In silico studies on 2,3-dihydro-1,5-benzothiazepines as cholinesterase inhibitors

Article abstract of DOI:10.1007/s00044-011-9754-6

In vitro studies on cholinesterase inhibitory potential on the three sets of 2,3-dihydro-1,5-benzothiazepines have been carried out. The compounds in Set 1 were unsubstituted on ring A, while those in Sets 2 and 3 had a 2′- and 3′-hydoxy substituent, respectively, in ring A. These studies revealed that they are mixed inhibitors of both AChE and BChE as reflected from their IC₅₀ values. It was further observed that 3′-hydroxy substituted benzothiazepines (Set 3) were found to have stronger affinity for both AChE and BChE compared with those of Sets 1 and 2. Moreover, all the compounds in Set 3 were found to be stronger BChE inhibitors than AChE. These experimental observations were rationalized by conducting in silico studies using molecular docking tool of Molecular Operating Environment (MOE) software, thereby, a good correlation was observed between IC₅₀ values and their binding interactions within the enzyme active site. We have observed that these interactions were electrostatic and hydrophobic in nature besides hydrogen bonding. The high BChE inhibitory potential of 3′-hydroxy substituted benzothiazepines was found to be cumulative effect of hydrogen bonding and π-π interactions between the ligand and BChE. These findings may serve as a guideline for synthesizing more potent ChE inhibitors for the treatment of Alzheimer's disease and related dementias. Springer Science+Business Media, LLC 2011.

Full text of DOI:10.1007/s00044-011-9754-6

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res (2012) 21:2329–2339
DOI 10.1007/s00044-011-9754-6
ORIGINAL RESEARCH
In silico studies on 2,3-dihydro-1,5-benzothiazepines
as cholinesterase inhibitors
•
Farzana Latif Ansari Saima Kalsoom
•
•
Zaheer-ul-Haq Zahra Ali Farukh Jabeen
•
Received: 1 March 2011 / Accepted: 18 July 2011 / Published online: 3 August 2011
Ó Springer Science+Business Media, LLC 2011
Abstract In vitro studies on cholinesterase inhibitory
potential on the three sets of 2,3-dihydro-1,5-benzothiaze-
pines have been carried out. The compounds in Set 1 were
unsubstituted on ring A, while those in Sets 2 and 3 had a
20- and 30-hydoxy substituent, respectively, in ring A.
These studies revealed that they are mixed inhibitors of
both AChE and BChE as reflected from their IC50 values. It
was further observed that 30-hydroxy substituted ben-
zothiazepines (Set 3) were found to have stronger affinity
for both AChE and BChE compared with those of Sets 1
and 2. Moreover, all the compounds in Set 3 were found to
be stronger BChE inhibitors than AChE. These experi-
mental observations were rationalized by conducting in
silico studies using molecular docking tool of Molecular
Operating Environment (MOE) software, thereby, a good
correlation was observed between IC50 values and their
binding interactions within the enzyme active site. We
have observed that these interactions were electrostatic and
hydrophobic in nature besides hydrogen bonding. The high
BChE inhibitory potential of 30-hydroxy substituted ben-
zothiazepines was found to be cumulative effect of
hydrogen bonding and p–p interactions between the ligand
and BChE. These findings may serve as a guideline for
synthesizing more potent ChE inhibitors for the treatment
of Alzheimer’s disease and related dementias.
Keywords Cholinesterase inhibitors ꢀ MOE docking ꢀ
1,5-benzothiazepines
Introduction
Alzheimer’s disease (AD) is a neurodegenerative disorder
which is characterized by the loss of cholinergic activity in
patient’s brain (Terry and Buccafusco, 2003; Silman and
Sussman, 2005; Nachon et al., 2005); Acetylcholinesterase
(AChE) is one of nature’s most elegantly engineered pro-
teins. The physiological role of the enzyme is the accelera-
tion of the hydrolysis of neurotransmitter acetylcholine at
nerve–nerve and neuromuscular junctions. The enzyme is
ideally suited for this role, because it possesses one of the
fastest turnover rates known (Carreiras and Marco, 2004;
Shen et al., 2005). ACh is a neurotransmitter that is
hydrolyzed by acetylcholinesterase (EC 3.1.1.7) and butyr-
ylcholinesterase (EC 3.1.1.8) (Rodriguez et al., 2005; Zah-
eer-ul-Haq et al., 2003); Hence, the most promising
therapeutic strategy for activation of the central cholinergic
function has been the use of cholinomimetic agents. The
function of cholinesterase inhibitors is to boost up the
endogenous level of ACh in the brain of patients suffering
from AD, thereby, increasing cholinergic neurotrans-
mission.
Acetylcholinesterase (AChE) and butyrylcholinesterase
(BChE) are found to be quite similar, possessing 53%
sequence homology and both have their active sites at the
F. L. Ansari (&) ꢀ S. Kalsoom ꢀ Z. Ali ꢀ F. Jabeen
Department of Chemistry, Quaid-i-Azam University,
Islamabad 45320, Pakistan
˚
base of enzyme gorge of depth 20 A. Recent research has
revealed that in the brain of patients suffering from AD, the
level of AChE is considerably reduced whereas that of
BChE increases, thus, aggravating the toxicity of b-amy-
loid peptide. In such instances, it is possible that BChE
may be more suitable target than AChE (Harel et al.,
e-mail: fla_qau@yahoo.com
Zaheer-ul-Haq
Dr. Panjwani Center for Molecular Medicine and Drug Research,
International Center for Chemical and Biological Sciences,
University of Karachi, Karachi 75270, Pakistan
123

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Med Chem Res (2012) 21:2329–2339
1993). It is not surprising that cholinesterase inhibitors
have shown better results in the treatment of AD than any
other strategy explored (Kryger et al., 1998); (Ansari et al.,
2005). Hence, the search for the discovery of novel cho-
linesterase inhibitors (ChE) is expected to continue in
future since the current ChEs inhibitors are reported to
have some side effects too (Ansari et al., 2008). The
availability of several crystal structures of both ChEs in
complexes with different inhibitors provides the possibility
to apply docking protocol to explore for protein inhibitor
complexes in terms of the nature of their interactions
(MOE 2005-06; Ellman et al., 1961).
which was found to be primarily based on the presence and
interaction of the ligands with Trp84 and Arg515 in the
active sites of AChE and BChE, respectively.
Materials and methods
Solution phase synthesis of 2,3-dihydro-1,5-
benzothiazepines of Sets 1 and 2 (Ansari et al., 2005)
To a solution of chalcone (20 mmol) in dry acidic metha-
nol acidified by adding few drops of glacial acetic acid, o-
aminothiophenol (20 mmol) was added. The mixture was
refluxed until a crystalline solid separated out. After
cooling, the solid product was collected and washed with
diethyl ether and cold methanol. The crude solid was
recrystallized from ethanol.
We have reported earlier the synthesis of a variety of
2,3-dihydro-1,5-benzothiazepines by a [4?3] annulation of
a,b-unsaturated ketones with o-aminothiophenol (Esposito
et al., 2000; Kuntz, 1992). The elements of diversity were
introduced by substitution both on the rings A and B that
led to the following three sets of compounds.
N
OH
N
N
S
S
S
HO
R
R
R
Set 1
Set 2
Set 3
Solid phase synthesis of 2,3-dihydro-1,5-
It was interesting to note that all the compounds in Set 2
were inactive, while only one benzothiazepines (4) was
active in Set 1; however, quite a significant number of
compounds were found to possess strong BChE inhibitory
potential. Another striking observation was that ben-
zothiazepines having a 30-hydroxy substituent on ring A
were found to be more potent than either the unsubstituted-
or 20-hydroxy substituted analogs. These observations point
toward the necessity of having a deeper insight into their
mechanism of cholinesterase inhibition. The knowledge of
this mechanistic detail would help in developing new
potential drugs for treating AD and related dementias.
Herein, we report a comparative study of cholinesterase
inhibitory potential of the three sets of compounds syn-
thesized earlier by conducting molecular docking studies
using Molecular Operating Environment (MOE) software
(Morris et al., 1996). The main purpose of this study was to
provide a possible relationship between the experimentally
determined IC50 values and the docking interactions of
benzothiazepines 1–27 in the active site of ChEs. The
docking of these benzothiazepines with AChE and BChE
has provided valuable information about the nature of
binding interactions of these ligands with both these
enzymes. These results provide a rationale for the greater
inhibitory potential of some compounds than the others,
benzothiazepines of Set 3 (Ansari et al., 2008)
Solid phase synthesis of 2,3-dihydro-1,5-benzothiazepines
(12–27) has been carried out on Wang resin using chal-
cones as precursors which in turn were synthesized on the
same solid support through Claisen-Schmidt condensation
of 3-hydroxyacetophenone with different substituted
aldehydes.
Cholinesterase inhibition assay
Electric-eel acetylcholinesterase (EC 3.1.1.7), acetylthi-
ocholine iodide, butyrylthiocholine chloride, 5,5-dithio-
bis-[2-nitrobenzoic acid] (DTNB), galanthamine, and
eserine were purchased from Sigma. All the other chemi-
cals were of analytical grade. AChE and BChE inhibitory
activities were determined according to the reported pro-
tocol (Abdel-Hamid et al., 2007). Acetylthiocholine iodide
and butyrylthiocholine chloride were used as substrates to
evaluate AChE and BChE, respectively. Sodium phosphate
buffer (pH 8.0; 150 ll, 100 mM), DNTB, test-compound
solution (10 ll, in EtOH), and AChE (or BChE) solution
(20 ll) were mixed and incubated for 15 min at 25°C. The
reactionwas then initiatedbythe additionofacetylthiocholine
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Med Chem Res (2012) 21:2329–2339
2331
(or butyrylthiocholine) (10 ll), their hydrolysis was moni-
tored by the formation of a yellow 5-thio-2-nitrobenzoate
anion which was traced by UV/VIS at 412 nm. All the reac-
tions were performed in triplicate in a 96-well micro-titer
plate. The percentage inhibition was calculated as (E - S)/
E 9 100,whereEistheactivityoftheenzyme withoutthetest
compound, and S is the activity of enzyme with the test
compound.
model was then used for calculating the energy parameters
using MMFF94x force field energy calculation and pre-
dicting the ligand–enzyme interactions at the active site.
Results and discussion
The solution phase synthesis of benzothiazepines of Sets 1
and 3 (1–11) was carried out by the reaction of a,b-
unsaturated ketones using o-aminothiophenol (Scheme 1).
Benzothiazepines of Set 3 (12–27) were synthesized by the
same synthetic strategy in the solid phase using Wang resin
as a solid support as shown in Scheme 2.
Docking procedure
Docking programs are broadly classified into two classes
i.e., direct and unbiased; the former have the advantage of
having higher speed, but they have the disadvantage of
making assumptions about potential energy landscape to
save computational time (Stewart, 1989). The unbiased
methods such as Autodock and MOE Dock have abilities to
cover up disadvantages of direct docking methods.
Although both MOE and Autodock are quite similar,
however, the former has the advantage of graphical inter-
face as well as other modules such as analysis, molecular
mechanics, and molecular dynamics (Nawaz et al., 2008);
(Carolan et al., 2008). MOE-Dock by Chemical Computing
Group Inc was used in the current study. All the docking
studies were carried out on a Pentium 1.6 GHz worksta-
tion, 512 MB memory using the Windows Operating
system.
In vitro cholinesterase inhibition studies were carried
out on unsubstituted bezothiazepines (Set 1) and different
substituted 20-, and 30-hydroxybenzothiazepines (Sets 2 and
3) and the IC50 values of the screened compounds were
determined along with that of standard galanthamine as
shown in Table 1. It is evident that none of the ben-
zothiazepines in Set 2 was found to be active against any of
the ChEs. However, one benzothiazepines 4 in Set 1 was
found to be active against BChE with an IC50 value of
60 lM; the same compound was found to be only weakly
active against AChE (IC50 = 102 lM).
Parallel studies when carried out on 30-hydroxy-substi-
tuted benzothiazepines Set 3 revealed that quite a few
compounds were found to have potential as ChE inhibitors
with IC50 values varying from 5.9 to 102 lM against
AChE, and 3.97–142 lM against BChE. A comparison of
the inhibitory potential of these compounds against AChE
and BChE also revealed that the number of compounds
active against BChE was greater than that of those active
against AChE. Furthermore, compounds 17, 19, and 27
with IC50 values 4.70, 4.65, and 3.97 lM, respectively
were found to be even more active than the standard
galanthamine (IC50 = 8.0 lM). Therefore, it deemed nec-
essary to explain the possible ligand–enzyme interactions
in these three sets of benzothiazepines and to have a deeper
insight into the mechanism of their interaction to help in
the designing of new potent ChEs inhibitors.
The crystal structure of protein complexes of AChE and
BChE were obtained from Protein Data Bank (PDB id
1ACl and 1POI, respectively), since they represent the
pharmacological target for the development of new drugs
to cure the Alzheimer disease. The edited crystal structure
after removing water molecules was imported into MOE
and all the hydrogen atoms were added to the structure with
their standard geometry followed by their energy minimi-
zation using MOPAC 7.0. The resulting model was sub-
jected to systematic conformational search at default
parameters with RMS gradient of 0.01 kcal/mol using Site
Finder. Enzymes were searched for their active sites and
dummy atoms were created from the resulting alpha
spheres. The backbone and residues were kept fixed, and
the energy minimization was performed. Root mean square
deviation values (RMSD) were used to compare the ligand
between the predicted and its corresponding crystal struc-
ture. The resulting docked poses with RMSD less than
More recently, we reported the cholinestrersase inhibi-
tory potential of 2,3,4,5-tetrahydrobenzothiazepines; the
reduced analogs of 2,3-dihydrobenzothiazepines with the
observation that the former set of compounds was much
more active than the latter. For getting further structural
insight into the inhibition mechanism of these compounds,
docking studies were carried out using SYBYL and the
results were reported. Herein, we report the theoretical and
computational studies on the mechanism of ChE inhibition
by the three sets of compounds through molecular docking
studies using MOE software.
˚
1.5 A were clustered together. The lowest energy mini-
mized pose was used for further analysis.
All the benzothiazepines were docked following the
same procedure. Ten different conformations were selected
for each ligand. All the other parameters were maintained
at their default settings. The best conformation of each of
the ligand–enzyme complex was selected based on the
energetic grounds. The resulting ligand–enzyme complex
Protein data bank contains several AChE and BChE
complexes with small molecules. The major difference in
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Med Chem Res (2012) 21:2329–2339
CH3
C
O
Scheme 1 Solution phase
synthesis of 2,3-dihydro-1,5-
benzothiazepines (1–11)
O
H
C
O
NaOH
R2
R1
+
R1
SH
H / MeOH
NH2
N
S
R1
R2
Set 1
No. R1
Set 2
R2
H
No. R1
R2
H
4-Cl
4-F
3-OMe
4-OMe
4-Me
1
2
3
4
H
H
H
H
5
6
7
8
9
10
11
2-OH
4-Me
3,4-diOMe
4-NO2
2-OH
2-OH
2-OH
2-OH
2-OH
2-OH
3,4-OCH2O
Scheme 2 Solid phase
synthesis of 30-hydroxy-2,3-
dihydro-1,5-benzothiazepines
(Set 3)
O
O
O
OH
'3
HO
'3
CH3
O
O
DIAD, TPP,NMM
48 hrs.
CH3
A
A
O
Ring B
CH
KOH 6 equiv.
SH
O
N
S
i)
'3
O
O
NH2
Ring B
HO '3
2
4
A
Ring B
3
ii)
A
50% TFA
Set 3
Ring B
Ring B
12 = Phenyl
20 = 3,4-OMe2C6H4
21 = 4-OH-3-OMeC6H3
22 = 3-NO2C6H4
23 = 4-NO2C6H4
24 = 4-MeC6H4
25 = 4-NMe2C6H4
26 = Pyridin-3-yl
27 = Thien-2-yl
13 = 2-ClC6H4
14 = 3-ClC6H4
15 = 4-ClC6H4
16 = 2-FC6H4
17 = 3-OHC6H4
18 = 4-OHC6H4
19 = 2-OCH3C6H4
their complexes is the orientation of one or more amino
acids. In order to control the performance of our docking
approach in the case of AChE, we have selected the crystal
structure of AChE cocrystallized with decamethonium. The
˚
active site is found to be located at 20 A from the protein
surface at the bottom of a deep and narrow gorge. The
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Med Chem Res (2012) 21:2329–2339
2333
Table 1 In vitro cholinesterase inhibitory activities and binding energies of 2,3-dihydro-1,5-benzothiazepines (1–27)
Compound
R1
R2
AChE
BChE
IC50 (lM)
Binding energy (KJ/mol)
IC50 (lM)
Binding energy (KJ/mol)
1
H
H
–
-10.45
-10.53
-11.13
-10.82
-10.77
-11.16
-11.22
-11.20
-10.94
-11.08
-11.71
-11.16
-11.00
-11.23
-11.44
-11.47
-11.80
-11.88
-11.77
-10.85
-11.80
-11.63
-11.66
-11.26
-11.31
-11.46
-11.49
-11.51
–
-9.06
-9.19
2
H
4-CH3
3,4-OCH2O
4-NO2
H
–
–
3
H
–
–
-10.56
-9.72
4
H
102
–
60.0
–
5
2-OH
2-OH
2-OH
2-OH
2-OH
2-OH
2-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
3-OH
-9.54
6
4-Cl
–
–
-10.48
-9.84
7
4-F
–
–
8
3-OCH3
4-OCH3
4-CH3
3,4-OCH2O
H
–
–
-10.09
-10.54
-9.87
9
–
–
10
–
–
11
–
–
-10.63
-10.37
-10.54
-10.25
-11.01
-10.41
-10.69
-11.78
-10.56
-10.55
-11.71
-11.27
-11.06
-10.64
-10.79
-10.09
-10.20
-10.04
12
40.0
–
142.1
–
13
2-Cl
14
3-Cl
–
29.4
76.9
–
15
4-Cl
18.8
70.6
71.1
–
16
2-F
17
3-OH
4.7
–
18
4-OH
19
2-OCH3
3,4-diCH3
3-OCH3-4-OH
3-NO2
4-NO2
4-CH3
4-NMe2
3-Pyridyl
2-Thiophen
–
4.65
19.7
–
20
–
21
–
22
–
–
23
–
–
24
49.8
–
75.2
34.4
–
25
26
–
27
5.9
0.5
3.97
8.0
Galanthamine
details of the docking procedure are given in the ‘‘Materials
and methods’’ section. Using docking protocol, the active
site of AChE enzyme was obtained, which contained
decamethonium as a ligand. Both the ligand and the side
chain of the binding site were relaxed, taking into account
direct contact through hydrogen bonding and p–p interac-
tions with these two residues.
All the synthesized benzothiazepines 1–27 were docked
in the active site of AChE and BChE and ten conformations
for each ligand were saved. Finally, the best conformation
was selected based on the lowest binding energy (Table 1).
The binding free energy data obtained after docking pro-
cedure did not show any apparent correlation between
docked binding energies and experimentally determined
IC50 values, which could be due to the neglect of aqueous
environment. Another reason could be that the experi-
mental data may not be precise enough to give consistent
values within the standard confidence limits.
˚
the amino acids mobility within 5 A around the ligand,
whereby nine amino acids Tyr70, Ser81, Trp84, Tyr121,
Glu199, Phe330, Phe331, Tyr334, and His440 were found
in the active site of AChE. The docking of all the com-
pounds with BChE (PDB id = 1POI) was done following
the same docking protocol as for AChE using the crystal
structure of BChE cocrystallized with 2-(N-morpholino)-
ethanesulfonic acid (MES). The active site, in this case,
˚
was also found to be located 20 A deep from the protein
˚
surface at the bottom. The amino acids within 5 A in the
Docking calculations also allow the prediction of the
structures of all the complexes between the enzymes and
ligands, thereby, suggesting different kinds of interactions.
These interactions were based on possible H-bonding and
p–p interactions between ligands and amino acids present
active sites were Lys323, Asp334, Tyr420, Val436,
Leu514, and Arg515. Most of the compounds were found
to bind with BChE between Lys323 and Arg515 making
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Med Chem Res (2012) 21:2329–2339
˚
within 5 A. Besides, the activity of compounds was also
found to be dependent on the presence or absence of one or
more amino acids in the active site. Moreover, all the
compounds were found to bind in the same pocket of active
site as shown by their superimposition in Fig. 1. The binding
of all the compounds in the active site of AChE could be due
to the fact that they have aromatic rings and a nitrogen atom,
which is generally common to the other AChE inhibitors.
However, the difference in their binding affinity could be
due to the difference in their substitution pattern.
Geometrical features of ligand–enzyme complexes were
correlated on the basis of their morphology and amino acids
environment in the macromolecular cavity. It was observed
that p–p interactions played an important role in stabilizing
ligand–enzyme complexes. The hydrogen bonding and p–p
interactions were the main features of interaction of mostly
ligands with Trp84 of AChE as shown in Table 2. Figure 2
confirms the key contributing role of Trp84 in the ligand
pocket. It is evident from Table 2 that benzothiazepine 27
was most potent AChE inhibitor of the series (IC50
=
5.9 lM). This could be rationalized on the basis of strong
hydrogen bonding (64% with Glu199 with a distance of
0
˚
2.9 A). The H-bonding was found between the 3 -hydroxyl
group of compound 27 and the carbonyl oxygen of Glu199 at
˚
Fig. 1 Superimposition of some benzothiazepines of Sets 1–3 in the
active site of AChE enzyme
the distance of 2.9 A (Fig. 3). The p–p interactions were
Table 2 Binding interactions observed in 1,5-benzothiazepines (1–27) with AChE
Compounds
IC50 (lM)
H-bonding
p–p interactions
˚
Distance (A)
Score (%)
Amino acid
1
–
–
–
–
–
2
–
–
–
–
Trp84, Tyr 334
Trp84
3
–
–
–
–
4
102
–
–
–
–
Trp84, Tyr 334
Trp84, Tyr 334
Phe331
Trp84
5
–
–
–
6
–
–
–
–
7
–
–
–
–
8
–
–
–
–
Trp84
9
–
–
–
–
Tyr334
Tyr334
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
–
–
–
–
–
–
–
–
40.0
–
2.07
–
45
–
His 440
Trp84
–
Phe330
Phe330
Trp84
–
–
–
–
18.8
70.6
71.1
–
3.5
3.65
2.9
–
37
20
25
–
Tyr 130
Ser 81
Trp84
Ser 81
Trp84, Phe330
Trp 84
Tyr334
Tyr334
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Tyr334
Trp84
49.8
2.52
21
Tyr 121
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Med Chem Res (2012) 21:2329–2339
2335
Table 2 continued
Compounds
IC50 (lM)
H-bonding
p–p interactions
˚
Distance (A)
Score (%)
Amino acid
25
26
27
–
–
–
–
Tyr334
Phe330
Trp84
–
–
–
–
5.9
1.49
2.5
2.63
2.84
2.0
46
77
93
65
55
Trp 84
Tyr 121
Tyr121
Tyr121
Asn85
Galanthamine
0.5
Trp84
Fig. 2 Key contribution of Trp84 in the active site of AChE enzyme with a 16, b 27
found with Trp84 and Phe330. The other active ben-
zothiazepines 15 (IC50 = 18.0 lM) also showed hydrogen
bonding (37%) with Tyr130 and p–p interaction with Trp84.
The equipotency of benzothiazepines 16 and 17 was evident
from the same type of H-bonding with Ser81 in both the
compounds. Table 3 showed the amino acid environment
˚
around the ligand within 5 A. It is evident that Trp84, being
present in all the ligand–enzymes complexes could be con-
sidered as activity determinant amino acids as the same
amino acid was also found in galanthamine–enzyme
complex.
The fact that benzothiazepines were found to be stronger
inhibitor of BChE compared with AChE may be rational-
ized on the basis of the observation that numerous docked
poses of these compounds scored very high in terms of
H-bonding (Table 4). For example in benzothiazepines 27,
the extent of H-bonding was found to be 96% (Fig. 4)
besides having very strong p–p interaction with Arg515.
The equipotent benzothiazepines 17 and 19 bind to BChE
with the same degree of p–p interaction with Arg515 and
the same extent of H-bonding (81%) with Try500 and
Lys323, respectively.
Another remarkable observation was that none of the
benzothiazepines in Set 2 was found to be active against
Fig. 3 3D docking pose of 27 (ball & stick) at AChE active site,
showing H-bonding (purple) with Glu199 (Color figure online)
123

2336
Med Chem Res (2012) 21:2329–2339
˚
Table 3 Hydrophobic interactions of 1,5-benzothiazepines (1–27) with different amino acids in the active site of AChE within 5 A radius
Compound
Tyr70
Ser81
Trp84
Glu199
Phe330
Phe331
Tyr334
His440
1
?
?
?
-
?
?
?
?
?
?
?
-
?
?
-
-
-
?
?
?
?
?
?
-
?
?
-
-
?
-
-
-
?
-
?
?
-
-
-
-
?
-
?
-
-
-
?
?
?
-
?
?
?
-
-
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
-
?
?
?
-
?
?
?
-
?
-
-
-
?
?
-
-
-
?
-
-
-
?
-
?
?
?
-
?
?
-
?
?
?
?
?
?
?
?
?
?
?
?
-
?
-
?
?
?
-
?
?
?
-
?
?
?
?
-
-
?
?
?
?
?
?
?
?
?
?
?
-
?
?
?
?
?
?
?
?
?
-
?
-
?
-
?
?
-
?
?
?
-
?
-
?
-
?
?
?
?
?
-
?
?
?
?
-
?
?
?
-
-
?
-
-
-
?
?
2
3
4
5
6
7
8
9
?
?
?
?
?
-
?
?
?
?
?
-
?
-
-
?
-
?
?
?
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Galanthamine
?
Table 4 Binding interactions of 1,5-benzothiazepines (1–27) with BChE
Compound
IC50 (lM)
H-bonding
p–p interactions
˚
Distance (A)
Score (%)
Amino acid
1
–
–
–
–
–
65
–
–
–
–
–
–
–
–
–
–
Arg 515
Arg 515
Lys 323
Arg 515
–
2
–
–
–
3
–
–
–
4
60.0
2.78
–
Tyr 420
5
–
–
–
–
–
–
–
–
–
–
6
–
–
–
7
–
–
–
8
–
–
Arg 515
Arg 515
Arg 515
9
–
–
10
11
12
13
–
–
–
–
142.1
–
–
Arg 515
–
123

Med Chem Res (2012) 21:2329–2339
2337
Table 4 continued
Compound
IC50 (lM)
H-bonding
p–p interactions
˚
Distance (A)
Score (%)
Amino acid
14
29.4
2.23
2.5
3
93
12
96
–
Lys 323
Tyr 420
–
–
15
16
17
76.9
–
Arg 515
–
–
–
4.7
2.6
2.7
3.0
–
81
41
23
–
Tyr 500
Lys 323
Lys 323
–
Arg 515
18
19
–
–
4.65
2.28
2.55
2.7
–
81
17
96
–
Lys 323
Tyr 420
Tyr 420
–
Arg 515
20
19.7
–
Arg 515
Arg 515
–
21
22
–
–
–
–
23
–
–
–
–
24
75.2
34.4
–
3.04
2.47
–
25
49
–
Lys 323
Gln 518
–
Arg 515
Arg 515
Arg 515
Arg 515
Arg 515
25
26
27
3.97
8.0
2.4
2.1
96
76
Tyr 420
Tyr240
Galanthamine
Fig. 5 Comparison of orientation pattern of unsubstituted (blue), 20-
hydroxy (white), and 30-hydroxy (green) substituted benzothiazepines
(Color figure online)
Fig. 4 3D docking pose of 27 (ball & stick) at BChE active site,
showing H-bonding (purple) with Tyr500 (Color figure online)
AChE and BChE. This could be explained on the basis of
the difference in the orientation and fitting in mechanism of
all the benzothiazepines in Set 2 compared with those in
Set 1 and 3. A careful glance at Fig. 5 reveals that the
orientation of ring A in these compounds does not allow
penetration of molecules deep into the gorge, thereby,
hindering their interactions with the amino acids in the
active site.
Based on these in silico studies, it was also possible to
predict that a benzothiazepine without a hydroxyl group on
ring A and with a thiophene moiety as ring B (shown in
Fig. 6 as white) could also be a strong candidate as BChE
123

2338
Med Chem Res (2012) 21:2329–2339
docking studies was to explore the possible docking mode
of active compounds within the active site of AChE and
BChE. The docking results of these compounds show that
the active compounds bind to AChE and BChE with the
orientation and position very close to that resulting from
crystallographic analysis in both ChEs with their actual
ligands DME and MES, respectively. The stabilizing
interactions between the protein and ligands are p–p inter-
actions, H-bonding and of hydrophobic nature, and the high
affinity of these compounds are the results of cumulative
effect of all these interactions. In our docking experiments,
a good correlation of IC50 values with binding interaction
was found, evidencing the significant role of Trp84 and
Arg515 in AChE and BChE enzymes, respectively.
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