J = 7.0 Hz, H-6), 7.96 (1H, t, J = 7.5 Hz, H-7), 7.50 (2H, d, J = 8.0 Hz, 2
H-15), 7.12 (2H, s, CH2), 2.46 (3H, s, CH3); 13C-NMR (DMSO-d6)
190.2 (C = O keto), 148.2
(C-2), 145.6 (C-16), 142.4 (C-4), 140.0 (C-10a), 138.0 (C-9), 131.1 (C-13), 130.8 (C-8a), 130.1
(C-7), 130.0 (C-6), 129.5 (2 C-15), 129.4 (C-8), 128.8 (2 C-14), 127.9 (C-4a), 127.8 (C-4b),
,
J = 6.0 Hz, H-3), 8.28 (2H, m, H-8, H-10), 8.08 (2H, d, J = 8.0 Hz, 2
×
H-14), 8.02
×
δ
×
×
124.2 (C-5), 122.8 (C-3), 116.3 (C-10), 63.8 (CH2), 21.4 (CH3); Anal. Calcd. for C22H18BrNO
C, 67.36; H, 4.62; N, 3.57. Found C, 67.47; H, 4.52; N, 3.47.
3.2.9. 1-(4-Methoxyphenacyl)benzo[f]quinolin-1-ium bromide (3i)
Yellowish powder; yield: 70%; m.p. 139–142 ◦C; IR,
ν
2968, 2900, 1673, 1598, 1418,
max
1344, 1237, 1179, 1021 cm−1; 1H-NMR (DMSO-d6)
d, J = 5.5 Hz, H-2), 9.15 (1H, d, J = 8.0 Hz, H-5), 8.62 (1H, d, J = 9.5 Hz, H-9), 8.46 (1H, t, J
= 8.0 Hz, J = 6.5 Hz, H-3), 8.27 (2H, at, J = 8.0 Hz, H-8, H-10), 8.15 (2H, d, J = 8.5 Hz, 2
H-14), 8.03 (1H, t, J = 8.0 Hz, J = 7.0 Hz, H-6), 7.96 (1H, t, J = 7.5 Hz, J = 7.0 Hz, H-7), 7.22
(2H, d, J = 8.5 Hz, 2
H-15), 7.08 (2H, s, CH2), 3.92 (3H, s, CH3); 13C-NMR (DMSO-d6)
188.9 (C = O keto), 164.4 (C-16), 148.2 (C-2), 142.4 (C-4), 140.0 (C-10a), 138.0 (C-9), 131.2 (2
C-14), 130.8 (C-8a), 130.1 (C-7), 130.0 (C-6), 129.4 (C-8), 127.9 (C-4a), 127.8 (C-4b), 126.4
δ 10.25 (1H, d, J = 8.5 Hz, H-4), 9.52 (1H,
×
×
δ
×
(C-13), 124.2 (C-5), 122.8 (C-3), 116.3 (C-10), 114.3 (2 × C-15), 63.5 (CH2), 55.9 (CH3); Anal.
Calcd. for C22H18BrNO2 C, 64.72; H, 4.44; N, 3.43. Found C, 64.62; H, 4.40; N, 3.63.
3.2.10. 1-(4-Phenylphenacyl)benzo[f]quinolin-1-ium bromide (3j)
Yellowish powder; yield: 80%; m.p. 242–245 ◦C; IR,
ν
3016, 2895, 1689, 1601, 1339,
max
1225 cm−1; 1H-NMR (DMSO-d6)
δ 10.28 (1H, d, J = 8.5 Hz, H-4), 9.56 (1H, add, J = 6.5 Hz,
H-2), 9.17 (1H, d, J = 8.5 Hz, H-5), 8.64 (1H, d, J = 10 Hz, H-9), 8.48 (1H, dd, J = 8.5 Hz,
J = 6.0 Hz, H-3), 8.36 (1H, d, J = 10.0 Hz, H-10), 8.30 (1H, d, J = 7.5 Hz, H-8), 8.26 (2H, d,
J = 8.5 Hz, 2
7.84 (2H, d, J = 8.5 Hz, 2
J = 8.5 Hz, J = 7.0 Hz, H-20), 7.18 (2H, s, CH2); 13C-NMR (DMSO-d6)
148.2 (C-2), 146.0 (C-16), 142.5 (C-4), 140.1 (C-10a), 138.5 (C-17), 138.9 (C-9), 132.4 (C-13),
130.8 (C-8a), 130.1 (C-6), 130.0 (C-7), 129.4 (2 C-14, C-8), 129.2 (2 C-19), 128.8 (C-20),
127.9 (C-4a), 127.8 (C-4b), 127.1 (2 C-18, 2 C-15), 124.3 (C-5), 122.8 (C-3), 116.4 (C-10),
×
H-14), 8.03 (3H, m, 2
×
H-15, H-6), 7.97 (1H, atd, J = 8.0 Hz, J = 7.5 Hz, H-7),
H-19), 7.48 (1H, atd,
190.3 (C = O keto),
×
H-18), 7.55 (2H, t, J = 8.0 Hz, J = 7.0 Hz, 2
×
δ
×
×
×
×
63.9 (CH2); Anal. Calcd. for C27H20BrNO C, 71.37; H, 4.44; N, 3.08. Found C, 71.47; H, 4.34;
N, 3.18.
3.2.11. 1-(4-Cyanophenacyl)benzo[f]quinolin-1-ium bromide (3k)
Yellow powder; yield: 85%; m.p. 199–202 ◦C; IR,
ν
3015, 2952, 2232, 1706, 1401,
max
1340, 1214 cm−1; 1H-NMR (DMSO-d6)
Hz, H-2), 9.17 (1H, d, J = 8.5 Hz, H-5), 8.64 (1H, d, J = 9.5 Hz, H-9), 8.48 (1H, atd, J = 8.5 Hz,
J = 6.0 Hz, H-3), 8.41 (1H, d, J = 9.5 Hz, H-10), 8.31 (3H, t, J = 8.5 Hz, H-8, 2 H-14), 8.20
δ 10.28 (1H, d, J = 8.5 Hz, H-4), 9.51 (1H, d, J = 6.0
×
(2H, d, J = 8.5 Hz, 2
×
H-15), 8.04 (1H, t, J = 8.0 Hz, J = 7.5 Hz, H-6), 7.98 (1H, t, J = 8.0 Hz,
J = 7.0 Hz, H-7), 7.15 (2H, s, CH2); 13C-NMR (DMSO-d6)
δ
190.3 (C = O keto), 148.2 (C-2),
142.6 (C-4), 140.2 (C-10a), 138.1 (C-9), 136.9 (C-13), 132.9 (2
×
C-15), 130.8 (C-8a), 130.1 (C-7,
C-6), 129.4 (C-8), 129.3 (2
×
C-14), 127.9 (C-4a), 127.8 (C-4b), 124.3 (C-5), 122.8 (C-3), 118.0
(CN), 116.4 (C-10), 116.3 (C-16), 64.1 (CH2); Anal. Calcd. for C22H15BrN2O C, 65.52; H, 3.75;
N, 6.95. Found C, 65.62; H, 3.70; N, 6.85.
3.2.12. 1-(4-Nitrophenacyl)benzo[f]quinolin-1-ium bromide (3l)
Yellowish powder; yield: 74%; m.p. 156–158 ◦C; IR,
ν
3023, 2963, 1717, 1599, 1520,
max
1349, 1217 cm−1; 1H-NMR (DMSO-d6)
H-2), 9.17 (1H, d, J = 8.5 Hz, H-5), 8.65 (1H, d, J = 9.5 Hz, H-9), 8.52 (2H, d, J = 9.0 Hz, 2
H-15), 8.48 (1H, at, J = 8.5 Hz, J = 6.0 Hz, H-3), 8.43 (1H, d, J = 10.0 Hz, H-10), 8.40 (2H, d,
J = 8.5 Hz, 2 H-14), 8.13 (1H, d, J = 7.5 Hz, H-8), 8.50 (1H, t, J = 8.0 Hz, J = 7.0 Hz, H-6),
7.98 (1H, t, J = 7.5 Hz, J = 7.0 Hz, H-7), 7.18 (2H, s, CH2); 13C-NMR (DMSO-d6)
190.1 (C =
δ 10.29 (1H, d, J = 8.5 Hz, H-4), 9.52 (1H, d, J = 5.5 Hz,
×
×
δ

Pharmaceuticals 2021, 14, 335
17 of 21
O keto), 150.6 (C-16), 148.2 (C-2), 142.7 (C-4), 140.2 (C-10a), 138.3 (C-13), 138.1 (C-9), 130.8
(C-8a), 130.2 (2
×
C-14), 130.1 (C-7, C-6), 129.4 (C-8), 128.0 (C-4a), 127.8 (C-4b), 124.2 (C-5),
124.0 (2 C-15), 122.8 (C-3), 116.4 (C-10), 64.1 (CH2); Anal. Calcd. for C21H15BrN2O3 C,
×
59.59; H, 3.57; N, 6.62. Found C, 59.49; H, 3.50; N, 6.52.
3.2.13. 1-(4-Bromophenacyl)benzo[f]quinolin-1-ium bromide (3m)
Cream-colored powder; yield: 80%; m.p. 229–232 ◦C; IR,
νmax 3018, 2911, 1696, 1583,
1347, 1223 cm−1; 1H-NMR (DMSO-d6)
δ
10.27 (1H, d, J = 9.0 Hz, H-4), 9.52 (1H, d, J = 6.0 Hz
,
H-2), 9.16 (1H, d, J = 8.5 Hz, H-5), 8.63 (1H, d, J = 10.0 Hz, H-9), 8.47 (1H, atd, J = 8.5 Hz,
J = 6.0 Hz, H-3), 8.35 (1H, d, J = 9.5 Hz, H-10), 8.29 (1H, d, J = 7.5 Hz, H-8), 8.10 (2H, d,
J = 8.5 Hz, 2
7.94 (2H, d, J = 8.5 Hz, 2
keto), 148.2 (C-2), 142.5 (C-4), 140.1 (C-10a), 138.0 (C-9), 132.7 (C-13), 132.1 (2
(C-8a), 130.6 (2 C-14), 130.1 (C-7), 130.0 (C-6), 129.4 (C-8), 128.9 (C-16), 127.9 (C-4a), 127.8
×
H-14), 8.36 (1H, t, J = 7.5 Hz, J = 7.0 Hz, H-6), 7.97 (1H, t, J = 7.5 Hz, H-7),
H-15), 7.11 (2H, s, CH2); 13C-NMR (DMSO-d6)
190.1 (C = O
C-15), 130.8
×
×
δ
×
(C-4b), 124.2 (C-5), 122.8 (C-3), 116.4 (C-10), 63.8 (CH2); Anal. Calcd. for C21H15Br2NO C,
55.17; H, 3.31; N, 3.06. Found C, 55.07; H, 3.36; N, 3.16.
3.2.14. 1-(4-Chlorophenacyl)benzo[f]quinolin-1-ium bromide (3n)
Yellowish powder; yield: 79%; m.p. 192–196 ◦C; IR,
ν
3020, 2931, 1688, 1586, 1341,
max
1088, 759 cm−1; 1H-NMR (DMSO-d6)
H-2), 9.18 (1H, d, J = 8.5 Hz, H-5), 8.64 (1H, d, J = 9.5 Hz, H-9), 8.47 (1H, atd, J = 8.5 Hz
δ
10.28 (1H, d, J = 8.5 Hz, H-4), 9.47 (1H, d, J = 5.5 Hz
,
,
J = 6.0 Hz, H-3), 8.36 (1H, d, J = 9.5 Hz, H-10), 8.31 (1H, d, J = 8.0 Hz, H-8), 8.18 (2H,
d, J = 8.5 Hz, 2 H-14), 8.05 (1H, t, J = 8.0 Hz, J = 7.0 Hz, H-6), 7.99 (1H, t, J = 8.0 Hz,
J = 7.0 Hz, H-7), 7.80 (2H, d, J = 8.5 Hz, 2
H-15), 7.08 (2H, s, CH2); 13C-NMR (DMSO-d6)
189.9 (C = O keto), 148.2 (C-2), 142.6 (C-4), 140.1 (C-10a), 139.6 (C-16), 138.1 (C-9), 132.4
×
×
δ
(C-13), 130.8 (C-8a), 130.6 (2
(C-4a), 127.9 (C-4b), 124.3 (C-5), 122.8 (C-3), 116.4 (C-10), 63.8 (CH2); Anal. Calcd. for
C21H15BrClNO C, 61.11; H, 3.66; N, 3.39. Found C, 61.01; H, 3.60; N, 3.29.
×
C-14), 130.1 (C-7, C-6), 129.4 (C-8), 129.2 (2
×
C-15), 128.0
3.2.15. 1-(4-Fluorophenacyl)benzo[f]quinolin-1-ium bromide (3o)
Cream-colored powder; yield: 87%; m.p. 143–145 ◦C; IR,
1335, 1221, 1152 cm−1; 1H-NMR (DMSO-d6)
10.27 (1H, d, J = 8.5 Hz, H-4), 9.51 (1H, d,
J = 6.0 Hz, H-2), 9.16 (1H, d, J = 8.0 Hz, H-5), 8.63 (1H, d, J = 10.0 Hz, H-9), 8.47 (1H, atd,
J = 8.5 Hz, J = 6.5 Hz, H-3), 8.34 (1H, d, J = 10.0 Hz, H-10), 8.28 (3H, m, H-8, 2 H-14), 8.04
(1H, t, J = 7.5 Hz, H-6), 7.97 (1H, t, J = 7.5 Hz, H-7), 7.56 (2H, t, J = 8.5 Hz, J = 9.0 Hz, 2
H-15), 7.11 (2H, s, CH2); 13C-NMR (DMSO-d6)
189.5 (C = O keto), 165.93 (d, J = 101 Hz
C-16), 148.2 (C-2), 142.5 (C-4), 140.1 (C-10a), 138.1 (C-9), 131.9 (d, J = 4.0 Hz, 2 C-14),
130.8 (C-8a), 130.4 (ad, J = 2.0 Hz, C-13), 130.1 (C-7, C-6), 129.4 (C-8), 128.0 (C-4a), 127.9
(C-4b), 124.3 (C-5), 122.8 (C-3), 116.4 (C-10), 116.26 (d, J = 9.0 Hz, 2 C-15), 63.8 (CH2);
νmax 3048, 2969, 1694, 1593,
δ
×
×
δ
,
×
×
Anal. Calcd. for C21H15BrFNO C, 63.65; H, 3.82; N, 3.53. Found C, 63.75; H, 3.77; N, 3.43.
3.3. Antimicrobial Assay
3.3.1. Disk-Diffusion Method
For inoculum preparation, reference microbial cultures of bacteria (Staphylococcus
aureus ATCC 25923, Escherichia coli ATCC 25922) and fungi (Candida albicans ATCC 10231)
were employed. A number of approximately five colonies from each type of culture were
used to inoculate 10 mL of Mueller Hinton (MH) agar (for antibacterial tests) and Sabouraud
agar (for antifungal tests). Using a DU 730 spectrophotometer (Beckman Coulter, city, state
abbrev if USA, country,
McFarland standard (1–2
and the inoculum was transferred, in a 1 mL volume, onto the surface of the growth media
specific for bacteria (MH) and fungi (Sabouraud). Once the inoculum was absorbed, sterile
λ
= 600 nm), the turbidity of the inoculum was adjusted to a 0.5
×
108 CFU/mL for bacteria and 1–5 106 CFU/mL for Candida),
×
paper disks of approximately 6 mm in diameter and impregnated with 10
µL of antibacterial
compound (dissolved in 3% DMSO) were placed on the surface of the culture media; for

Pharmaceuticals 2021, 14, 335
18 of 21
all the tested compounds, the concentration used was 25 mg/mL. Following incubation
at the optimal temperatures for bacteria and fungi, of 37 ◦C and 28 ◦C, respectively, for
24 h (bacteria) and 72 h (fungi), the diameters of the inhibition zones were measured using
a ruler. The controls were prepared in the same growth conditions (i.e., C+: sterile filter
paper disks impregnated with antibiotics inducing sensitivity in the organisms under
investigation, namely penicillin 10 IU for Staphylococcus aureus, carbenicillin 100 µg/mL
for Escherichia coli and nystatin 500,000 IU for Candida albicans, and C : sterile filter paper
−
disks with no antimicrobial compounds).
3.3.2. Broth Microdilution Method for Determining the Minimum Inhibitory
Concentration (MIC)
The working technique involves the use of a 96-well microtiter plate (microdilution).
In each well of the plate, 80 µL of growth medium MH, 10 µL of microbial inoculum
(Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922, or Candida albicans ATCC
10231) prepared in the same manner as in the diffusion test (i.e., by diluting the standardized
microbial suspension adjusted to a 0.5 McFarland standard), and 100 µL of antimicrobial
substance to be tested were transferred by pipetting, in different concentrations. For this
purpose, double dilutions of the antimicrobial agent were made in the DMSO 3%, starting
with the 25
0.78 µg/mL and so on). For each tested microorganism, a positive control C+ (containing
80 µL of MH growth medium, 10 L of diluted microbial culture, 100 L successive double
dilutions of antibiotic) and a negative one C (containing 80 L of MH growth medium
and 10 L of diluted microbial culture) were prepared. Following the incubation of the
microplates at 37 C for 24 h (for Staphylococcus aureus ATCC 25923 and Escherichia coli ATCC
25922) and at 28 ◦C for 72 h (for Candida albicans ATCC 10231), 10
L of resazurin were
µg/mL dilution (e.g., 12.5 µg/mL, 6.25 µg/mL, 3.12 µg/mL, 1.56 µg/mL,
µ
µ
−
µ
µ
◦
µ
added in each well. The samples were incubated once again at the optimal temperature for
each microorganism for one hour. The colour of the indicator turned from purple to pink.
Resazurin is a colorimetric indicator for cell viability widely applied for monitoring cell
proliferation. The redox dye, resazurin, enters the cytosol in the oxidized form (purple-blue)
and is converted to the reduced form, resorufin (pink).
4. Conclusions
We report herein the design, synthesis, experimental and in silico evaluation of the
antibacterial and antifungal activity of some new benzo[f]quinoline derivatives. Two classes
of benzo[f]quinolinium derivatives (salts BQS and cycloadducts PBQC) were designed
and obtained via a direct and facile two step procedure: quaternization followed by a
cycloaddition reaction. The synthesized compounds were characterized by elemental
1
and spectral analysis (FT-IR, H-NMR, 13C-NMR). The antifungal assays revealed that
the BQS salts have an excellent quasi-nonselective antifungal activity against the fungus
Candida albicans, some of them higher that the control drug nystatin. The antibacterial assay
revealed that the BQS salts have a very good antibacterial activity against the Gram positive
germ Staphylococcus aureus while the activity against the Gram negative germ Escherichia
coli is negligible. The cycloadducts PBQC 4 are inactive. Analysis of the biological data
reveals some interesting SAR correlations between the structures and their antimicrobial
activity. The in silico studies furnished important data concerning the pharmacodynamics,
pharmacokinetics and ADMET parameters of the BQS salts. Study of the de interaction
of each BQS salt 3a-o with ATP synthase in the formed complex, reveal that salts 3j, 3i,
and 3n have the best-fit in complex with ATP synthase. Study of the de interaction of each
BQS salt 3a-o with TOPO II in the formed complex, revealed that salts 3j and 3n have the
best-fit in complex with TOPO II. The in silico ADMET studies reveal that the BQS salts
have excellent drug-like properties, low toxicity profiles, excellent blood—brain barrier
permeability, an excellent oral absorption and a good solubility. Overall, the experimental
and in silico studies indicated that compounds 3e and 3f (from the aliphatic series) and 3i
3j and 3n (from the aromatic series), are promising leading drug candidates.
,

Pharmaceuticals 2021, 14, 335
19 of 21
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10
.3390/ph14040335/s1, Figures S1–S3: The antibacterial activity for BQS salts 3i (OL22), 3h (OL23),
and QBSC cycloadducts 4e1 (VB10-29), 4g1 (VB10-30), Tables S1–S4: Docking data used in ligand
evaluation and QSAR model, Table S5: Molecular descriptors used in QSAR models evaluation,
Table S6: Screening results for the best fit hypothesis of BQS3 interaction with ATP synthase and
1
Topoisomerase II, Figures S4–S33: H-NMR spectra of compounds 3a–o.
Author Contributions: Design, conception and writing were performed by G.Z. and I.I.M. Biological
assay was performed by S.D. Molecular docking was performed by C.N.L. Synthesis, structure
elucidation, biological data analysis and molecular docking interpretations were performed by all
authors, which also reviewed and approved the final version. All authors have read and agreed to
the published version of the manuscript.
Funding: This work was supported by a grant of the Romanian Ministry of Education and Research,
CNCS—UEFISCDI, project number PN-III-P4-ID-PCE-2020-0371, within PNCDI III.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: Authors are also gratefully to CERNESIM center, for NMR experiments.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
WHO Global Strategy for Containment of Antimicrobial Resistance. Available online: https://www.who.int/drugresistance/
WHO_Global_Strategy_English.pdf (accessed on 24 February 2021).
Silverman, R.B.; Holladay, M.W. The Organic Chemistry of DRUG design and Drug Action, 3rd ed.; Academic Press: London, UK,
2014; ISBN 9780123820303.
Brunton, L.; Knollmann, B.; Hilal-Dandan, R. Goodman & Gilman’s the Pharmacological Basis of Therapeutics, 13th ed.; McGraw-Hill:
New York, NY, USA, 2013; ISBN 9781259584732.
Eicher, T.; Hauptmann, S.; Speicher, A. The Chemistry of Heterocycles: Structures, Reactions, Synthesis, and Applications, 3rd ed.;
Wiley-VCH: Weinheim, Germany, 2013; ISBN 978-3-527-32747-8.
Kumari, L.S.; Mazumder, A.; Kumar, V.; Gupta, S. Synthesis and biological potentials of quinoline analogues: A review of
literature. Mini-Rev. Org. Chem. 2019, 16, 653–688. [CrossRef]
Song, Y.; Xu, H.; Chen, W.; Zhan, P.; Liu, X. 8-Hydroxyquinoline: A privileged structure with a broad-ranging pharmacological
potential. Med. Chem. Commun. 2015, 6, 61–74. [CrossRef]
Kalaria, P.N.; Karad, S.C.; Raval, D.K. A review on diverse heterocyclic compounds as the privileged scaffolds in antimalarial
drug discovery. Eur. J. Med. Chem. 2018, 158, 917–936. [CrossRef]
Hu, Y.Q.; Gao, C.; Zhang, S.; Xu, L.; Xu, Z.; Feng, L.S.; Wu, X.; Zhao, F. Quinoline hybrids and their antiplasmodial and
antimalarial activities. Eur. J. Med. Chem. 2017, 139, 22–47. [CrossRef]
Mantu, D.; Antoci, V.; Moldoveanu, C.; Zbancioc, G.; Mangalagiu, I.I. Hybrid imidazole (benzimidazole)/pyridine(quinoline)
derivatives and evaluation of their anticancer and antimycobacterial activity. J. Enz. Inhib. Med. Chem. 2016, 31, 96–103. [CrossRef]
10. Afzal, O.; Kumar, S.; Haider, M.R.; Haider, R.; Ali, R.; Kumar, R.; Jaggi, M.; Bawa, S. A review on anticancer potential of bioactive
heterocycle quinoline. Eur. J. Med. Chem. 2015, 97, 871–910. [CrossRef] [PubMed]
11. Pinz, M.P.; Reis, A.S.; Oliveira, R.L.; Voss, G.T.; Vogt, A.G.; Sacramento, M.D.; Roehrs, J.A.; Alves, D.; Luchese, C.; Wilhelm,
E.A. 7-Chloro-4-phenylsulfonylquinoline, a new antinociceptive and anti-inflammatory molecule: Structural improvement of a
quinoline derivate with pharmacological activity. Regul. Toxicol. Pharmacol. 2017, 90, 72–77. [CrossRef]
12. Zhong, F.; Geng, G.; Chen, B.; Pan, T.; Li, Q.; Zhang, H.; Bai, C. Identification of benzenesulfonamide quinoline derivatives as
potent HIV-1 replication inhibitors targeting Rev protein. Org. Biomol. Chem. 2015, 13, 1792–1799. [CrossRef]
13. Diaconu, D.; Mangalagiu, V.; Amariucai-Mantu, D.; Antoci, V.; Giuroiu, C.L.; Mangalagiu, I.I. Hybrid quinoline-sulfonamide
complexes (M2+) derivatives with antimicrobial activity. Molecules 2020, 25, 2946. [CrossRef]
14. Zhang, J.; Wang, S.; Ba, Y.; Xu, Z. 1,2,4-Triazole-quinoline/quinolone hybrids as potential anti-bacterial agents. Eur. J. Med. Chem.
2019, 174, 1–8. [CrossRef] [PubMed]
15. Al Matarneh, C.; Sardaru, M.; Apostu, M.; Rosca, I.; Ciobanu, C.; Mangalagiu, I.I.; Danac, R. Synthesis and antibacterial evaluation
0
0
0
0
of new pyrrolo[3 ,4 :3,4]pyrrolo[1,2-a]quinoline and pyrrolo[3 ,4 :3,4] pyrrolo[1,2-a]isoquinoline derivatives. Studia UBB Chemia
2019, LXIV, 67–80. [CrossRef]
16. Zhang, G.; Zhang, S.; Pan, B.; Liu, X.; Feng, L.S. 4-Quinolone derivatives and their activities against Gram positive pathogens.
Eur. J. Med. Chem. 2018, 143, 710–723. [CrossRef] [PubMed]

Pharmaceuticals 2021, 14, 335
20 of 21
17. Ajani, O.O.; Iyaye, K.T.; Audu, O.Y.; Kuye, A.O.; Olanrewaju, I.O. Microwave assisted synthesis and antimicrobial potential of
quinoline based 4-hydrazide-hydrazone derivatives. J. Heterocycl. Chem. 2018, 55, 302–312. [CrossRef]
18. Nainwal, L.M.; Tasneem, S.; Akhtar, W.; Verma, G.; Khan, M.F.; Parvez, S.; Shaquiquzzaman, M.; Akhter, M.; Alam, M.M. Green
recipes to quinoline: A review. Eur. J. Med. Chem. 2019, 164, 121–170. [CrossRef]
19. Gattu, R.; Bagdi, P.R.; Sidick Basha, R.; Khan, A.T. Camphorsulfonic acid catalyzed one-pot three-component reaction for the
synthesis of fused quinoline and benzoquinoline derivatives. J. Org. Chem. 2017, 82, 12416–12429. [CrossRef]
20. Lungu, L.; Ciocarlan, A.; Simigon, C.; Ozer, I.; Shova, S.; Gutu, I.; Vornicu, N.; Mangalagiu, I.I.; D’Ambrosio, M.; Aricu, A.
Synthesis and evaluation of biological activity of homodrimane sesquiterpenoids bearing 1,3,4-oxadiazole or 1,3,4-thiadiazole
units. Chem. Heterocycl. Compd. 2020, 56, 578–585. [CrossRef]
21. Aricu, A.; Ciocarlan, A.; Lungu, L.; Barba, A.; Shova, S.; Zbancioc, G.; Mangalagiu, I.I.; D’Ambrosio, M.; Vornicu, N. Synthesis
of new antibacterial and antifungal drimane sesquiterpenoids with azaheterocyclic units. Med. Chem. Res. 2016, 25, 2316–2323.
[CrossRef]
22. Balan, A.M.; Miron, A.; Tuchilus, C.; Rotinberg, P.; Mihai, C.T.; Mangalagiu, I.I.; Zbancioc, G. Synthesis and in vitro analysis of
novel dihydroxyacetophenone derivatives with antimicrobial and antitumor activities. Med. Chem. 2014, 10, 476–483.
23. Kuchkova, K.; Aricu, A.; Barba, A.; Vlad, P.; Shova, S.; Secara, E.; Ungur, N.; Tuchilus, C.; Zbancioc, G.; Mangalagiu, I.I. Design,
syntheses and antimicrobial activity of some novel homodrimanese squiterpenoids with diazine skeleton. Med. Chem. Res. 2014
,
23, 1559–1568. [CrossRef]
24. Tucaliuc, R.; Cotea, V.; Niculaua, M.; Tuchilus, C.; Mantu, D.; Mangalagiu, I.I. New pyridazine–fluorine derivatives: Synthesis,
chemistry and biological activity. Part II. Eur. J. Med. Chem. 2013, 67, 367–372. [CrossRef]
25. Balan, A.M.; Florea, O.; Moldoveanu, C.; Zbancioc, G.; Iurea, D.; Mangalagiu, I.I. Diazinium salts with dihydroxyacetophenone
skeleton: Syntheses and antimicrobial activity. Eur. J. Med. Chem. 2009, 44, 2275–2279. [CrossRef] [PubMed]
26. Larsen, A.K.; Grondard, L.; Couprie, J.; Desoize, B.; Comoe, L.; Jardillier, J.C.; Riou, J.F. The antileukemic alkaloid fagaronine is an
inhibitor of DNA topoisomerases I and II. Biochem. Pharmacol. 1993, 46, 1403–1412. [CrossRef]
27. Van, D.N.; Rucinschi, E.; Druta, I.; Zugravescu, I. Reaction de benzo[f]quinoline avec sels. Bul. Inst. Politeh. Iasi Sect. 2 Chim. 1977
,
23, 51–57.
28. Mangalagiu, I.I.; Mangalagiu, G.; Roman, M.; Olariu, I.; Petrovanu, M. Synthesis and spectral characterization of some new
diazine salts. An. Stiint. Univ. Al. I. Cuza Iasi 1999, 7, 137–142.
29. Georgescu, E.; Draghici, C.; Iuhas, P.C.; Georgescu, F. A new approach for the synthesis of benzo[f]pyrrolo [1,2-a]-quinolines.
Arkivoc 2005, X, 95–104. [CrossRef]
30. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically, 11th ed.; Approved Standard, CLSI Document
M07-A11; Clinical and Laboratory Standards Institute: Wayne, PA, USA, 2018; Available online: https://clsi.org/media/1928
/m07ed11_sample.pdf (accessed on 24 February 2021).
31. Konaté, K.; Mavoungou, J.F.; Lepengué, A.N.; Aworet-Samseny, R.R.R.; Hilou, A.; Souza, A.; Dicko, M.H.; M’Batchi, B.
Antibacterial activity against β- lactamase producing Methicillin and Ampicillin-resistants Staphylococcus aureus: Fractional
Inhibitory Concentration Index (FICI) determination. Ann. Clin. Microbiol. Antimicrob. 2012, 11, 1–12. [CrossRef]
32. Kavanagh, A.; Ramu, S.; Gong, Y.; Copper, M.A.; Blaskovich, M.A.T. Effects of microplate type and broth additives on microdilu-
tion MIC susceptibility assays. Antimicrob. Agents Chemother. 2019, 63, 1–17. [CrossRef]
33. Balouiri, M.; Sadiki, M.; Ibnsouda, K.S. Methods for in vitro evaluating antimicrobial activity. J. Pharm. Anal. 2016, 6, 71–79.
[CrossRef]
34. Thakur, D.; Sahani, K. In vitro antimicrobial activity and MIC of the extracellular ethyl acetate crude extract of endophytic fungi
Fusarium sp. isolated from Tephrosia purpurea root. Int. J. Pharm. Pharm. Sci. 2019, 11, 48–53. [CrossRef]
35. Osaka, I.; Hefty, P.S. Simple resazurin-based microplate assay for measuring chlamydia infections. Antimicrob. Agents Chemother.
2013, 57, 2838–2840. [CrossRef]
36. Hunter Lindsey, R.; Bromberg, K.D.; Felix, C.A.; Osheroff, N. 1,4-Benzoquinone is a topoisomerase II poison. Biochemistry 2004
43, 7563–7574. [CrossRef] [PubMed]
,
37. Lakshmanan, M.; Xavier, A.S. Bedaquiline—The first ATP synthase inhibitor against multi drug resistant tuberculosis. J. Young
Pharm. 2013, 5, 112–115. [CrossRef]
38. SwissTargetPrediction. Available online: http://www.swisstargetprediction.ch/ (accessed on 4 December 2020).
39. Wang, L.; Wu, D.; Robinson, C.V.; Wu, H.; Fu, T.M. Structures of a complete human V-ATPase reveal mechanisms of its assembly.
Mol. Cell. 2020, 80, 501–5011. [CrossRef]
40. Laponogov, I.; Pan, X.S.; Veselkov, D.A.; McAuley, K.E.; Fisher, L.M.; Sanderson, M.R. Structural basis of gate-DNA breakage and
resealing by Type II Topoisomerases. PLoS ONE 2010, 5, e11338. [CrossRef]
41. Dos Santos Maia, M.; Soares Rodrigues, G.C.; de Sousa, N.F.; Scotti, M.T.; Scotti, L.; Mendonça-Junior, F.J.B. Identification of new
targets and the virtual screening of lignans against Alzheimer’s disease. Oxid. Med. Cell. Longev. 2020, 2020, 3098673. [CrossRef]
[PubMed]
42. Levy, Y.; Wolynes, P.G.; Onuchic, J.N. Protein topology determines binding mechanism. Proc. Natl. Acad. Sci. USA 2004, 101,
511–516. [CrossRef] [PubMed]
43. Vilar, S.; Cozza, G.; Moro, S. Medicinal chemistry and the molecular operating environment (MOE): Application of QSAR and
molecular docking to drug discovery. Curr. Top Med. Chem. 2008, 8, 1555–1572. [CrossRef]

Pharmaceuticals 2021, 14, 335
21 of 21
44. Rollinger, J.M.; Stuppner, H.; Langer, T. Virtual screening for the discovery of bioactive natural products. In Natural Compounds as
Drugs Volume I; Progress in Drug Research; Springer: Berlin/Heidelberg, Germany, 2008; Volume 65, pp. 211–249.
45. Bohacek, R.S.; McMartin, C.; Guida, W.C. The art and practice of structure-based drug design: A molecular modeling perspective.
Med. Res. Rev. 1999, 16, 3–50. [CrossRef]