Synthesis and biological evaluation of novel triazole-biscoumarin conjugates as potential antitubercular and anti-oxidant agents

Article abstract of DOI:10.1007/s11164-018-3490-1

Abstract: The synthesis of a new series of triazole-biscoumarin conjugates by using a molecular hybridization approach is described. The newly synthesized compounds 6a–k were evaluated for their in vitro antitubercular activity against active and dormant

Full text of DOI:10.1007/s11164-018-3490-1

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3490-1
Synthesis and biological evaluation of novel triazole-
biscoumarin conjugates as potential antitubercular
and anti-oxidant agents
Ashruba B. Danne1 Amit S. Choudhari2 Dhiman Sarkar2
•
•
•
Jaiprakash N. Sangshetti3 Vijay M. Khedkar Bapurao B. Shingate
4
1
•
•
Received: 8 March 2018 / Accepted: 22 May 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract The synthesis of a new series of triazole-biscoumarin conjugates by using a
molecular hybridization approach is described. The newly synthesized compounds 6a–
k were evaluated for their in vitro antitubercular activity against active and dormant Mtb
H37Ra and anti-oxidant activity against DPPH radical scavenging. Molecular docking
simulations for the antitubercular activity showed that the conjugates 6a–k bind in the
pocket of the DprE1 enzyme. Most of the conjugates displayed good antitubercular
activity against both the active and dormant Mtb H37Ra strain. The compound 6h
displayed very good antitubercular activity against dormant Mtb H37Ra with an IC50
value of 1.44 lg/mL. Most of the synthesized conjugates exhibit excellent anti-oxidant
activity with an IC50 of less than the standard BHT. Compound 6b is the most active
among all the conjugates with an IC50 value of 08.17 0.11 lg/mL. The molecular
docking study shows good agreement between the observed antitubercular activity and
the binding affinity.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3490-1) contains supplementary material, which is available to authorized users.
&
Bapurao B. Shingate
bapushingate@gmail.com
1
2
Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University,
Aurangabad 431004, India
Combi-Chem Bio-Resource Center, Organic Chemistry Division, CSIR-National Chemical
Laboratory, Pune 411008, India
3
4
Y. B. Chavan College of Pharmacy, Dr. Rafiq Zakaria Campus, Aurangabad 431001, India
Department of Pharmaceutical Chemistry, Shri Vile Parle Kelavani Mandal’s Institute of
Pharmacy, Dhule, Maharashtra 424001, India
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A. B. Danne et al.
Graphical Abstract
Keywords Antitubercular activity Á Anti-oxidant activity Á 1,2,3-Triazoles Á
Biscoumarins Á Molecular docking
Introduction
The coumarin moiety has been used as a pharmacophore unit for the last two
centuries. Many drugs bearing the coumarin moiety are used as anticoagulants,
antibiotics, insecticides and pesticides. Synthetic organic chemists and biologists
have paid attention to the synthesis of coumarin derivatives, because they exhibit
various biological activities such as anticancer [1], anti-breast cancer [2], anti-
Alzheimers [3], anti-oxidant [4], anti-influenza [5], antidepressant [6], antihepatitis
C [7], anti-inflammatory [8] and antitubercular [9]. Synthetic and naturally
occurring biscoumarin scaffolds have attracted considerable attention in medicinal
chemistry. The biscoumarin derivatives are of biological importance because of
their ability to form hydrogen bonding via two hydroxyl groups and show various
biological activities including a-glucosidase [10], anticoagulant [11], antimicrobial
[12], (HIV-1 IN) inhibitor [13], urease inhibitors [14], enzyme inhibitory activity
[15], antibacterial, antitumor [16], anti-oxidant [17] and c-Met inhibitors [18].
Synthesis of coumarin fused or linked with different heterocyclic derivatives has
been gaining importance over the last few decades in medical and chemical research
[19].
The term, ‘‘Click Chemistry’’ was introduced for the first time by the
Sharpless research group [20]. It is a 1,3-dipolar cycloaddition reaction between
alkyne and azide catalyzed by Cu(I), selectively giving 1, 4-disubstituted-1,2,3-
triazoles [21–24]. The 1,2,3-triazole pharmacophore unit containing the drugs
5-amino-1-(3,5-dichloro-4-(4-chlorobenzoyl)benzyl)-1H-1,2,3-triazole-4-carbox-
amide (CAI), Cefatrizine and Tazobactam are available in the market (Fig. 1).
The 1,2,3-triazole is a bioisostere of the amide [25–28]. The 1,2,3-triazole
nucleus has broad pharmaceutical and medicinal applications including a-
123

Synthesis and biological evaluation of novel triazole…
HO
N
O
N
O
S
N
Cl
O
H
O
HN
O
N
N
S
O
N
N
N
N
O
N
OH
O
S
Cl
Cl
N
H
H2N
H
OH
H2N
O
H2N
CAI
Tazobactam
Cefatrizine
Fig. 1 Structures of 1,2,3-triazole-containing drugs
glycosidases [29], antimicrobial [30], antifungal [31], antiviral [32] and antitu-
bercular activity [33–38].
There are several reports on the synthesis of 1,2,3-triazole-coumarin conjugates
and their biological evaluation, such as antifungal, anti-oxidant [39], antitumor [40],
antimicrobial [41–43], 5-lipoxygenase inhibitor [44], anti-inflammatory [45] and
antimalarial [46] activities. The molecules (A–D) [47–50] bearing coumarin and
1,2,3-triazole as pharmacophoric groups, the complex of biscoumarin (E) [51] and
other related compounds [52] display better antitubercular activity (Fig. 2).
Tuberculosis (TB) is a chronic contagious airborne disease caused by the
mycobacterial pathogen Mtb [53]. According to WHO, 1.5 million peoples died and
9 million new TB cases were reported in 2013 [53]. The Bacilli Calmette–Guerin
vaccine is used as chemotherapy for the treatment of TB [54], and the drugs
Bedaquiline and Delamanid are also used for the treatment of multidrug-resistant
(MDR) TB. However, the surge of MDR-TB and extensively drug-resistant TB does
not respond to the first-line drugs and also the totally drug-resistant TB has been
reported [55, 56]. This brings challenges for the researcher to design hybrid
Fig. 2 1,2,3-Triazole-coumarin conjugates (A–D) and biscoumarin (E) shows good antitubercular
activity
123

A. B. Danne et al.
molecules by molecular hybridization of different bioactive substances as a newer,
potent and more effective prototype antitubercular agent having a novel mode of
action.
Free radicles are generated during aerobic respiration. The ROS acting as free
radicles play a vital role in the human body, but may also exert toxic effects [57].
Our body develops a defence system to minimize the damage done by ROS by
producing free radical scavengers including peroxidase, dismutase, catalase
enzymes, cytochrome and glutathione [57]. There are several natural hydrophilic
anti-oxidants such as sinapic, p-coumaric, caffeic and ferulic acids which are
present in vegetables, fruits, herbs and spices [58]. Due to this, there is a need to
develop better new anti-oxidant agents.
In view of the above and in continuation [39, 48, 59–66] of our program towards
the development of new bioactive molecular hybrids, and the reported lead
conjugates (A, D and E) having coumarin and triazole as a pharmacophoric groups,
and the advanced molecular hybridization approach (Fig. 3), here, we would like to
report the synthesis of novel 1,2,3-triazole-incorporated biscoumarin conjugates and
the evaluation of their antitubercular and anti-oxidant activity. A molecular docking
study of the synthesized conjugates were also performed.
Experimental
Materials and methods
Reagents were purchased from Spectrochem, Sigma Aldrich, and Alfa Aesar and
used without further purification. Analytical thin-layer chromatography (TLC) was
monitored with Merck Silica Gel 60 F254 and the spots were visualized using
Fig. 3 Molecular design strategy for 1,2,3-triazole-coumarin conjugates
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Synthesis and biological evaluation of novel triazole…
ultraviolet (UV) light with an excitation wavelength of 254 nm. Melting points
were obtained on an open capillary electrothermal apparatus and are uncorrected.
Infrared (IR) spectra were measured on an Alpha Brucker FT-IR spectropho-
1
tometer. The identity of the compounds were confirmed by NMR. H and 13C
NMR spectra were obtained on Bruker Advance 400 and 100 MHz spectropho-
tometers, respectively. The compounds were solubilized in CDCl3 and DMSO-d6
with TMS used as an internal standard. The chemical shifts (d) are expressed in
parts per million (ppm). Multiplicity of the signals is shown as singlet (s), doublet
(d), triplet (t), quartet (q), multiplet (m), and doublet of doublets (dd), and
coupling constants J are given in Hz. High-resolution mass spectrometry (HRMS)
experiments were performed for the exact mass determination of synthesized
compounds.
General procedure for preparation of 6a–k
In a round-bottom flask equipped with a magnetic stirring bar, a mixture of triazole
aldehyde 4a–k (1.0 mmol) and 4-hydroxycoumarin 5 (2.0 mmol) in ethanol-acetic
acid was refluxed for 4–6 h. The progress of the reaction was monitored by TLC.
After completion of the reaction, the resulting mixture was concentrated under
reduced pressure followed by the addition of 30 mL ice water. The obtained solid
was filtered, dried and recrystallized from ethanol–acetic acid to obtain the pure
products 6a–k in good yields.
Spectral data of compounds
3,30-((1-Phenyl-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-chromen-2-one)
(6a) The compound 6a was prepared according to general procedure using
aldehyde 4a and 4-hydroxycoumarin 5, in 86% yield. Mp 246–248 °C. IR m max/
cm-1 3363, 1655, 1606, 1539, 756. 1H NMR (400 MHz, DMSO-d6) d 8.17 (s, 1H),
7.85 (t, J = 12.0 Hz, 4H), 7.53–7.47 (m, 4H), 7.40 (t, J = 7.3 Hz, 1H), 7.23 (d,
J = 8.3 Hz, 4H), 6.38 (s, 1H). 13C NMR (100 MHz, CDCl3 ? DMSO-d6) d 167.9,
164.5, 152.5, 149.5, 136.8, 130.8, 129.5, 127.9, 124.2, 122.7, 120.1, 120.0, 119.6,
115.3, 102.8, 29.4. HRMS (ESI-TOF, [M ? H]?): m/z calcd for C27H17N3O6,
480.1195; found, 480.1193.
3,30-((1-(p-Tolyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-chromen-2-
one) (6b) The compound 6b was prepared according to general procedure from
aldehyde 4b and 4-hydroxycoumarin 5, in 82% yield. Mp 166–168 °C. IR m max/
cm-1 3314, 3065, 1659, 1614, 1565, 760. 1H NMR (400 MHz, CDCl3) d 8.06–7.86
(m, 3H), 7.56–7.41 (m, 4H), 7.35–7.05 (m, 6H), 6.27 (s, 1H), 2.36 (s, 3H). 13C NMR
(100 MHz, CDCl3) d 170.1, 167.8, 167.7, 164.5, 152.4, 138.9, 137.6, 134.5, 132.5,
130.0, 124.4, 120.6, 120.4, 116.4, 103.7, 30.2, 21.1.
3,30-((1-(2-Methoxyphenyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-
chromen-2-one) (6c) The compound 6c was prepared according to general
procedure from aldehyde 4c and 4-hydroxycoumarin 5, in 80% yield. Mp
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A. B. Danne et al.
1
above 270 °C. IR m max/cm-1 3397, 1650, 1606, 1534, 756. H NMR (400 MHz,
DMSO-d6) d 8.06 (s, 1H), 7.89 (d, J = 7.6 Hz, 2H), 7.86 (s, 1H), 7.62 (d,
J = 7.7 Hz, 1H), 7.49–7.40 (m, 3H), 7.22 (d, J = 6.9 Hz, 3H), 7.16 (d, J = 8.2 Hz,
1H), 7.08 (t, J = 7.5 Hz, 1H), 6.45 (s, 1H), 3.82 (s, 3H). 13C NMR (100 MHz,
CDCl3 ? DMSO-d6) d 168.3, 164.9, 152.5, 151.1, 147.9, 130.8, 129.8, 126.1,
125.1, 124.2, 123.9, 122.8, 120.7, 120.0, 115.4, 112.6, 103.1, 55.9, 29.4. HRMS
(ESI-TOF, [M ? H]?): m/z calcd for C28H19N3O7, 510.1301; found, 510.1296.
3,30-((1-(4-Methoxyphenyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-
chromen-2-one) (6d) The compound 6d was prepared according to general
procedure from aldehyde 4d and 4-hydroxycoumarin 5, in 90% yield. Mp
1
170–172 °C. IR m max/cm-1 3462, 3076, 1657, 1610, 762. H NMR (400 MHz,
CDCl3) d 7.99 (s, 2H), 7.86 (s, 1H), 7.71–7.48 (m, 4H), 7.40–7.14 (m, 4H), 6.94 (s,
2H), 6.22 (s, 1H), 3.83 (s, 3H). 13C NMR (100 MHz, CDCl3) d 167.8, 167.7, 159.8,
152.4, 132.8, 132.7, 130.3, 124.6, 124.4, 122.1, 120.8, 117.1, 116.5, 114.6, 103.9,
55.6, 30.4.
3,30-((1-(2-Chlorophenyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-chro-
men-2-one) (6e) The compound 6e was prepared according to general procedure
from aldehyde 4e and 4-hydroxycoumarin 5, in 82% yield. Mp 146–148 °C. IR m
max/cm-1 3338, 1653, 1604, 1540, 754. 1H NMR (400 MHz, DMSO-d6) d 8.01 (s,
1H), 7.97 (d, J = 5.4 Hz, 2H), 7.63–7.61 (m, 2H), 7.57–7.49 (m, 4H), 7.31–7.28 (m,
4H), 6.53 (s, 1H). 13C NMR (100 MHz, CDCl3) d 172.1, 165.9, 165.1, 152.4, 144.3,
142.5, 134.8, 132.5, 130.7, 130.6, 128.7, 127.8, 126.1, 124.6, 124.5, 116.5, 103.8,
30.2. HRMS (ESI-TOF, [M ? H]?): m/z calcd for C27H16ClN3O6, 514.0806; found,
514.0804.
3,30-((1-(3-Chlorophenyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-chro-
men-2-one) (6f) The compound 6f was prepared according to general procedure
from aldehyde 4f and 4-hydroxycoumarin 5, in 86% yield. Mp 188–190 °C. IR m
max/cm-1 3482, 3175, 1654, 1606, 760. 1H NMR (400 MHz, DMSO-d6) d 8.21 (s,
1H), 8.05 (s, 1H), 7.93 (s, 1H), 7.89 (d, J = 8.1 Hz, 2H), 7.81 (d, J = 8.1 Hz, 1H),
7.47 (d, J = 8.0 Hz, 2H), 7.37 (d, J = 8.0 Hz, 1H), 7.21 (d, J = 7.7 Hz, 4H), 6.43 (s,
1H). 13C NMR (100 MHz, CDCl3 ? DMSO-d6) d 168.4, 165.1, 152.9, 150.2,
138.3, 134.7, 131.3, 130.9, 127.9, 124.6, 122.9, 120.5, 120.5, 119.8, 118.3, 115.7,
103.2, 29.8. HRMS (ESI-TOF, [M ? H]?): m/z calcd for C27H16ClN3O6, 514.0806;
found, 514.0804.
3,30-((1-(4-Chlorophenyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-chro-
men-2-one) (6g) The compound 6g was prepared according to general procedure
from aldehyde 4g and 4-hydroxycoumarin 5, in 88% yield. Mp 204–206 °C. IR m
max/cm-1 3341, 1605, 1547, 755. 1H NMR (400 MHz, CDCl3) d 7.99 (s, 4H), 7.89
(s, 1H), 7.62 (s, 4H), 7.42 (s, 2H), 7.35 (s, 2H), 6.20 (s, 1H). 13C NMR (100 MHz,
CDCl3) d 190.5, 167.9, 165.7, 152.4, 144.9, 135.3, 134.6, 133.0, 129.9, 124.8,
124.5, 121.7, 120.5, 116.6, 103.7, 30.4. HRMS (ESI-TOF, [M ? H]?): m/z calcd for
C27H16ClN3O6, 514.0806; found, 514.0799.
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Synthesis and biological evaluation of novel triazole…
3,30-((1-(3-Bromophenyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-chro-
men-2-one) (6h) The compound 6h was prepared according to general procedure
from aldehyde 4h and 4-hydroxycoumarin 5, in 80% yield. Mp 205–206 °C. IR m
1
max/cm-1 3118, 1654, 1606, 755. H NMR (400 MHz, CDCl3) d 11.66 (s, 2H),
8.04 (d, J = 7.9 Hz, 2H), 7.89 (d, J = 11.4 Hz, 2H), 7.71 (d, J = 7.4 Hz, 1H), 7.63
(t, J = 7.4 Hz, 2H), 7.56 (d, J = 7.7 Hz, 1H), 7.41–7.36 (m, 5H), 6.14 (s, 1H). 13C
NMR (100 MHz, CDCl3) d 168.0, 167.9, 165.2, 152.5, 144.8, 137.9, 133.2, 131.9,
131.1, 125.0, 124.5, 123.6, 123.3, 120.3, 119.2, 116.7, 103.9, 30.6. HRMS (ESI-
TOF, [M ? H]?): m/z calcd for C27H16BrN3O6, 558.0300; found, 558.0295.
3,30-((1-(2-Nitrophenyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-chro-
men-2-one) (6i) The compound 6i was prepared according to general procedure
from aldehyde 4i and 4-hydroxycoumarin 5, in 86% yield. Mp 180–182 °C. IR m
1
max/cm-1 3361, 1648, 1533, 755. H NMR (400 MHz, CDCl3) d 11.79 (s, 2H),
8.09 (d, J = 8.1 Hz, 1H), 8.04 (d, J = 7.9 Hz, 2H), 7.80 (t, J = 7.1 Hz, 1H), 7.75 (s,
1H), 7.70 (t, J = 6.8 Hz, 2H), 7.63 (t, J = 7.1 Hz, 2H), 7.43–7.37 (m, 4H), 6.19 (s,
1H). 13C NMR (100 MHz, CDCl3) d 190.7, 165.2, 163.8, 152.5, 144.5, 133.9, 133.1,
130.9, 130.3, 128.4, 125.7, 125.0, 124.5, 124.0, 119.9, 116.7, 104.0, 30.6. HRMS
(ESI-TOF, [M ? H]?): m/z calcd for C27H16N4O8, 525.1046; found, 525.1048.
3,30-((1-(3-Nitrophenyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-chro-
men-2-one) (6j) The compound 6j was prepared according to general procedure
from aldehyde 4j and 4-hydroxycoumarin 5, in 85% yield. Mp 180–183 °C. IR m
1
max/cm-1 3389, 1658, 1610, 757. H NMR (400 MHz, DMSO-d6) d 8.73 (s, 1H),
8.59 (s, 1H), 8.36 (s, 1H), 8.22 (s, 1H), 7.92 (d, J = 7.4 Hz, 2H), 7.79 (t, J = 8.0 Hz,
1H), 7.53 (s, 2H), 7.27 (d, J = 9.5 Hz, 3H), 7.04 (s, 1H), 6.46 (s, 1H). 13C NMR
(100 MHz, CDCl3 ? DMSO-d6) d 166.9, 164.8, 152.5, 149.3, 148.5, 137.6, 131.2,
131.1, 125.5, 124.2, 123.1, 122.2, 120.7, 119.2, 115.6, 114.2, 102.9, 29.5.
3,30-((1-(4-Nitrophenyl)-1H-1,2,3-triazol-4-yl)methylene)bis(4-hydroxy-2H-chro-
men-2-one) (6k) The compound 6k was prepared according to general procedure
from aldehyde 4k and 4-hydroxycoumarin 5, in 80% yield. Mp 204–206 °C. IR m
1
max/cm-1 3400, 1602, 749. H NMR (400 MHz, CDCl3) d 11.68 (s, 2H), 8.40 (d,
J = 8.0 Hz, 2H), 8.05 (d, J = 8 Hz, 2H), 7.97 (d, J = 8 Hz, 3H), 7.65 (t, J = 8 Hz,
2H), 7.41 (t, J = 8.2 Hz, 4H), 6.16 (s, 1H). 13C NMR (100 MHz, CDCl3) d 168.1,
165.2, 152.5, 147.2, 146.9, 145.4, 141.1, 133.2, 125.5, 125.1, 124.4, 120.5, 120.1,
116.7, 103.8, 30.6. HRMS (ESI-TOF, [M ? H]?): m/z calcd for C27H16N4O8,
525.1046; found, 525.1060.
Pharmacology
Antitubercular activity
All the chemicals such as sodium salt XTT and MTT, DMSO and rifampicin were
purchased from Sigma-Aldrich, USA. Synthesized compounds were dissolved in
DMSO and it was used as a stock solution (10 mg/mL) for further biological testing.
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A. B. Danne et al.
A microbial strain, Mtb H37Ra (ATCC 25177), was obtained from Astra Zeneca,
India. The stock culture was maintained at - 80 °C and subcultured once in a liquid
medium before inoculation into an experimental culture. For the antitubercular
assay, M. pheli medium (minimal essential medium) was used. It contains 0.5 g
KH2PO4, 0.25 g trisodium citrate, 60 mg MgSO4, 0.5 g asparagine and 2 mL
glycerol in distilled water (100 mL) followed by pH adjustment to 6.6. It takes at
least 8–10 days for OD 1 at 620 nm. The antitubercular assay was performed in 96-
well plates for the active as well as the dormant stages. All the synthetic compounds
were screened for their in vitro activity against Mycobacterium tuberculosis H37Ra
(ATCC 25177) using dilutions ranging from 20 to 0.625 lg/mL, in order to
determine the actual minimum inhibitory concentration (MIC). The screening of M.
tuberculosis H37Ra has been carried out by XTT Reduction Menadione Assay
(XRMA) by reading absorbance at 470 nm. The optical density was read on a
micro-plate reader with a 470-nm filter for XTT against a blank prepared from cell-
free wells. Absorbance given by cells treated with the vehicle (DMSO) alone was
taken as 100% cell growth. Initially, primary screening was carried out at 20, 10 and
5 lg/mL. Compounds showing more inhibition of MTB at or lower than 20 lg/mL
were selected for further dose response curves. All the experiments were performed
in duplicate and the quantitative value was expressed as the average standard
deviation. MIC and IC50 values of the selected compounds were calculated from
their dose response curves by using Origin 8 software. Percentage inhibition was
calculated using the formula: %inhibition ¼ ½ðcontrol À CMPÞ=ðcontrol À
blankފ  100 where ‘control’ is the activity of Mycobacterium without compounds,
‘CMP’ is the activity of the Mycobacterium in the presence of compounds and
‘blank’ is the activity of the culture medium without Mycobacterium.
1,1-Diphenyl-1-picrylhydrazyl radical scavenging activity
The bleaching of the purple-colored methanol solution of DPPH radical scavenging
activity was used for the determination of the hydrogen atoms or electron donation
ability of the compounds. The stable radical DPPH was used as a reagent in
spectrophotometric assay. Next, 1 mL of various concentrations of the test
compounds (5, 10, 25, 50 and 100 lg/mL) in methanol was added to 4 mL of
0.004% (w/v) methanol solution of DPPH. After a 30-min incubation period at room
temperature, the absorbance was measured against blank at 517 nm. The percent
inhibition (I%) of free radical production from DPPH was calculated by the
following equation:
% of scavenging ¼ ½ðA control À A sampleÞ=A blankŠ  100
where ‘‘A control’’ is the absorbance of the control reaction (containing all reagents
except the test compound) and ‘‘A sample’’ is the absorbance of the test compound.
Tests were carried out in triplicate.
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Synthesis and biological evaluation of novel triazole…
Molecular docking studies
To study the binding mode of the title compounds into the active site of the DprE1
enzyme, the Grid-Based Ligand Docking with Energetics (GLIDE) module
integrated in the Small Drug Discovery Suite of the Schrodinger molecular
modeling software [67] was used. It involved mainly enzyme selection and
preparation, receptor grid generation, ligand building and preparation, and docking
and analysis of docking studies. With this purpose, the three-dimensional X-ray
structure of M. tuberculosis DprE1 (Accession number 4FDO) in complex with its
inhibitor was retrieved from the Protein Data Bank. The protein structure was
prepared for docking simulation using the Protein Preparation Wizard which
involved removing the crystallographically observed water molecules (as none of
them was found to be conserved in the interaction with the protein), and adding the
missing hydrogen/side chain atoms corresponding to pH 7.0 which was considered
the appropriate ionization state for the acidic as well as the basic amino acid
residues. After assigning appropriate charge and protonation states to the obtained
structure, the energy minimization with a root mean square deviation (RMSD) value
˚
of 0.30 A was then carried out using the OPLS-2005 force field to relieve the steric
clashes between the residues caused due to the addition of hydrogen atoms and this
structure was used for further docking studies. Molecules to be docked were drawn
in 3D form with the build panel in Maestro and refined by the LigPrep module. The
molecules were subjected to the OPLS-2005 force field which involves addition of
hydrogen, adjusting realistic bond lengths and angles, correct chiralities, ionization
states, tautomers, stereo chemistries and ring conformations and assigning partial
charges. The ligand structures thus obtained were finally optimized by energy
˚
minimization until it reached a RMSD cutoff of 0.01 A. The active site of DprE1 for
docking was defined using the Receptor Grid Generation panel in Glide. A grid box
˚
of 12 9 12 9 12 A dimensions centered on the centroid of the native ligand in the
crystal complex was generated which was sufficient to explore a larger region of the
enzyme structure. The co-crystallized ligand serves as the reference coordinate
signifying the active site for an inhibitor molecule with respect to the biological
target. The optimized ligand structures were then subjected to docking calculation
against the defined active site using the extra precision (i.e., GlideXP) scoring
function to rank the docking poses and to gauge their binding affinities towards the
DprE1 enzyme. The output files generated in the form of the docking poses were
visualized and analyzed using the Maestro’s Pose Viewer utility for the key
elements of interaction with the active site residues.
Results and discussion
Chemistry
The synthetic route for the designed 1,2,3-triazole-incorporated biscoumarin
conjugates using the molecular hybridization approach has been depicted in
Scheme 1. The commercially available anilines 1a–k were successively diazotized
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A. B. Danne et al.
N
N
O
OH
N
N
N
N
N3
NH2
(i)
(ii)
(iii)
R
R
R
R
4a-k
3a-k
1a-k
2a-k
a
b
c
d
e
f
g
h
i
j
k
R
H
3-Br
4-CH3 2-OCH3 4-OCH3 2-Cl
3-Cl
4-Cl
2-NO2 3-NO2 4-NO2
R
N
N
N
N
O
N
N
OH
O
OH
O
OH
(iv)
2
O
O
R
O O
6a-k
4a-k
5
Scheme 1 Synthetic route for 1,2,3-triazole-incorporated biscoumarin conjugates
followed by treatment with sodium azide resulting in the corresponding aryl azides
2a–k. The aryl azides 2a–k on 1,3-dipolar cycloaddition reaction (click reaction)
with propargyl alcohol using copper sulfate and sodium ascorbate in t-butanol:H2O,
resulted in 1,4-disubstituted 1,2,3-triazolyl methanols 3a–k in quantitative yields.
The triazolyl methanols 3a–k were subjected to oxidation using Collins reagent
(CrO3:2Py) in dichloromethane affording the corresponding triazolyl aldehydes 4a–
k in 55–65% isolated yield. The 1,2,3-triazolyl aldehydes 4a–k on reaction with
commercially available 4-hydroxycoumarin in ethanol and acetic acid resulted in
the corresponding 1,2,3-triazole-incorporated biscoumarin conjugates 6a–k in
80–90% yield (Scheme 1). The structures of all the conjugates were confirmed by
1
physical data and spectral analysis. In the H NMR spectra for all the conjugates,
Reagents and conditions: (1) NaNO2, HCl, 0 °C; NaN3, 2–4 h, rt; (2) propargyl
alcohol, CuSO4, sodium ascorbate, t-butanol:H2O (1:1), 48–72 h, rt; (3) CrO3:2Py,
CH2Cl2, 3–6 h, rt; (4) EtOH–AcOH, reflux, 4–6 h.
The singlet signal observed at d 7.98–8.30 ppm was assigned to the triazole
proton. The other prominent singlet signal was observed at d 6.12–6.55 ppm and
assigned to the methine proton. In the 13C NMR spectra for all conjugates, the
signals appeared at d 29–31 ppm, and were assigned for methine carbon.
Biological activity
Antitubercular activity
The newly synthesized compounds 6a–k were initially screened for their in vitro
antitubercular activity against M. tuberculosis H37Ra (ATCC 25177) at three
different concentrations, 20, 10 and 5 lg/mL, using an established XRMA [68]
screening protocol, and the results are displayed in supplementary information
123

Synthesis and biological evaluation of novel triazole…
(Table S1). Rifampicin was used as the reference drug. Among the screened
compounds, 6b, 6c, 6e, 6f, 6g, 6h and 6k showed more than 77.84% inhibition
towards Mtb H37Ra which supports the antimicrobial nature of the biscoumarin
hybrids. The compounds were further evaluated in secondary screening at six
different concentrations, 20, 10, 5, 2.5, 1.25 and 0.625, in order to determine the
actual MIC. The obtained results of the screening are tabulated in Table 1.
Structure–activity relationship
The antitubercular activity was significantly affected by introducing various
substituents on the phenyl ring. Among the compounds 6a–k, compound 6h (R = 3-
Br) showed good antitubercular activity with an IC50 value of 1.44 lg/mL against
dormant Mtb H37Ra. Compounds 6f (R = 3-Cl) and 6g (R = 4-Cl) showed good
antitubercular activity against dormant Mtb H37Ra with IC50 values of 8.27 and
6.27 lg/mL, respectively. Compounds 6a, 6b, 6c, 6d, 6e, 6i and 6k having
substituents R = H, 4-CH3, 2-OCH3, 4-OCH3, 2-Cl, 2-NO2 and 4-NO2 showed
moderate antitubercular activity against dormant Mtb H37Ra with IC50 values of
13.1, 12.28, 14.28, 19.45, 10.42, 20.00 and 13.12 lg/mL, respectively. Hence,
among all the compounds 6a–k, the derivatives 6a, 6b, 6c, 6d, 6e, 6f, 6g, 6h, 6i and
6k showed good to moderate antitubercular activity against dormant Mtb H37Ra
(Table 1).
Compounds 6g and 6h showed good antitubercular activity against active Mtb
H37Ra with IC50 values of 7.48 and 1.93 lg/mL, respectively. Compounds 6a, 6b,
6c, 6e, 6f and 6k showed moderate antitubercular activity against active Mtb H37Ra
with IC50 values of 15.64, 13.81, 15.17, 11.38, 10.00 and 14.38 lg/mL,
respectively. Hence, among all the compounds, the hybrids 6a, 6b, 6c, 6e, 6f, 6g,
6h and 6k showed good to moderate antitubercular activity against active Mtb
H37Ra (Table 1). The presence of –CH3, –OCH3, –Cl, –Br and –NO2 groups on the
phenyl ring largely influence the antitubercular activity in the dormant and active
states of Mtb H37Ra. The electron-donating as well as electron-withdrawing groups
is responsible for antitubercular activity. The halogen groups are also responsible
for the antitubercular activity.
Anti-oxidant activity
Triazole–biscoumarin conjugates were evaluated for their in vitro free radical
scavenging activity. The anti-oxidant activity of compounds 6a–k resulted by using
the DPPH radical scavenging assay and the results are summarized in Table 1.
Butylated hydroxytoluene (BHT) was used as the standard drug having an IC50
value of 16.47 0.18 lg/mL. All the synthesized compounds showed excellent
anti-oxidant activity compared with BHT, except for 6e. Compound 6b showed
excellent anti-oxidant activity with an IC50 value of 8.17 0.11 lg/mL and
compounds 6j and 6g exhibited excellent anti-oxidant activity with IC50 values of
8.44 0.64 and 9.15 0.34 lg/mL, respectively. This shows that theompounds
6b, 6j and 6g are highly active copmared with the standard BHT drug. Compounds
6k, 6d, 6c and 6f exhibited excellent anti-oxidant activity with IC50 values of
123

A. B. Danne et al.
Table 1 In vitro antitubercular and anti-oxidant activities of triazole-biscoumarin conjugate 6a–k
Entry
M. tuberculosis H37Ra (lg/mL)
DPPH
IC50 (lg/mL)
Active
IC50
Dormant
IC90
IC50
13.1
IC90
6a
15.64
[ 20
[ 20
15.23 0.34
6b
13.81
[ 20
12.28
[ 20
08.17 0.11
6c
15.17
[ 20
14.28
[ 20
12.94 0.05
6d
–
–
19.45
[ 20
11.64 0.09
6e
11.38
10.00
[ 20
[ 20
10.42
[ 20
18.22 0.34
6f
8.27
20.00
13.33 0.47
123

Synthesis and biological evaluation of novel triazole…
Table 1 continued
Entry
M. tuberculosis H37Ra (lg/mL)
DPPH
IC50 (lg/mL)
Active
IC50
Dormant
IC50
IC90
17.5
IC90
6g
7.48
6.27
16.75
09.15 0.34
6h
1.93
15.13
1.44
9.99
16.13 0.87
6i
–
–
20.00
[ 20
14.08 0.78
6j
–
–
–
–
08.44 0.64
6k
14.38
[ 20
13.12
[ 20
11.15 0.76
RIF
–
–
0.020
–
0.53
–
0.0019
–
0.80
–
–
BHT
16.47 0.18
RIF rifampicin, BHT butylated hydroxy toluene, DPPH 2,2-diphenyl-1-picrylhydrazyl, – not calculated
11.15 0.76, 11.64 0.09, 12.94 0.05 and 13.33 0.47 lg/mL, respectively.
Compounds 6i, 6a and 6h showed anti-oxidant activity close to the standard drug
with IC50 values of 14.08 0.78, 15.23 0.34 and 16.13 0.87 lg/mL, respec-
tively (Table 1).
123

A. B. Danne et al.
Computational study
Molecular docking
In an effort to gain a better understanding of the potency of the 1,2,3-triazole–
coumarin conjugates and to elucidate the mechanism by which the compounds can
induce antitubercular activity. A molecular docking study was performed into the
binding site of DprE1 for the triazole–coumarin conjugates. The DprE1 enzyme is
one of the most vulnerable targets of M. tuberculosis. It is an oxidase enzyme
involved in arabinogalactan biosynthesis (cell wall biogenesis), shown to be a
crucial target for the survival of M. tuberculosis. It is a key enzyme in the
biosynthesis of the basic precursor for the mycobacterial cell wall core
decaprenylphosphoryl-D-arabinose, the only known donor of D-arabinofuranosyl
residues for the synthesis of arabinogalactan.
Results from the ensuing docking simulation revealed that the 1,2,3-triazole–
coumarin conjugates bind well to the active site of DprE1 by the formation of
several bonded and non-bonded interactions, occupying coordinates very close to
that of the native ligand with a varying magnitude of affinity. The docking score for
the most active compound, 6h, is - 8.935, while for the least active, 6j, it is
- 6.147. These theoretical predictions were in agreement with the experimental
antitubercular activity, the most active compounds exhibiting higher docking scores
while those with relatively least activity having a lower docking score. Further, a
detailed per-residual interaction analysis has been performed to gain an insight into
the thermodynamic elements governing the binding event of these molecules
through which we can speculate the binding patterns in the active site. The lowest
energy docked conformation of 6h was observed to be tightly bound to the active
site of DprE1, engaging in a series of steric and electrostatic interactions (Fig. 4).
These interactions contributed to the highest binding affinity observed for 6h.
The per-residue interaction analysis shows that the compound is stabilized in the
active site through significant van der Waals interactions with Lys418
(- 3.561 kcal/mol), Csy387 (- 2.859 kcal/mol), Asn385 (- 1.481 kcal/mol),
Fig. 4 Binding mode of 6h into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
123

Synthesis and biological evaluation of novel triazole…
Lys367 (- 2.459 kcal/mol), Val365 (- 2.964 kcal/mol), Leu363 (- 1.523 kcal/-
mol), Gln336 (- 1.921 kcal/mol), His132 (- 3.559 kcal/mol), and Pro116
(- 4.141 kcal/mol) residues via its 1-(3-bromophenyl)-1H-1,2,3-triazol-4-yl scaf-
fold, while the 4-hydroxy-2H-chromen-2-one component became engaged in
similar types of interactions with Ala417 (- 2.316 kcal/mol), Tyr415
(- 2.863 kcal/mol), Gly321 (- 1.984 kcal/mol), Phe320 (- 2.263 kcal/mol),
Asp318 (- 2.929 kcal/mol), Leu317 (- 3.259 kcal/mol), Csy129 (- 1.942 kcal/-
mol), Val121 (- 1.888 kcal/mol), Thr118 (- 3.513 kcal/mol), Gly117
(- 4.667 kcal/mol), Tyr60 (- 2.749 kcal/mol), Ser59 (- 1.967 kcal/mol), Arg58
(- 4.122 kcal/mol), Trpand 16 (- 2.063 kcal/mol) lining the active site of DprE1.
The high affinity observed for 6h is also attributed to the electrostatic interactions
shown by 6h with Lys418 (- 11.452 kcal/mol), Lys367 (- 2.039 kcal/mol),
Glu322
(- 2.908 kcal/mol),
Lys134
(- 1.974 kcal/mol),
and
Trp16
(- 1.495 kcal/mol) residues. The non-bonded interactions were seen to be the
principal driving force for anchoring the 6h to the enzyme cavity. Furthermore,
binding is stabilized by one hydrogen bond and two p stacking (p–p and p–cation
interactions). The hydroxyl group of the coumarin ring formed a crucial hydrogen
˚
bond with the carbonyl oxygen of Arg58 with a bonding distance of 2.07 A. One p–
p stacking interaction was observed between the bromo-phenyl ring and His132,
while a p–cation interaction was observed for the triazole ring and Lys418 with
bonding distances of 2.771 and 2.479 further stabilizing the ligand–enzyme
complex. Such bonding interactions serve as an anchor guiding the 3D orientation
of the ligand in the active pocket and further facilitate the non-bonded (steric and
electrostatic) interactions.
A similar binding mode and network of interaction were also observed for 6a
(Fig. 5), 6b (Fig. 6), 6c (Fig. 7), 6d (Fig. 8), 6e (Fig. 9), 6f (Fig. 10), 6g (Fig. 11),
6i (Fig. 12), 6j (Fig. 13) and 6k (Fig. 14), but diminishing regularly with observed
antitubercular activity. Comparing the results of 6h with the least active analogue,
6j, revealed that this compound was as well bound to the active site of DprE1 at the
same coordinate as 6h, engaging in a very similar network of non-bonded and
Fig. 5 Binding mode of 6a into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
123

A. B. Danne et al.
Fig. 6 Binding mode of 6b into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
Fig. 7 Binding mode of 6c into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
Fig. 8 Binding mode of 6d into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
123

Synthesis and biological evaluation of novel triazole…
Fig. 9 Binding mode of 6e into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
Fig. 10 Binding mode of 6f into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
Fig. 11 Binding mode of 6g into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
bonded interactions (H-bond and p stacking). However, it is noteworthy that the
per-residue interactions observed for 6j were significantly lower than for 6h, which
could be the fundamental cause for the weaker binding affinity observed for 6j,
123

A. B. Danne et al.
Fig. 12 Binding mode of 6i into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
Fig. 13 Binding mode of 6j into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
Fig. 14 Binding mode of 6k into the active site of DprE1 (right side: green lines signify p–p stacking
interactions while the pink lines represent the hydrogen bonding interactions)
which is in agreement with the experimental antitubercular activity. For the sake of
brevity of text, the detailed per-residue interaction analyses for other compounds
and binding modes are given in Table 2. Overall, it is evident from the docking
studies that the compounds possess a promising affinity for this crucial
123

Synthesis and biological evaluation of novel triazole…
Table 2 Quantitative per-residue interaction analysis of the molecular docking study on DprE1 for the
most active triazole–biscoumarin conjugate analogues
Entry Docking Glide
score interaction
energy
Per-residues interactions
Van der Waals
(kcal/mol)
Coulombic
(kcal/mol)
H-bonds p–p/p–
˚
(A)
cation
stacking
(kcal/mol)
˚
(A)
6a
- 7.271 - 44.796
Lys418 (- 2.626),
Ala417 (- 1.863),
Tyr415 (- 1.946),
Asp389 (- 1.483),
Csy387 (- 1.881),
Val365 (- 1.544),
Leu363 (- 1.211),
Asn324 (- 1.421),
Gly321 (- 1.629),
Phe320 (- 1.279),
Asp318 (- 1.771),
Leu317 (- 2.657),
Tyr314 (- 1.018),
His132 (- 2.79),
Thr118 (- 2.034),
Gly117 (- 2.652),
Pro116 (- 1.948),
Tyr60 (- 1.946),
Arg58 (- 1.582),
Trp16 (- 2.071)
Lys418 (- 9.657),
Asp389 (- 3.117),
Asn324 (- 0.953),
Glu322 (- 2.127),
Gly117 (- 1.674),
Pro116 (- 1.301),
Tyr60 (- 3.271)
Arg58
(2.73),
Tyr60
(1.79)
6b
- 7.726 - 49.412
Lys418 (- 3.069),
Ala417 (- 2.114),
Tyr415 (- 2.014),
Csy387 (- 2.133),
Asn385 (- 1.143),
Phe369 (- 1.214),
Lys367 (- 1.669),
Phe366 (- 1.198),
Val365 (- 2.089),
Leu363 (- 1.123),
Gln336 (- 1.693),
Gly321 (- 1.603),
Phe320 (- 1.923),
Leu317 (- 2.428),
Tyr314 (- 1.011),
His132 (- 1.363),
Val121 (- 1.232),
Thr118 (- 2.335),
Gly117 (- 2.819),
Pro116 (- 1.992),
Tyr60 (- 2.098),
Arg58 (- 2.294)
Lys418 (- 1.526),
Asp389 (- 3.219),
Arg119 (- 1.347),
Pro116 (- 1.954)
Lys418
(2.16)
His132
(2.49)/
Lys418
(2.16)
123

A. B. Danne et al.
Table 2 continued
Entry Docking Glide
score
Per-residues interactions
interaction
energy
(kcal/mol)
Van der Waals
(kcal/mol)
Coulombic
(kcal/mol)
H-bonds
˚
(A)
p–p/p–
cation
stacking
˚
(A)
6c
- 7.138 - 43.51
Lys418 (- 1.757),
Ala417 (- 1.574),
Tyr415 (- 1.911),
Asp389 (- 1.382),
Csy387 (- 1.753),
Asn385 (- 1.643),
Phe369 (- 1.165),
Phe366 (- 1.082),
Val365 (- 1.801),
Leu363 (- 1.173),
Gln336 (- 1.994),
Asn324 (- 1.331),
Gly321 (- 1.574),
Phe320 (- 1.908),
Leu317 (- 1.759),
His132 (- 1.018),
Ile131 (- 1.231),
Thr118 (- 1.061),
Gly117 (- 1.017),
Pro116 (- 1.589),
Tyr60 (- 1.337),
Arg58 (- 1.259)
Lys418 (- 7.096),
Asp389 (- 1.279),
Arg119 (- 1.126),
Arg58 (- 1.624)
Gly117
(2.54)
His132
(2.99),
Tyr415
(1.81)
6d
- 6.869 - 40.573
Lys418 (- 1.681),
Tyr415 (- 1.897),
Csy387 (- 1.484),
Leu363 (- 1.149),
Asn324 (- 1.284),
Glu322 (- 1.225),
Gly321 (- 1.673),
Phe320 (- 1.133),
Asp318 (- 1.082),
His132 (- 1.225),
Csy129 (- 1.335),
Val121 (- 1.602),
120 (- 1.348),
Lys418 (- 5.091),
Asp389 (- 1.446),
Lys367 (- 1.146),
Glu322 (- 2.057),
Asp318 (- 3.099),
Gly117 (- 2.8)
Gly117
(1.99),
Leu317
(1.91)
Lys418
(2.274)
Gly117 (- 1.294),
Pro116 (- 1.614),
Tyr60 (- 1.18),
Ser59 (- 1.352),
Arg58 (- 1.397),
Trp16 (- 1.726)
123

Synthesis and biological evaluation of novel triazole…
Table 2 continued
Entry Docking Glide
score interaction
energy
Per-residues interactions
Van der Waals
(kcal/mol)
Coulombic
(kcal/mol)
H-bonds
˚
(A)
p–p/p–
cation
stacking
(kcal/mol)
˚
(A)
6e
- 7.909 - 50.278
Lys418 (- 3.276),
Ala417 (- 2.154),
Tyr415 (- 2.044),
Asp389 (- 1.504),
Csy387 (- 2.157),
Asn385 (- 1.181),
Phe369 (- 1.242),
Lys367 (- 1.949),
Val365 (- 2.198),
Leu363 (- 1.316),
Gln336 (- 1.725),
Asn324 (- 1.211),
Gly321 (- 1.806),
Phe320 (- 2.149),
Leu317 (- 2.518),
Tyr314 (- 1.041),
228 (- 1.22), Lys134
(- 1.715), His132
(- 2.335), Val121
(- 1.257), Thr118
(- 3.123), Gly117
(- 3.625), Pro116
(- 2.935), Tyr60
(- 2.272), Arg58
(- 2.839)
Lys418
(- 6.834),
Asp389
Lys418
(2.12),
Leu317
(2.13)
–
(- 1.132),
Asp318
(- 1.327),
Leu317
(- 1.307)
6f
- 8.052 - 52.868
Lys418 (- 3.392),
Ala417 (- 2.109),
Tyr415 (- 2.162),
Csy387 (- 2.145),
Asn385 (- 1.345),
Lys367 (- 2.113),
Val365 (- 2.134),
Leu363 (- 1.336),
Gln336 (- 1.815),
Asn324 (- 1.425),
Gly321 (- 1.444),
Phe320 (- 2.119),
Asp318 (- 2.125),
Leu317 (- 2.672),
His132 (- 2.371),
Csy129 (- 1.843),
Val121 (- 1.473),
Thr118 (- 3.192),
Gly117 (- 4.264),
Pro116 (- 3.359),
Tyr60 (- 2.456),
Ser59 (- 1.898),
Arg58 (- 3.114),
Trp16 (- 1.588)
Lys418
(- 8.155),
Glu322
Arg58
(2.07)
His132
(2.76)/
Lys418
(2.47)
(- 2.303),
Lys134
(- 1.621),
Trp16
(- 1.492)
123

A. B. Danne et al.
Table 2 continued
Entry Docking Glide
score
Per-residues interactions
interaction
energy
(kcal/mol)
Van der Waals
(kcal/mol)
Coulombic
(kcal/mol)
H-bonds p–p/p–
˚
(A)
cation
stacking
˚
(A)
6g
- 8.366 - 55.768
Lys418 (- 3.418),
Ala417 (- 2.161),
Tyr415 (- 2.137),
Csy387 (- 2.146),
Asn385 (- 1.442),
Lys367 (- 2.139),
Val365 (- 2.203),
Leu363 (- 1.434),
Gln336 (- 1.835),
Gly321 (- 1.534),
Phe320 (- 2.239),318
(- 2.148), Leu317
(- 2.936), His132
(- 2.999), Csy129
(- 1.985), Val121
(- 1.565), Thr118
(- 3.222), Gly117
(- 4.315), Pro116
(- 3.443), Tyr60
(- 2.572), Ser59
(- 1.929), Arg58
(- 3.089), Trp16
(- 1.692)
Lys418 (- 8.271), Arg58
Lys367
His132
(2.53)/
Lys418
(2.50)
(2.73),
Tyr60
(1.79)
(- 1.495),
Lys134
(- 1.645)
6h
- 8.935 - 60.153
Lys418 (- 3.561),
Ala417 (- 2.316),
Tyr415 (- 2.863),
Csy387 (- 2.859),
Asn385 (- 1.481),
Lys367 (- 2.459),
Val365 (- 2.964),
Leu363 (- 1.523),
Gln336 (- 1.921),
Gly321 (- 1.984),
Phe320 (- 2.263),
Asp318 (- 2.929),
Leu317 (- 3.259),
His132 (- 3.559),
Csy129 (- 1.942),
Val121 (- 1.888),
Thr118 (- 3.513),
Gly117 (- 4.667),
Pro116 (- 4.141),
Tyr60 (- 2.749),
Ser59 (- 1.967),
Arg58 (- 4.122),
Trp16 (- 2.063)
Lys418
(- 11.452),
Lys367
Arg58
(2.07)
His132
(2.771)/
Lys418
(2.479)
(- 2.039),
Glu322
(- 2.908),
Lys134
(- 1.974),
Trp16 (- 1.495)
123

Synthesis and biological evaluation of novel triazole…
Table 2 continued
Entry Docking Glide
score interaction
energy
Per-residues interactions
Van der Waals (kcal/mol) Coulombic
(kcal/mol)
H-
bonds
˚
(A)
p–p/p–
cation
stacking
(kcal/mol)
˚
(A)
6i
- 6.285 - 38.62
Lys418 (- 1.318),
Ala417 (- 1.133),
Tyr415 (- 1.196),
Asp389 (- 1.881),
Csy387 (- 1.185),
Asn385 (- 1.419),
Phe369 (- 1.057),
Phe366 (- 1.154),
Val365 (- 1.31),
Leu363 (- 1.198),
Gln336 (- 1.973), 334
(- 1.078), Asn324
(- 1.517), Gly321
(- 1.479), Phe320
(- 1.176), Leu317
(- 1.662), His132
(- 1.025), Ile131
(- 1.114), Thr118
(- 1.011), Gly117
(- 1.165), Pro116
(- 1.681), Tyr60
(- 1.575), Arg58
(- 1.125),
Lys418
(- 4.672),
Asp389
Gly117 His132
(2.48) (3.053)
(- 1.73),
Arg119
(- 0.931),
Arg58
(- 1.359)
6j
- 6.147 - 35.872
Lys418 (- 1.189),
Ala417 (- 1.13),
Tyr415 (- 1.935),
Csy387 (- 1.606),
Asn385 (- 1.967),
Lys367 (- 1.461),
Val365 (- 1.297),
Gln336 (- 1.498),
Asp318 (- 1.932),
Leu317 (- 1.026),
His132 (- 1.274),
Csy129 (- 1.28),
Val121 (- 1.448),
Thr118 (- 1.364),
Gly117 (- 1.043),
Pro116 (- 1.421),
Tyr60 (- 1.616), Ser59
(- 1.412), Arg58
(- 1.626), Trp16
(- 1.141),
Lys418 (- 4.773) Arg58
(2.27)
His132
(2.73)/
Lys418
(2.47)
123

A. B. Danne et al.
Table 2 continued
Entry Docking Glide
score
Per-residues interactions
interaction
energy
(kcal/mol)
Van der Waals
(kcal/mol)
Coulombic (kcal/mol) H-
p–p/p–
cation
stacking
bonds
˚
(A)
˚
(A)
6k
- 7.279 - 45.253
Lys418 (- 2.469),
Ala417 (- 2.063),
Tyr415 (- 2.046),
Csy387 (- 1.959),
Asn385 (- 1.111),
Phe369 (- 1.201),
Lys367 (- 1.553),
Phe366 (- 1.139),
Val365 (- 1.732),
Leu363 (- 1.123),
Gln336 (- 1.456),
Gly321 (- 1.182),
Phe320 (- 1.442),
Leu317 (- 2.129),
Tyr314 (- 1.063),
Lys134 (- 2.006),
Gly133 (- 1.414),
Ile131 (- 1.1720,
Csy129 (- 1.341),
Gly125 (- 1.333),
Val121 (- 1.102),
Thr118 (- 1.895),
Gly117 (- 2.645),
Tyr60 (- 2.026),
Ser59 (- 1.647),
Arg58 (- 1.468),
Trp16 (- 2.094)
Lys418 (- 5.777),
Lys367 (- 2.488),
Glu322 (- 2.122),
Lys134 (- 2.586),
Gly117 (- 1.03),
Pro116 (- 1.52)
Lys418 His132
(2.09) (2.37)
mycobacterial enzyme DprE1, providing ample opportunity to nurture this scaffold
further through the structure-based drug design approach to arrive at high-affinity
candidates.
In silico ADME predictions
We have performed a computational study for the prediction of ADME properties.
The compounds exhibited a good % absorption (% ABS) ranging from 47.790 to
63.598% (Table 3) and calculated by % ABS = 109 - (0.345 9 TPSA). We have
calculated molecular weight (MW), molecular volume (MV), number of hydrogen
bond donors (n-OHNH), number of hydrogen bond acceptors (n-ON), logarithm of
partition coefficient (milog P), topological polar surface area (TPSA), number of
rotatable bonds (n-ROTB) and Lipinski’s rule of five [69] using the Molinspiration
online property calculation toolkit [70]. The drug-likeness model score has been
calculated by Molsoft software [71] (Table 3).
123

Synthesis and biological evaluation of novel triazole…
123

A. B. Danne et al.
Conclusions
We have demonstrated the synthesis of novel triazole-incorporated biscoumarin
conjugates by using the molecular hybridization approach for the first time. The
synthesized conjugates were evaluated for their in vitro antitubercular activity
against dormant and active Mtb H37Ra strains. Most of the conjugates displayed
good activity with IC50 values ranging from 1.44 to 20.00 lg/mL against both the
Mtb H37Ra strains. In particular, compound 6h exhibited excellent antitubercular
activity with an IC50 value of 1.44 lg/mL against dormant Mtb H37Ra. A molecular
docking study was performed against the mycobaterial target DprE1 to investigate
the possible binding mode of the compounds. In addition, the conjugates showed
excellent anti-oxidant activity with lower IC50 values than the standard drug BHT.
These findings provide an opportunity to explore this scaffold as a pertinent starting
point for structure-based lead optimization.
Acknowledgements One of the authors, A.B.D., is very grateful to the University Grants Commission,
New Delhi, for the award of a research fellowship. The authors are also grateful to the Department of
Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad-431 004, India, for providing
necessary laboratory facilities. The authors are also grateful to Schrodinger, Inc. for providing the Demo
license of the Schrodinger Molecular Modeling Suite to perform the molecular modeling studies.
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