Synthesis, biological evaluation, molecular docking and structure-activity relationship studies of halogenated quinone and naphthoquinone derivatives

Article abstract of DOI:10.1016/j.molstruc.2019.06.002

Protein glycation, oxidative stress and acetyl cholinesterase enzyme (AChE) are closely implicated in the pathogenesis of diabetic complications and Alzheimer's disease. In this study, we synthesized a series of previously unexplored molecular analogues of halogenated quinone and naphthoquinone to investigate their therapeutic potency against protein glycation, free radical production and acetyl cholinesterase enzyme. The study on protein glycation resulted in the identification of two novel antiglycation agents such as 2,3-dichloronaphthalene-1,4-bistriflate (3) and 2,3-dibromonaphthalene-1,4-bis (trifluoromethanesulfonate) (6) with IC₅₀ values less than 16 micromolar range. Moreover, 2,3-dichloro-1,4- naphthoquinone (DCNQ) (1), 2,3-dibromonaphthoquinone (4), 2,3-dibromonaphthalene-1,4-diol (5) and 2,3,5,6-tetrachlorobenzene-1,4-bis(trifluoromethane- sulfonate) (8) exhibited potent antiglycation activity with IC₅₀ values of 54 ± 1.02, 43 ± 0.5, 65 ± 1.00 and 60 ± 1.25 μM, respectively as compared to the standard inhibitor rutin with an IC₅₀ value of 98 ± 1.50 μM. The structure-activity relationship (SAR) demonstrated that the presence of bis-(trifluoromethanesulfonate) as substituents alongwith halogens moieties appreciably influenced inhibitory potential and enhanced the antiglycation potential significantly. On free radical DPPH (2,2-diphenyl-1-picrylhydrazyl) scavenging assay, 2,3,5,6-tetrachlorobenzene-1,4-bis(trifluoromethane- sulfonate) (8), showed significant antioxidant activity with IC₅₀ values 52 ± 1.50 μM. On acetyl cholinesterase inhibition assay, compound 8 and 2,3-dichloronaphthalene-1,4-bistriflate 3, exhibited considerable inhibition with IC₅₀ values of 98 ± 1.00 μM and 124 ± 1.50 μM, respectively. The SAR studies against AChE revealed that the presence of chlorine and trifluoromethanesulfonate both combined together in a compound enhanced its inhibitory activity several folds. In support of experimental observations, molecular docking studies with the active site of acetyl cholinesterase also confirmed the stronger inhibition of compounds 3, and 8 compared to others derivatives. These inhibitors demonstrated hydrogen bonding with Tyr124 and π-π stacking with Tyr341 of the active site residues. In this study, galantamine, rutin and propyll gallate were used as standard inhibitors, respectively.

Full text of DOI:10.1016/j.molstruc.2019.06.002

Journal of Molecular Structure 1195 (2019) 462e469
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, biological evaluation, molecular docking and structure-
activity relationship studies of halogenated quinone and
naphthoquinone derivatives
,
b
,
, c,
Ghulam Abbas a *, Zahid Hassan b **, Ahmed Al-Harrasi b, Ajmal khan b,
Ahmed Al-Adawi a, Majid Ali d
a Department of Biological Sciences and Chemistry, University of Nizwa, P.O Box 33, 616, Birkat Al Mauz, Nizwa, Oman
b Natural and Medical Sciences Research Center, University of Nizwa, P.O Box 33, 616, Birkat Al Mauz, Nizwa, Oman
c Institute for Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, Karlsruhe, Germany
d Department of Chemistry, COMSATS Institute of Technology, Abbottabad, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history:
Protein glycation, oxidative stress and acetyl cholinesterase enzyme (AChE) are closely implicated in the
pathogenesis of diabetic complications and Alzheimer's disease. In this study, we synthesized a series of
previously unexplored molecular analogues of halogenated quinone and naphthoquinone to investigate
their therapeutic potency against protein glycation, free radical production and acetyl cholinesterase
enzyme.
Received 24 March 2019
Received in revised form
24 May 2019
Accepted 2 June 2019
Available online 5 June 2019
The study on protein glycation resulted in the identification of two novel antiglycation agents such as
2,3-dichloronaphthalene-1,4-bistriflate
(3)
and
2,3-dibromonaphthalene-1,4-bis
(tri-
Keywords:
Protein glycation
Free radical scavenging
Acetyl cholinesterase enzyme
Quinone and naphthoquinone derivatives
Molecular docking
fluoromethanesulfonate) (6) with IC50 values less than 16 micromolar range. Moreover, 2,3-dichloro-1,4-
naphthoquinone (DCNQ) (1), 2,3-dibromonaphthoquinone (4), 2,3-dibromonaphthalene-1,4-diol (5) and
2,3,5,6-tetrachlorobenzene-1,4-bis(trifluoromethane- sulfonate) (8) exhibited potent antiglycation ac-
tivity with IC50 values of 54 1.02, 43 0.5, 65 1.00 and 60 1.25
standard inhibitor rutin with an IC50 value of 98 1.50 M. The structure-activity relationship (SAR)
demonstrated that the presence of bis-(trifluoromethanesulfonate) as substituents alongwith halogens
moieties appreciably influenced inhibitory potential and enhanced the antiglycation potential signifi-
cantly. On free radical DPPH (2,2-diphenyl-1-picrylhydrazyl) scavenging assay, 2,3,5,6-
tetrachlorobenzene-1,4-bis(trifluoromethane- sulfonate) (8), showed significant antioxidant activity
mM, respectively as compared to the
m
Structure-activity relationship
with IC50 values 52 1.50
dichloronaphthalene-1,4-bistriflate 3, exhibited considerable inhibition with IC50 values of
98 1.00 M and 124 1.50 M, respectively. The SAR studies against AChE revealed that the presence of
mM. On acetyl cholinesterase inhibition assay, compound 8 and 2,3-
m
m
chlorine and trifluoromethanesulfonate both combined together in a compound enhanced its inhibitory
activity several folds. In support of experimental observations, molecular docking studies with the active
site of acetyl cholinesterase also confirmed the stronger inhibition of compounds 3, and 8 compared to
others derivatives. These inhibitors demonstrated hydrogen bonding with Tyr124 and p-p stacking with
Tyr341 of the active site residues. In this study, galantamine, rutin and propyll gallate were used as
standard inhibitors, respectively.
© 2019 Elsevier B.V. All rights reserved.
1. INTRODUCTION
The process of protein glycation is one of the major causes of
diabetic complications which adversely affect vital organs of the
human body [1]. During protein glycation, amino groups of protein
react with carbonyl functionality of reducing sugar thus forming
Schiff bases which are converted into stable Amadori products after
* Corresponding author. Department of Biological Sciences and Chemistry, Uni-
versity of Nizwa, P.O Box 33, 616, Birkat Al Mauz, Nizwa, Oman.
** Corresponding author. Natural and Medical Sciences Research Center, Univer-
sity of Nizwa, P.O Box 33, 616, Birkat Al Mauz, Nizwa, Oman.
E-mail addresses: abbashej@unizwa.edu.om (G. Abbas), zahid.hassan@kit.edu
(Z. Hassan).
https://doi.org/10.1016/j.molstruc.2019.06.002
0022-2860/© 2019 Elsevier B.V. All rights reserved.

G. Abbas et al. / Journal of Molecular Structure 1195 (2019) 462e469
463
rearrangement. These Amadori compounds are finally converted
into protein cross linked products called advanced glycation end-
products (AGEs) through dehydration and rearrangement in the
final step of protein glycation. The process of AGEs formation is
associated with major diabetic complications which damages vital
organs, such as kidney (nephropathy), eyes (cataract), nerves
(neuropathy), blood vessels (atherosclerosis) and causes impaired
wound healing. Identification of new antiglycation agents with
potent inhibitory potential is one of the most effective approaches
to manage the damage due to diabetic complications [2,3].
The process of protein glycation and oxidative stress are closely
associated with the onset of diabetes. Actually, oxidative stress is an
imbalance between the production of free radicals e reactive ox-
ygen species (ROS) and the antioxidant defense system [4]. Reactive
oxygen species including oxygen ions, free radicals and peroxides
are produced in the body as a result of several biological processes.
These ROS have been associated with many health problems such
as carcinogenesis, coronary heart disease, and ageing problems
[5e7]. Oxidative stress also leads to an accumulation of advanced
glycation endproducts and causes the vascular changes in the
nervous system that are found in Alzheimer's disease [8,9]. Syn-
thetic inhibitors of ROS also known as antioxidants, have been used
for the inhibition of radical formation to prevent serious health
problems. These antioxidants exert their effects by scavenging or
preventing the generation of ROS to control the formation of free
radicals and thus retard the progress of many chronic diseases such
as cancer, cardiovascular problems, inflammation and neurode-
generative diseases and Alzheimer's disease [10e12].
efficient lead molecules against important biological disorders. We
intended to investigate whether a particular halogen atom or group
(F, Cl, Br, and CF3 substituent) at the specific position in a molecule
renders it more active or more selective towards enzyme. In
particular, we are interested to study if systemic insertion of
halogen sites into small biologically active molecules could effec-
tively influence their inhibitory potential against pharmacologi-
cally important enzyme, protein glycation and oxidative stress. In
this study, molecular analogues of halogenated quinone and
naphthoquinone were synthesized and subjected to antiglycation
assay, radical scavenging assay and acetyl cholinesterase enzyme
inhibition assay. Some encouraging results were obtained in this
study.
2. Material and methods
2.1. Chemicals
Required chemicals and reagents such as bovine serum albumin
(BSA) was purchased from Research Organics (USA), anhydrous D-
glucose from Fisher Scientific (UK), sodium azide and trichloro
acetic acid (TCA) from Scharlau (Spain) and rutin from Carl Roth
GmbH
& Co (Germany). DPPH (l,l-Diphenyl-2-picrylhydrazyl
radical) and dimethyl sulfoxide (DMSO) were also purchased
from Sigma Aldrich. Acetyl cholinesterase (AChE) type VI-S, from
electric eel, acetylthiocholine iodide (ATCI), 5,50-dithiobis[2-
nitrobenzoic acid] (DTNB), TriseHCl buffer of pH 8 were pur-
chased from Sigma Aldrich.
Although enzymes are extremely vital for life, dysregulated
enzyme activity leads to the disease states. Enzyme inhibition is the
process in which certain compounds influence the enzyme-
substrate reaction due to their covalent or non-covalent in-
teractions with enzyme active sites. It is therefore convincing that
inhibition of specific enzyme will remain a key focus of pharma-
ceutical research for future therapy [13]. The acetyl cholinesterase
(EC 3.1.1.7) is a pharmacologically important enzyme found in the
blood and neural synapses to regulate the acetylcholine level in the
brain [14,15]. Deficiency of acetylcholine causes a common type of
dementia in the aging population called Alzheimer's disease (AD).
Enhancement of acetylcholine levels in the brain is an effective
approach to treat Alzheimer's disease [16]. The inhibition of acetyl
cholinesterase (AChE) enzyme remains a highly accessible and
viable therapeutic strategy for the symptomatic improvement in
Alzheimer's disease (AD). The acetyl cholinesterase enzyme in-
hibitors actually block the cholinesterase enzyme to break down
acetylcholine. In this way both the level and duration of the
neurotransmitter action is increased in the body [17].
Spectrophotometers (Molecular Devices, CA, USA) was used to
measurement absorption.
2.2. Antiglycation assay
Same assay procedure was followed as described by Matsuda
et al., 2003; Matsuura et al., 2002; Nakagawa et al., 2002 [20e22]
with slight modifications since we used 150 mL sample in each well
of micro titer plate. The comparison of fluorescence intensity at
370 nm excitations and emission at 440 nm was obtained by using
spectrofluorimeter (RF-1500, Shimadzu, Japan) Rutin was used as
standard inhibitor in this assay [23]. The percentage inhibition was
calculated using the equation:
% inhibition ¼ 100ꢀ (sample reading/ control reading)100
2.2.1. Statistical analysis
The results were expressed as mean SEM while the EZ-fit
software (Perrella Scientific Inc., Amherst, U.S.A.) was used to
calculate the IC50 values (mg/mL). IC50 values were measured by
using different concentrations of the active samples.
Since small molecules such as halogenated phenylcoumarins,
diphenyl ethers, and
b-lactams have previously shown great
pharmacological and enzyme inhibitory potentials, therefore
various classes of these compounds are common targets of the
pharmaceutical companies for new drug discovery [13]. The in-
hibitors of acetyl cholinesterase can still be considered as important
drug candidates in the overall multitarget profile of Alzheimer's
disease. Previously, several quinones-based series of hybrids con-
taining a central benzoquinone core were evaluated against acetyl
cholinesterase enzyme. These compounds showed good inhibitory
potencies against human (AChE), in the micromolar range [18].
Similarly, 2,3-dimethoxy-5-methyl-1,4-benzoquinones and 2-
methyl-1,4-naphthoquinones exhibited promising antiglycation
2.3. DPPH-radical scavenging assay
Free radical scavenging activities of these compounds were
determined by measuring the change in absorbance of DPPH (l,l-
diphenyl-2-picrylhydrazyl radical) at 515 nm by the spectrophoto-
metric method described by S. K. Lee [24].
Reaction mixture comprised of 95
DPPHֹ and 5 L of the test sample (1 mM) dissolved in DMSO. The
final concentrations of 300 M and 1000 M of DPPH and test
mL of ethanolic solution of
activity with IC50 value around 50
mM [19]. These findings inspired
m
us to design and investigate new series of quinone and naph-
thoquinone derivatives for their therapeutic application against
protein glycation, free radical production and acetyl cholinesterase.
Present study is in continuation of our efforts to identify new
m
m
compounds were used, respectively. The reaction mixture was then
incubated at 37 ꢁC for 30 min. After incubation, decrease in ab-
sorption was measured at 515 nm by using spectrophotometers

464
G. Abbas et al. / Journal of Molecular Structure 1195 (2019) 462e469
(Molecular Devices, CA, USA). The control contained 5
instead of the test compound. The reactions were performed in
triplicates.
m
L of DMSO,
were carried out in oven-dried reaction pressure tubes under inert
atmosphere (purged with argon before use).
In the present study, halogenated hydroxy-quinone and 1,4-
naphthoquinone derivatives (B) were synthesized from inexpen-
sive, commercially available 2,3-dichloro-1,4-naphthoquinone
(DCNQ), 2,3-dibromo-1,4-naphthoquinone, chloranil and broma-
nil by reduction with aqueous Na2S2O4 in high yields (up to 94%),
followed by transformation to the corresponding bis-triflates (C)
(up to 96%) Fig. 1 (Scheme1). The structures of these compounds
were characterized by IR, 1HNMR, 13CNMR and 19FNMR
spectroscopy.
2.4. Acetyl cholinesterase enzyme inhibition assay
Inhibition of acetylcholinesterase enzyme was determined by
using Ellman's colorimetric method as modified by Eldeen and co-
workers [25].
In this assay 96-well plate was used and each reaction well
contained 25
DTNB in Buffer (50 mM TriseHCl, pH 8), and 25
concentration dissolved in DMSO. In control sample 25
ple was replaced by 25 L of DMSO. 96-well plate was then incu-
bated at 25 ꢁC for 15 min. Thereafter, 25
L AChE (0.2 U/mL) was
m
L of 15 mM ATCI dissolved in water, 125
L of 1 mM sample
L of sam-
mL of 3 mM
m
2.6.1. General procedure A for the synthesis of 2
m
A suspension of 1 (2.0 g) in 30 ml of diethyl ether was shaken in
a separatory funnel with a freshly prepared solution of Na2S2O4
(5.00 g, in excess) in 10 ml of water. After the mixture was shaken
for 1h, the organic layer was separated, washed with brine
(1 ꢂ 25 ml), dried over Na2SO4, and then concentrated using a ro-
tary evaporator to obtain the product 2.
m
m
added to the wells and the absorbance measured five times
consecutively every 45 s. The reaction was monitored for 5 min at
405 nm [16,25,26]. The percentage inhibition was calculated using
the following equation:
Inhibition % ¼ 1ꢀ (blank sample reading/control reading) ꢂ 100
2.6.2. Synthesis of 2,3-dichloronaphthalene-1,4-diol (2)
A suspension of compound 1 (2.0 g) in diethyl ether (30 ml) was
shaken in a separatory funnel with a freshly prepared solution of
Na2S2O4 (5.00 g) in water (10 ml). After the mixture was shaken, the
organic layer was separated, washed with brine (2 ꢂ 25 ml), dried
over Na2SO4, then concentrated under reduced pressure following
previously reported general procedure to obtain 2 as a brown solid
(2.20 g, 90%).1H NMR (600 MHz, CDCl3): TM ¼ 5.69 (s, 2H),
7.52e7.55 (m, 2H), 8.13e8.16 (m, 2H); 13C NMR (150 MHz, CDCl3):
TM ¼ 111.1 (C), 122.1 (CH),123.2 (C) 126.9 (CH), 142.1 (C). IR (cmꢀ1):
2.5. Molecular docking studies
Crystal structure of the enzyme, acetylcholinesterase (PDB:
4EY7) was retrieved from Protein Data Bank [27]. The co-
crystallized ligand in the active site of AChE (donepezil) (PDB:
E20) was taken as possible binding site. The molecular construction
of the compounds was performed using ChemBioDraw Ultra 14
suite [28]. Molecular docking studies of the compounds were car-
ried out using MOE (Molecular Operating Environment, Chemical
computing group Inc. 2016). The structures of the compounds were
energy minimized using MMFF94x forcefield and gradient: 0.05.
Enzyme was prepared by the addition of hydrogen. All other pa-
rameters were used with the default settings. Galantamine and
donepezil were taken as reference ligands to compare the results.
For each ligand ten conformations were generated. The images in
2D were captured using MOE ligand binding interaction and 3D
images were taken using UCSF Chimera 1.11 software [29].
n
¼ 1538, 1274 (m), 1077, 883, 757 (s).
2.6.3. Synthesis of 2,3-dichloronaphthalene-1,4-diyl bis-
(trifluoromethanesulfonate) (3)
To a solution of 2 (1.0 g, 1.0 equiv.) in CH2Cl2 (25 ml) was added
pyridine (2.0 equiv.) at ꢀ50 ꢁC under an argon atmosphere. After
stirring the solution for 10 min at ꢀ50 ꢁC, Tf2O (2.5 equiv.) was
added. The mixture was stirred for further 3 h and was allowed to
warm to 20 ꢁC and stirred for further 3h. The reaction mixture was
filtered and the filtrate was concentrated under reduced pressure.
The residue was directly purified by chromatography without
aqueous work up (flash silica gel, Hexane/EtOAc) to obtain 3 as a
white crystalline solid. Yield: 1.80 g (96%). 1H NMR (600 MHz,
CDCl3): TM ¼ 7.79e7.80 (m, 2H), 8.13e8.15 (m, 2H); 19F NMR
2.6. Synthetic material and methodology
(564.6 MHz, CDCl3):
d
¼ ꢀ71.7; 13C NMR (150 MHz, CDCl3):
¼ 116.1
d
All starting materials and anhydrous solvents were purchased
commercially and used without further purification. NMR spectra
were recorded on Bruker Avance 600 MHz instruments. Chemical
shifts given in ppm and coupling constants J in Hz are referenced to
signals of the deuterated solvent. Purification of the synthesized
compounds was performed by column chromatography using a
flash silica gel 60 chromatography system. TLC was performed us-
ing silica gel 60 F254 and visualized with a UV lamp. All reactions
(C), 118.8 (q, JC-F ¼ 319.5 Hz, CF3), 121.8 (CH), 125.6 (C), 126.6 (C),
130.1 (CH), 141.5 (C). IR (cmꢀ1):
n
¼ 1738 (s), 1366, 1216 (m) (s).
2.6.4. Synthesis of 2,3-dibromonaphthalene-1,4-diol (5)
Starting with 4 (2.0 g, 1.0 equiv.), Na2S2O4 (5.00 g), following
general procedure A, compound 5 was obtained as a brown solid
(1.9 g, 94%); 1H NMR (300 MHz, CDCl3): TM ¼ 5.63 (s, 2H), 7.47e7.50
Fig. 1. (Scheme 1). Synthesis of halogenated hydroquinones (B) and bis-triflates (C): Reagents and Conditions: (i), A (1.0 equiv.), Na2S2O4 (3.0 equiv.), 1 h; (ii), B (1.0 equiv.), Tf2O (1.5
equiv. per OH transformation), pyridine (2.0 equiv.), CH2Cl2, 20 ꢁC, 6 h.

G. Abbas et al. / Journal of Molecular Structure 1195 (2019) 462e469
465
(m, 2H), 8.09e8.12 (m, 2H); 13C NMR (75.5 MHz, CDCl3): TM ¼ 104.1
aqueous Na2S2O4 in high yield (up to 94%), followed by trans-
formation to the corresponding bis-triflates. The structures of these
compounds were characterized by using various spectroscopic
techniques [46,47]. These compounds were then evaluated for
antiglycation activity, free radical scavenging potential and acetyl
cholinesterase enzyme inhibition.
(C), 122.5 (2CH), 127.1 (2CH), 134.5 (C), 143.3 (C); IR (cmꢀ1):
n
¼ 1738, (s), 1366, 1216 (m), 700 (w). HRMS: (EI, 70 eV): calcd for
C
10H6 Br2O2 [Mþ]: 315.2741; found: 315.2745.
2.6.5. General procedure B for the synthesis of triflate 6
To a solution of 5 (1.0 g, 1.0 equiv.) in CH2Cl2 (2.5 ml/mmol) was
added pyridine (2.0 equiv.) at 20 ꢁC under argon atmosphere. After
stirring for 10 min at 0 ꢁC, Tf2O (2.5 equiv.) was added. The mixture
was allowed to warm to 20 ꢁC and stirred for further 6h. The re-
action mixture was filtered and the filtrate was concentrated in
vacuo. The residue was directly purified by chromatography
without aqueous work up (flash silica gel, heptane/EtOAc) to obtain
6 as a white solid. Yield: 2.20 g (90%).
In this study, one of the important findings is the identification
of two novel and four potent antiglycation agents from the tested
series of halogenated compounds. During this study, compound 3
(2,3-dichloronaphthalene-1,4-bistriflate) and compound 6 (2,3-
dibromonaphthalene-1,4-diyl bis (trifluoromethanesulfonate)
exhibited novel antiglyaction activity with IC50 values less than 16
micromolar range while compound
1
(2,3-dichloro-1,4-
naphthoquinone), compound (2,3-dibromonaphthoquinone),
4
compound 5 (2,3-dibromonaphthalene-1,4-diol) and compound 8
(2,3,5,6-tetrachlorobenzene-1,4-bis(trifluoromethanesulfonate)
showed potent antiglycation activity with IC50 values of 54 1.02,
2.6.6. 2,3-Dibromonaphthalene-1,4-diyl
bis(trifluoromethanesulfonate) (6)
Starting with 5 (1.0 g, 1.0 equiv.) in CH2Cl2 (25 ml), pyridine
(1.0 ml, 1.5 mmol) and Tf2O (2.7 ml, 2.5 equiv.), following general
procedure B, compound 6 was obtained as a white crystalline solid
(1.77 g, 96%); 1H NMR (300 MHz, CDCl3): TM ¼ 7.69e7.72 (m, 2H),
43 0.5, 65 1.00 and 60 1.25 mM, respectively as shown in Fig. 2.
Moreover, compound 2 (2,3-dichloronaphthalene-1,4-diol), com-
pound 7 (2,3,5,6-tetrachloro (1,4) benzoquinone) and compound 9
(2,3,5,6-tetrabromo-(1,4) benzoquinone) also showed good anti-
8.10e8.13 (m, 2H); 19F NMR (282.4 MHz, CDCl3):
d
¼ ꢀ71; 13C NMR
glycation activity with IC50 values of 139 1.35,108 1.5
101 1.50 M, respectively.
mM and
(75.5 MHz, CDCl3):
d
¼ 116.1 (2C), 118.8 (q, JF-C ¼ 319.0 Hz, 2CF3),
m
122.2 (2-CH), 127.3 (2C), 129.9 (2-CH), 142.6; GC-MS: (EI, 70eV): m/z
(%) ¼ (Mþ, 582 (100), 449 (51), 385 (66), 357 (43), 237 (72), 207
The structure-activity relationship (SAR) studies with reference
to antiglycation activity of quinone and naphthoquinone de-
rivatives demonstrated that the compounds 3, 6 and 8 containing
bis-(trifluoromethanesulfonate) moieties (Fig. 3) exhibited most
(60), 181 (40); IR (cmꢀ1):
n
¼ 1738 (s), 1366 (m), 1205 (m).HRMS:
(EI, 70 eV): calcd for
581.7691.
C
12H4Br2F6O6S2 [Mþ]: 581.7694 found:
promising activities with IC50 values of <16, <16 and 60 1.25 mM,
respectively as shown in Table-1. Thus the presence of bis-(tri-
fluoromethanesulfonate) as substituent alongwith halogen moiety
appreciably influenced and enhanced antiglycation potency. In this
3. Results and discussion
Quinones are widely distributed in nature, the natural products
containing quinone moiety are associated with anticancer, anti-
bacterial, antimalarial, and fungicide activities [30]. Quinones and
its polysubstituted derivates are also effective against several bio-
logical disorders. They posses electron and proton transfer prop-
erties as a result they are the part of drug lead structures [31e33].
Previously, quinone derivatives isoxazoles- exhibited several bio-
logical activities such as, hypoglycemic, antiinflammatory, anti-
bacterial, analgesic, antifungal, and HIV-inhibitory activity [34].
Several 1,4-naphthoquinone were identified as monoamine oxidase
(MAO) inhibitors which play an important role in treating disorders
such as Parkinson's disease, depressive illness, congestive heart
failure and cancer [35]. Designing naphthoquinones with diverse
substitution patterns may therefore benefit for future therapeutic
applications. Halonatural products have been recognized as valu-
able precursors for pharmaceuticals as they possess a variety of
biological effects such as antibacterial, antifungal, antiviral, anti-
inflammatory, antiproliferative, cytotoxic, antifouling, antifeedant,
ichthyotoxic, and insecticidal activities [36e39].
assay, rutin with an IC50 value of 98 1.50
inhibitor.
mM is used as standard
Generally, quinone and naphthoquinone compounds govern
their biological activities through their redox ability, free radical
mechanism and non-covalent interactions [48]. Inhibition of pro-
tein glycation has multiple mechanistic pathways including, free
radical inhibition, strong nucleophilic attack on C]O group of
glucose, electrophilic reaction with NH2 groups of protein and in-
hibition of AGEs cross-linking [2]. In this study, the possible
mechanism of inhibition can be attributed that inhibitors have S]O
and C]O groups called “sugar competitors “which make them
more vulnerable for NH2-group of proteins to bind and thus
inhibiting protein glycation by making NH2-group unavailable to
attack on C]O group of glucose molecules while few exhibited
strong antioxidants activity via free radical inhibition pathway.
The multitude of effects that can arise from the introduction of
carbon-halogen bonding sites in small molecules in the context of
medicinal chemistry is of great interest [40e43]. Insertion of
carbon-halogen bonding sites can strongly influences a number of
variables, such as binding affinity, pharmacokinetic properties, and
bioavailability of a given drug candidate [44]. Modulation of lip-
ophilicity can be effectively achieved with incorporation of chlo-
rine, fluorine, in particular, CF3, SꢀCF3, and OꢀCF3 groups. Halogen
incorporation in natural products assembly profoundly influences
and alters properties because of its metabolic stability, lipophilicity
that can be consequential for determining the affinity and selec-
tivity of interactions with biological targets provide quite attractive
opportunities in drug design [45].
In the present study, halogenated quinone and 1,4-
naphthoquinone derivatives were synthesized by reduction with
Fig. 2. Dose-dependent antiglycation activity of potent compounds.

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G. Abbas et al. / Journal of Molecular Structure 1195 (2019) 462e469
activity relationship (SAR) studies on DPPH-radical scavenging
assay showed that compound (2) exhibited more inhibition
(IC50 ¼ 120 1.50) as compared to compound (5) (50% at 1 mM)
which revealed that chloro substituted derivatives acquired more
inhibitory potential than bromo-substitued derivatives. Further-
more, the presence of bis-(trifluoromethanesulfonate) as substit-
uent alongwith halogen moiety further enhanced the radical
scavenging activity (IC50 ¼ 52 1.50). Compounds 1, 3, and 4, were
found to be inactive against DPPH radical scavenging assay. Propyll
gallate was used as standard inhibitor in this assay with an IC50
value of 33.00 1.5 mM.
On acetyl cholinesterase inhibition assay, compound (8) 2,3,5,6-
tetrachlorobenzene-1,4 bis (trifluoromethanesulfonate), and com-
pound (3) 2,3-dichloronaphthalene-1,4-bistriflate, exhibited sig-
nificant inhibition with IC50 values 98 1.00 mM and 124 1.50 mM,
respectively. Similarly, compound (6) 2,3-dibromonaphthalene-
1,4-diyl bis (trifluoromethanesulfonate), and compound (7) 2,3,5,6-
tetrachloro-(1,4)benzoquinone, showed a moderate inhibition with
IC50 values 180 1.00
in Table-2. In this study galantamine was used as standard inhibitor
with an IC50 value of 1.2 0.21 M.
mM and 205 1.00 mM, respectively as shown
m
Structure-activity relationship (SAR) studies on various quinone
and naphthoquinone derivatives revealed that the presence of both
chlorine and bis-(trifluoromethanesulfonate) combined together in
a compound enhanced the inhibitory potential several folds against
the acetyl cholinesterase enzyme. For instance, compound 2,3-
dichloronaphthalene-1,4-diol (2) with no triflate moiety showed
a reduced inhibition (25% at 1 mM) as compared to compound 2,3-
dichloronaphthalene-1,4-bistriflate (3), has shown several times
Fig. 3. Chemical structures of halogenated quinone and naphthoquinone derivatives
evaluated against acetyl cholinesterase enzyme, protein glycation and free radical
production.
On DPPH-radical scavenging assay, 2,3,5,6-tetrachlorobenzene-
1,4 bis-(trifluoromethanesulfonate) compound (8) showed signifi-
more inhibition (85%) with IC50 ¼ 124 1.50
mM due to presence of
cant antioxidant activity with an IC50 value of 52 1.50
2,3-dichloronaphthalene-1,4-diol compound (2) showed good
antioxidant potential with an IC50 value of 120 1.50 M (Table-1).
mM while
bistriflate moieties as shown in Fig. 3. However, compound 2,3-
dibromonaphthalene-1,4-diol (5) having no triflate moiety was
found to be inactive as compared to compound 2,3-
m
In this assay, compound 5, compound 7, compound 9 showed
moderate inhibition within 50e53% range at 1 mM. Structure-
dibromonaphthalene-1,4-bis
(trifluoromethanesulfonate)
(6)
Table-1
Antiglycation and DPPH-radical scavenging potential of halogenated quinone and naphthoquinone derivatives.
S. No
Name
Antiglycation activity
% Inhibition (1 mM)
DPPH
% Inhibition (1 mM)
(IC50
¼
mM)
(IC50
¼
mM)
1
2
3
4
5
6
7
8
9
2,3-dichloro-1,4-naphthoquinone (DCNQ) (1)
2,3-dichloronaphthalene-1,4-diol (2)
2,3-dichloronaphthalene-1,4-bistriflate (3)
2,3-dibromonaphthoquinone (4)
71 1.5 (IC50 ¼ 54 1.02)
63 1.00 (IC50 ¼ 139 1.35)
84 15 (IC50¼ <16)
NA
75 1.00 (IC50 ¼ 120 1.50)
NA
NA
75 1.00 (IC50 ¼ 43 0.5)
68 1.00 (IC50 ¼ 65 1.00)
87 1.5 (IC50¼ <16)
2,3-dibromonaphthalene-1,4-diol (5)
50 1.00
30 1.50
52 2.00
2,3-dibromonaphthalene-1,4-bis (trifluoromethanesulfonate) (6)
2,3,5,6-tetrachloro-(1,4)benzoquinone (7)
2,3,5,6-tetrachlorobenzene-1,4-bis(trifluoromethanesulfonate) (8)
2,3,5,6-tetrabromo-(1,4) benzoquinone (9)
60 1.5 (IC50 ¼ 108 1.5)
73 1.00 (IC50 ¼ 60 1.25)
62 1.00 (IC50 ¼ 101 1.50)
80 1.00 (IC50 ¼ 52 1.50)
53 2.00
Table-2
Inhibitory potential of halogenated quinone and naphthoquinone derivatives against acetyl cholinesterase.
S. No
Name
Acetyl cholinesterase
Enzyme % Inhibition (1 mM)
(IC50 M)
¼
m
1
2
3
4
5
6
2,3-dichloro-1,4-naphthoquinone (DCNQ) (1)
2,3-dichloronaphthalene-1,4-diol (2)
2,3-dichloronaphthalene-1,4-bistriflate (3)
2,3-dibromonaphthoquinone (4)
2,3-dibromonaphthalene-1,4-diol (5)
2,3-dibromonaphthalene-1,4-bis (trifluoromethanesulfonate) (6)
25 1.00
25 1.50
85 1.50 (IC50 ¼ 124 1.50)
70 2.00 (IC50 ¼ 415 2.00)
NA
82 1.50
IC50 ¼ 170 1.00
72 1.50 (IC50 ¼ 205 1.00)
90 2.00 (IC50 ¼ 98 1.00)
24 1.50
7
8
9
2,3,5,6-tetrachloro-(1,4)benzoquinone (7)
2,3,5,6-tetrachlorobenzene-1,4-bis(trifluoromethanesulfonate) (8)
2,3,5,6-tetrabromo-(1,4) benzoquinone (9)

G. Abbas et al. / Journal of Molecular Structure 1195 (2019) 462e469
467
which exhibited 82 1.50% inhibition and IC50 values
170 1.00 M.
m
Moreover, it was found that chloro-substituted quinones were
more active than bromo-substitued quinone derivatives. In present
study, compounds 2,3-dichloro-1,4-naphthoquinone (DCNQ) (1),
2,3-dichloronaphthalene-1,4-diol (2) and 2,3,5,6-tetrabromo-(1,4)
benzoquinone (9), exhibited a poor inhibition (24e25%) while
compound 2,3-dibromonaphthalene-1,4-diol (5), was found to be
inactive against acetyl cholinesterase enzyme as shown in Fig. 4.
Molecular docking studies investigated the possible binding
mode of interactions of these compounds with the active site of
acetylcholinesterase (4EY7) using MOE (Molecular Operating
Environment) software. During this docking study co-crystallized
ligand (Donepezil) indicated RMSD value of 1.5. Docking studies
confirmed the greater inhibition potential of compound 8, 3 and 6
through their interaction with acetylchoinesterase. They indicated
better affinity than others, with binding energy values
of ꢀ10.2, ꢀ9.5 and ꢀ10.8 kcal.molꢀ1, respectively as shown in
Table-3.
Fig. 4. Acetyl cholinesterase enzyme inhibition of halogenated quinone and naph-
thoquinone derivatives.
The two dimensional interaction model of the compounds with
the active site of enzyme along with water, taken through MOE,
revealed that the most active compounds (8, 3 and 6) makes con-
Table-3
Molecular docking energy (S) of compounds with the active site of
acetylcholinesterase (4EY7) through MOE.
Compound
Energy (S) Kcal.molꢀ1
ventional hydrogen bond with Tyr124 and
Tyr341 whereas galantamine demonstrated hydrogen bonding
with Tyr72 and stacking with Trp286, as shown in Fig. 5.
p-p stacking with
3
4
6
7
ꢀ9.5
ꢀ5.9
ꢀ10.8
ꢀ5.0
ꢀ10.2
ꢀ7.0
ꢀ9.3
1.5
p-p
Some important interactions were also observed through water
bridging as depicted in Fig. 6.
8
Galantamine
Co-crystallized
RMSD
Fig. 5. 2D binding pose representation of compound 3 (a), compound 6 (b), galantamine (c) and compound 8 (d) with the active site residues of acetylcholinesterase (4EY7) chain A,
taken through MOE.

468
G. Abbas et al. / Journal of Molecular Structure 1195 (2019) 462e469
Fig. 6. 3D binding pose representation of compound 8 (cyan), compound 6 (magenta) and co-crystallized ligand e donepezil (yellow) with acetylcholinesterase (4EY7) in wire mesh
model (a) and hydrophobic surface model (b).
J. Clin. Biochem. Nutr. 40 (3) (2007) 163e173.
[4] J.E. Cambers, R.L. Carr, J.S. Boone, H.W. Chambers, The metabolism of orag-
nophosphorus insecticides, in: second ed., in: R.I. Krieger (Ed.), Handbook of
4. Conclusions
The present study on a series of halogenated compounds
resulted in the identification of six new potent antiglycation agents,
two new inhibitors of acetyl cholinesterase enzyme and a prom-
ising antioxidant from the halogenated quinone and naph-
thoquinone derivatives. Structure-activity relationship (SAR)
studies revealed that substitutions attached to the quinone and
naphthoquinone skeleton exerted imperative influence to enhance
inhibition potency against protein glycation and acetyl cholines-
Pesticide Toxicology, vol. 2, Academic Press, New York, 2001, pp. 919e927.
[5] I. Khan, S. Ali, S. Hameed, N.H. Rama, M.T. Hussain, A. Wadood, R. Uddin, Z. Ul-
Haq, A. Khan, S. Ali, M.I. Choudhary, Synthesis, antioxidant activities and
urease inhibition of some new 1, 2, 4-triazole and 1, 3, 4-thiadiazole de-
rivatives, Eur. J. Med. Chem. 45 (2010) 5200e5207.
[6] K. Uchida, Role of reactive aldehyde in cardiovascular diseases, Free Radic.
Biol. Med. 28 (2000) 1685e1696.
[7] E. Cadenas, K.J. Davies, Mitochondrial free radical generation, oxidative stress,
and aging, Free Radic. Biol. Med. 29 (2000) 222e230.
[8] T. Ninomiya, Diabetes mellitus and dementia, Curr. Diabetes Rep. 14 (2014)
487.
[9] S. Ahtiluoto, T. Polvikoski, M. Peltonen, et al., Diabetes, Alzheimer disease, and
vascular dementia: a population-based neuropathologic study, Neurology 75
(2010) 1195e1202.
[10] J.E. Kinsella, E. Frankel, B. German, J. Kanner, Possible mechanisms for the
protective role of antioxidants in wine and plant foods, Food Technol. 47
(1993) 85e89.
[11] N. Singh, P.S. Rajini, Free radical scavenging activity of an aqueous extract of
potato peel, Food Chem. 85 (2004) 611e616.
terase.
It
is
assumed
that
halogen
and
bis-(tri-
fluoromethanesulfonate) derivatives both combined together in a
compound are appropriately oriented to support inhibitor against
acetyl cholinesterase enzyme to inhibit the active site of the
enzyme. Molecular docking studies illustrated compounds inter-
action with Tyr124 and Tyr341 of the enzyme active sites through
hydrogen bonding and
p-p stacking, respectively. These easy and
[12] R.L. Prior, X. Wu, K. Schaich, Standardized methods for the determination of
antioxidant capacity and phenolics in foods and dietary supplements, J. Agric.
Food Chem. 53 (2005) 4290e4302.
[13] R.A. Copeland, Evaluation of enzyme inhibitors in drug discovery: a guide for
medicinal chemists and pharmacologists, Methods Biochem. Anal. 46 (2005)
1e265.
accessible halogenated quinone and naphthoquinone derivative
inhibitors are attractive compounds to be further developed for
therapeutic applications.
Declaration of interest
[14] R.T. Bartus, R.L. Dean, B. Beer, A.S. Lippa, The cholinergic hypothesis of geri-
atric memory dysfunction, Science 217 (1982) 408e414.
[15] E.K. Perry, The cholinergic hypothesis-ten years on, Br. Med. Bull. 4 (1986)
63e69.
The authors declare no conflict of interest.
[16] E.A. Adewusi, N. Moodley, V. Steenkamp, Antioxidant and acetylcholines-
terase inhibitory activity of selected southern African medicinal plants, South
Afr. J. Bot. 77 (2011) 638e644.
Acknowledgements
[17] M.B. Colovic, D.Z. Krstic, T.D. Lazarevic-Pasti, A.M. Bondzic, V.M. Vasic, Acetyl-
cholinesterase inhibitors: pharmacology and toxicology, Curr. Neuro-
pharmacol. 11 (2013) 315e335.
[18] F. Prati, M. Bartolini, E. Simoni, A. De Simone, A. Pinto, V. Andrisano,
M.L. Bolognesi, Quinones bearing non-steroidal anti-inflammatory fragments
as multitarget ligands for Alzheimer's disease, Bioorg. Med. Chem. Lett 23
(2013) 6254e6258.
[19] Y.S. Jung, B.Y. Joe, S.J. Cho, Y. Konishi, 2, 3-Dimethoxy-5-methyl-1, 4-
benzoquinones and 2-methyl-1, 4-naphthoquinones: glycation inhibitors
with lipid peroxidation activity, Bioorg. Med. Chem. Lett 4 (2005) 1125e1129.
[20] H. Matsuda, T. Wang, H. Managi, M. Yoshikawa, Structural requirements of
flavonoids for inhibition of protein glycation and radical scavenging activities,
Bioorg. Med. Chem. 11 (24) (2003) 5317e5323.
This study was performed at the University of Nizwa, Oman
partially supported by its internal research grant [Project reference
Number: A/13-14-UoN/11/DBCS-CAS/IF].
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2019.06.002.
[21] N. Matsuura, T. Aradate, C. Sasaki, H. Kojima, M. Ohara, J. Hasegawa,
M. Ubukata, Screening system for the Maillard reaction inhibitor from natural
product extracts, J. Health Sci. 48 (6) (2002) 520e526.
[22] T. Nakagawa, T. Yokozawa, K. Terasawa, S. Shu, L.R. Juneja, Protective activity
of green tea against free radical-and glucose-mediated protein damage,
J. Agric. Food Chem. 50 (8) (2002) 2418e2422.
References
[1] Y.M. Farag, M.R. Gaballa, Diabesity: an overview of a rising epidemic, Nephrol.
Dial. Transplant. 26 (1) (2010) 28e35.
[2] G. Abbas, A.S. Al-Harrasi, H. Hussain, J. Hussain, R. Rashid, M.I. Choudhary,
Antiglycation therapy: discovery of promising antiglycation agents for the
management of diabetic complications, Pharmaceut. Biol. 54 (2) (2016)
198e206.
[3] M. Modak, P. Dixit, J. Londhe, S. Ghaskadbi, T.P.A. Devasagayam, Recent ad-
vances in Indian herbal drug research guest editor: thomas Paul Asir Deva-
sagayam Indian herbs and herbal drugs used for the treatment of diabetes,
[23] M. Mosihuzzman, S. Naheed, S. Hareem, S. Talib, G. Abbas, S.N. Khan, M. Israr,
Studies on a-glucosidase inhibition and anti-glycation potential of Iris loczyi
and Iris unguicularis, Life Sci. 92 (3) (2013) 187e192.
[24] S.K. Lee, Z.H. Mbwambo, H. Chung, L. Luyengi, E.J. Gamez, R.G. Mehta, King-
horn AD, Pezzuto JM. Evaluation of the antioxidant potential of natural

G. Abbas et al. / Journal of Molecular Structure 1195 (2019) 462e469
469
products, Comb. Chem. High Throughput Screen. 1 (1998) 35e46.
[25] I.M. Eldeen, E.E. Elgorashi, J. Van Staden, Antibacterial, anti-inflammatory,
anti-cholinesterase and mutagenic effects of extracts obtained from some
trees used in South African traditional medicine, J. Ethnopharmacol. 102
(2005) 457e464.
[26] A. Ferreira, C. Proença, M.L. Serralheiro, M.E. Araujo, The in vitro screening for
acetylcholinesterase inhibition and antioxidant activity of medicinal plants
from Portugal, J. Ethnopharmacol. 108 (2006) 31e37.
S. Fustero, V.A. Soloshonok, H. Liu, Fluorine in pharmaceutical industry:
fluorine-containing drugs introduced to the market in the last decade
(2001e2011), Chem. Rev. 114 (2013) 2432e2506.
[37] F.H. Vaillancourt, E. Yeh, D.A. Vosburg, S. Garneau-Tsodikova, C.T. Walsh,
Nature's inventory of halogenation catalysts: oxidative strategies predomi-
nate, Chem. Rev. 106 (2006) 3364e3378.
[38] C. Paul, G. Pohnert, Production and role of volatile halogenated compounds
from marine algae, Nat. Prod. Rep. 28 (2011) 186e195.
[27] H.M. Berman, J. Westbrook, Z. Feng, G. Gilliland, T.N. Bhat, H. Weissig,
I.N. Shindyalov, P.E. Bourne, The protein Data Bank, Nucleic Acids Res. 28
(2000) 235e242.
[39] D. O'Hagan, H. Deng, Enzymatic fluorination and biotechnological de-
velopments of the fluorinase, Chem. Rev. 115 (2014) 634e649.
[40] G.W. Gribble, Naturally occurring organohalogen compounds–a survey, J. Nat.
Prod. 55 (1992) 1353e1395.
[41] G.W. Gribble, The diversity of naturally produced organohalogens, Chemo-
sphere 52 (2003) 289e297.
[42] G.W. Gribble, Natural organohalogens: a new frontier for medicinal agents?
J. Chem. Educ. 81 (2004) 1441.
[43] G.W. Gribble, Recently discovered naturally occurring heterocyclic organo-
halogen compounds, Heterocycles 84 (2012) 157e207.
[44] M.Z. Hernandes, S.M. Cavalcanti, D.R. Moreira, J. de Azevedo, W. Filgueira, ,
Leite AC. Halogen atoms in the modern medicinal chemistry: hints for the
drug design, Curr. Drug Targets 11 (2010) 303e314.
[45] P. Metrangolo, H. Neukirch, T. Pilati, G. Resnati, Halogen bonding based
recognition processes: a world parallel to hydrogen bonding, Acc. Chem. Res.
38 (2005) 386e395.
[28] N. Mills, ChemDraw Ultra 10.0, J. Am. Chem. Soc. 128 (2006) 13649e13650;
J.W. Blunt, B.R. Copp, W.P. Hu, M.H. Munro, P.T. Northcote, M.R. Prinsep,
Marine natural products, Nat. Prod. Rep. 24 (2007) 31e86.
[29] E.F. Pettersen, T.D. Goddard, C.C. Huang, G.S. Couch, D.M. Greenblatt,
E.C. Meng, T.E. Ferrin, UCSF Chimerada visualization system for exploratory
research and analysis, J. Comput. Chem. 25 (2004) 1605e1612.
[30] P.J. O'brien, Molecular mechanisms of quinone cytotoxicity, Chem. Biol.
Interact. 80 (1991) 1e41.
[31] P.R. Dandawate, A.C. Vyas, S.B. Padhye, M.W. Singh, J.B. Baruah, Perspectives
on medicinal properties of benzoquinone compounds, Mini Rev. Med. Chem.
10 (2010) 436e454.
[32] S. Patai, Z. Rappoport, The Chemistry of the Quinonoid Compounds, John
Wiley & Sons, 1988.
[33] S. Sasaki, K. Ogawa, M. Watanabe, M. Yoshifuji, Synthesis and properties of
sterically crowded triarylphosphines bearing naphthoquinone moieties, Or-
ganometallics 29 (2010) 757e766.
[34] P.M. Dewick, in: Medicinal Natural Products, second ed., Wiley, Chichester,
UK, 2002.
[46] Z. Hassan, S. Al-Shidhani, A. Al-Ghafri, A. Al-Harrasi, J. Hussain, R. Csuk,
Palladium-catalyzed chemo-and regioselective cross-coupling reactions of 2,
3-dichloronaphthalene-1, 4-bistriflate, Tetrahedron Lett. 56 (2015)
7141e7144.
[47] Z. Hassan, I. Ullah, I. Ali, I. Knepper, T. Patonay, A. Villinger, P. Langer, Syn-
thesis of tetraaryl-p-benzoquinones and 2,3-diaryl-1,4-naphthoquinones via
Suzuki-Miyaura cross-coupling reactions, Tetrahedron 69 (2013) 460e469.
[48] D. Choudhari, D.N. Lande, A. Bagade, S.P. Gejji, D. Chakravarty, K.M. Kodam,
S. Salunke-Gawali, Molecular structures and biological activities of (N)-n-
alkylammonium 2-chloro-3-oxido-1, 4-naphthoquinone salts, J. Mol. Struct.
1145 (2017) 309e320.
[35] L.A. Decosterd, I.C. Parsons, K.R. Gustafson, J.H. Cardellina, J.B. McMahon,
G.M. Cragg, Y. Murata, L.K. Pannell, J.R. Steiner, J. Clardy, M.R. Boyd, Structure,
absolute stereochemistry, and synthesis of conocurvone, a potent, novel HIV-
inhibitory naphthoquinone trimer from a Conospermum sp, J. Am. Chem. Soc.
15 (1993) 6673e6679.
ꢀ
ꢀ
[36] J. Wang, M. Sanchez-Rosello, J.L. Acenea, C. del Pozo, A.E. Sorochinsky,