Allylxanthone derivatives as xanthine oxidase inhibitors: Synthesis, biological evaluation and molecular docking study

Article abstract of DOI:10.13005/ojc/340104

Gout is one of the most severe health problems of the aged and is caused by high levels of uric acid in the blood. The inhibition of Xanthine oxidase (XO) is one strategy to retain gout disease. Oxygenated xanthones and derivatives have been shown many important biological activities. However, some xanthones have the small amount of nature and its sulfur analogs, thioxanthone has not been well studied in their bioactivity. A series of hydroxyxanthones and allylxanthones analogous 3-5 have been synthesized and screened for their anti-XO activity. Leads to the discovery of 2,4-diallyl-1,3-dihydroxythioxanthone (5b) as the most active inhibitor with IC₅₀ = 0.69±0.02 mM. Consequent molecular docking analysis by AutoDock 4.2 indicated that the most active compound (5b) inhibits XO by accommodated at the binding site of Xanthine oxidase.

Full text of DOI:10.13005/ojc/340104

ISSN: 0970-020 X
CODEN: OJCHEG
2017, Vol. 33, No.(6):
Pg. 38-44
ORIENTAL JOURNAL OF CHEMISTRY
An International Open Free Access, Peer Reviewed Research Journal
www.orientjchem.org
Allylxanthone Derivatives as Xanthine Oxidase Inhibitors:
Synthesis, Biological Evaluation and Molecular
Docking Study
THONGCHAIKHAMMEE1*, WARAKORNJONGSU1, MAYUSOKUNO2 and SUNITSUKSAMRARN2
1Department of Chemistry, Faculty of Science and Technology, Phranakhon Rajabhat University,
Bangkok, 10220, Thailand.
2Department of Chemistry, Faculty of Science, Srinakharinwirot University, Bangkok, 10110, Thailand.
*Corresponding author E-mail: thongchai.k@pnru.ac.th
http://dx.doi.org/10.13005/ojc/340104
(Received: November 17, 2017; Accepted: December 01, 2017)
ABSTRACT
Gout is one of the most severe health problems of the aged and is caused by high levels
of uric acid in the blood. The inhibition of Xanthine oxidase (XO) is one strategy to retain gout
disease. Oxygenated xanthones and derivatives have been shown many important biological
activities. However, some xanthones have the small amount of nature and its sulfur analogs,
thioxanthone has not been well studied in their bioactivity. A series of hydroxyxanthones and
allylxanthones analogous 3-5 have been synthesized and screened for their anti-XO activity.
Leads to the discovery of 2,4-diallyl-1,3-dihydroxythioxanthone (5b) as the most active inhibitor
with IC50 = 0.69 0.02 mM. Consequent molecular docking analysis by AutoDock 4.2 indicated that
the most active compound (5b) inhibits XO by accommodated at the binding site of
Xanthine oxidase.
Keywords: Xanthine oxidase inhibitors, xanthone, thioxanthone, molecular docking.
INTRODUCTION
inflammatory agents to relieve the symptoms of the
disease and XO inhibitors. XO inhibitors are utilized
to treat gout by blocked the synthesis of uric acid.
Among of XO inhibitors, allopurinol is a standard
drug to treat gout and acts as XO inhibitor. The use
of allopurinol is restricted by adverse side effects
including hypersensitivity reaction, renal toxicity2,
Steven-Johnson syndrome and epidermal
Xanthine oxidase (XO) is a vital enzyme
that catalyzes the oxidation of hypoxanthine to
xanthine and the oxidation of xanthine to uric acid
in the purine pathway and plays a significant role
in hyperuricemia and gout1. There are two
conventional treatments for gout that are used anti-
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39
KHAMMEE et al., Orient. J. Chem., Vol. 33(6), 38-44 (2017)
necrolysis3. Therefore, the search for new of salicylic acid (1, 1.38 g, 10 mmol), phloroglucinol
pharmacophores for XO inhibitor remains to (2, 1.26 g. 10.0 mmol) and heating at 80 °C for 30
continue. Phenolic-containing compounds are one minutes. The progression of the reaction was
of the largest classes widely found in natural monitored by TLC. After completion of the reaction,
sources and have been described to hold promise the mixture was poured into ice-water to give the
for human health benefits4. Hydroxyxanthones are orange solid. The obtained solid was collected by
a class of naturally occurring polyphenols that have filtration, and washed with distilled water, dried in
shown to possess a broad range of biological air. The orange residue was then purified by flash
activities5, including their XO inhibitory activity6. CC eluting with 10% acetone-hexane obtained a
However, its sulfur analogs, thioxanthone, have yellow solid of the desired products (3) in 71.0 %
never been reported. The bioactive thioxanthone7 yield10. 1,3-Dihydroxyxanthone (3), Yield: 71%; m.p.:
and some xanthones are found in a limited amount 254-255 °C, lit. 255-256 °C 10, Rf: 0.50 (30%
in nature. Our interest in the bioactive compound acetone-hexane), 1H-NMR [acetone-d6] δ: 6.25 (d, J
along with their safety concern8 prompted us to = 2.0 Hz, 1H, H-2), 6.41 (d, J = 2.0 Hz, 1H, H-4), 7.43
explore the new anti-XO substances. In this report, (ddd, J = 8.6, 8.1, 0.9 Hz, 1H, H-7), 7.50 (dd, J = 7.1,
we have synthesized a series of hydroxyxanthones 0.9 Hz, 1H, H-5), 7.81 (ddd, J = 8.6, 7.1, 1.6 Hz, 1H,
and hydroxythioxanthones and screened for their H-6), 8.16 (dd, J = 8.1, 1.6 Hz, 1H, H-8), 12.89 (s,
anti-XO activity. The compound with the highest 1H, 1-OH), HR-TOF-MS (ESI, +ve) m/z: 229.0492
activity was then selected for molecular docking [M+H]+ (calcd 229.0495 for C13H8O3S +H).
analysis to observe its preferred binding orientation
Preparation of 1,3-dihydroxythioxanthone (5)
to the enzyme target.
A mixture of MsOH (14.6 g, 10 mL, 0.1 mol)
and alumina (Al2O3 acidic type, 0.3 g, 2.9 mmol)
was added into thiosalicylic acid (4, 1.34 g, 0.09
mmol) and then phloroglucinol (2, 1.26 g, 10.0
mmol). The mixture was heated in an oil bath at 110
°C with stirring for 20 minute. The reaction progress
was monitored by TLC. After completion of the
reaction, the mixture was poured into water and
extracted with EtOAc (3 × 20 mL). The EtOAc layer
was washed with a saturated aq. NaHCO3 (50 mL).
The organic layer was dried over Na2SO4 and
evaporated in vacuo to give a red residue.
Chromatography of the crude product was
performed on a column of silica gel eluted with 5%
acetone-hexane to give a yellow solid of
desired compounds (5) in 65% yield11. 1,3-
dihydroxythioxanthone (5); m.p.: 228-230 °C, lit. 232–
233 °C11, Rf: 0.35 (30% acetone-hexane), 1H NMR
[Acetone-d6] δ: 6.36 (d, J = 2.2 Hz, 1H, H-2), 6.64 (d,
J = 2.2 Hz, 1H, H-4), 7.55 (ddd, J = 8.2, 6.8, 1.4 Hz,
1H, H-7), 7.68 (dd, J = 7.9, 1.4 Hz, 1H, H-5), 7.68
(ddd, J = 7.9, 1.4, 0.5 Hz, 1H, H-6), 8.50 (dd, J = 8.2,
0.5 Hz, 1H, H-8), 9.99 (br s, 1H, 3-OH), 14.40 (s, 1H,
1-OH), HR-TOF-MS (ESI, +ve) m/z: 245.0266 [M+H]+
(calcd 245.0266 for C13H8O3S +H).
MATERIAL AND METHODS
General methods
1H- and 13C-NMR spectra were recorded
on a Bruker Avance 300 FT-NMR spectrometer,
operating at 300 MHz (1H) and 75 MHz (13C). The
NMR chemical shifts were referenced to the residual
solvent peaks (δH 7.24 and δC 77.00 for CDCl3, δH
2.04 and δC 29.8 for acetone-δ6). ESI-HR-TOF-MS
analyses were conducted using a Bruker maXis™
4G mass spectrometer. All flash column
chromatography (CC) was performed on Merck
silica gel 60 (particle size of 230-400 mesh). All
reagents were purchased from commercial sources
and used without further purification. The reactions
progression were monitored by TLC which was
operated on precoated silica gel GF254 preparative
TLC plates. The spots on TLC were detected under
UV light and by spraying with anisaldehyde-H2SO4
reagent followed by heating. Melting points were
examined with Griffin melting point apparatus, and
are uncorrected.
Preparation of 1,3-Dihydroxyxanthone (3)
Methanesulfonic acid (36.5 g, 25 mL, 0.4
mol) was added in a round-bottom flask (250 mL). Preparation of 1,3-diallyloxyxanthone analogues
Phosphorus pentoxide (P2O5, 3.6 g, 25.4 mmol) was To stirred suspension of 1,3-
a
added in one portion which was dissolved in dihydroxyxanthone analogues (2 mmol), allyl
1-2 hour9.Then the solution was added to a mixture bromide (0.5 mL, 994 mg, 4.9 mmol) and K2CO3

40
KHAMMEE et al., Orient. J. Chem., Vol. 33(6), 38-44 (2017)
(750 mg 5.4 mmol) in dry DMF for 24 hours. The
reaction was monitored by TLC. After the reaction
was completed, the reaction was diluted in EtOAc
(25 mL) and washed with water (3 x 15 mL). The
organic layers were collected and dried over
Na2SO4. Evaporation afforded a yellow residue.
Next the residue was purified by flash column
chromatography (10% acetone-hexane) to give
desired products in 64-75 % yield.
2,4-diallyl-1,3-dihydroxyxanthone (3b),
pale yellow needle, m.p.: 136-138 °C, Rf: 0.50 (30%
1
acetone-hexane), H NMR [CDCl3] δ: 3.53 (d, J =
6.14 Hz, 2H, H-1′′), 3.62 (d, J = 6.10 Hz, 2H, H-1′),
4.97-5.40 (m, 4H, H-3′a), 4.97-5.40 (m, 4H, H-3′′a),
4.97-5.40 (m, 4H, H-3′′b), 4.97-5.40 (m, 4H, H-3 ),
5.89-6.08 (m, 2H, H-2′), 5.89-6.08 (m, 2H, H-2′b′),
6.10 (s, 1H, 3-OH), 7.35 (t, J = 7.55 Hz, 1H, H-7),
7.44 (d, J = 8.39 Hz, 1H, H-5), 7.70 (ddd, J = 8.39,
1.2 Hz, 1H, H-6), 8.24 (dd, J= 7.9, 1.2 Hz 1H, H-8), 13.25
(s, 1H, 1-OH), HR-TOF-MS (ESI, +ve) m/z : 309.1124
[M+H]+ (calcd 309.1121 for C19H16O4 +H).
1,3-diallyloxyxanthone (3a), pale yellow
needle, 74.6% yield, m.p.: 98-100 °C, Rf: 0.35 (30%
1
acetone-hexane), H NMR [CDCl3] δ : 4.59 (d, J =
2,4-diallyl-1,3-dihydroxythioxanthone
5.26 Hz, 2H, H-1′′), 4.65 (d, J = 5.26 Hz, 2H, H-1′),
5.35 (d, J = 5.84 Hz, 1H, H-3′b), 5.29-5.34 (m, 1H,
H-3′′b), 5.68 (d, J = 17.28 Hz, 1H, H-3′a), 5.43 (d, J =
17.24 Hz, 1H, H-3′′a), 5.95-6.19 (m, 2H, H-2′), 5.95-
6.19 (m, 2H, H-2′′), 6.34 (s, 1H, H-2), 6.46 (s, 1H, H-
4), 7.19-7.36 (m, 2H, H-5), 7.19-7.36 (m, 2H, H-7),
7.59 (t, J = 7.76 Hz, 1H, H-6), 8.25 (d, J = 7.92 Hz,
1H, H-8), HR-TOF-MS (ESI, +ve) m/z : 309.1197
[M+H]+ (calcd 309.1121 for C19H16O4+H).
(5b), pale yellow needle, m.p.: 98-100 °C, Rf: 0.45
(30% acetone-hexane), H NMR [CDCl3] δ: 3.57
1
(d, J = 5.9 Hz, 1H, H-1′), 4.62 (d, J =4. 7 Hz, 1H, H-
1′′), 5.08 (d, J = 12.0 Hz, 1H, H-3′′b), 5.16 (d, J =26.1
Hz, 1H, H-3′′a), 5.17 (d, J = 17.4 Hz, 1H, H-3′b),
5.22 (d, J = 25.7 Hz, 1H, H-3′a), 6.08 (s, 1H, 3-OH),
6.95 (m, 1H, H-2′), 6.95 (m, 1H, H-2′′), 7.43 (t, J =
8.0 Hz, 1H, H-7), 7.55 (m, 1H, H-5), 7.55 (m, 1H, H-
6), 8.53 (d, J = 8.0 Hz, 1H, H-8), 15.13 (s, 1H, 1-
OH), HR-TOF-MS (ESI, +ve) m/z : 325.0896 [M+H]+
(calcd. 325.0892 for C19H16O3S+H.
1,3-diallyloxythioxanthone (5a), yellow
solid, 64.3 % yield, m.p.: 60-61 °C, Rf: 0.55 (30%
acetone-hexane), H NMR [CDCl3] δ: 4.59 (d, J =
2-allyl-3-allyloxy-1-hydroxyxanthone (3c),
1
yellow solid, m.p.: 118-200 °C, Rf: 0.62 (30%
1
7.9 Hz, 1H, H-1′′), 4.68 (d, J = 4.8 Hz, 1H, H-1′),
5.35 (dd, J = 10.5, 1.5 Hz, 1H, 3′b), 5.35 (dd, J =
10.5, 1.3 Hz, 1H, H-3′′b), 5.64 (dd, J = 17.2, 1.5 Hz,
1H, 3′a), 5.64 (dd, J = 17.2, 1.3 Hz, 1H, H-3′′a), 6.09
(m, 1H, 2′), 6.09 (m, 1H, H-2′′), 6.46 (d, J = 2.3 Hz,
1H, H-2), 6.56 (d, J = 2.3 Hz, 1H, H-4), 7.42 (m, 1H,
H-5), 7.46 (t, 1H, H-6), 7.42 (m, 1H, H-7), 8.46 (d, J
=7.9Hz, 1H, H-8), HR-TOF-MS(ESI, +ve)m/z :325.0913
[M+H]+ (calcd 325.0892 for C19H16O3S +H).
acetone-hexane), H NMR [CDCl3] d: 3.45 (d, J =
6.22 Hz, 2H, H-1′) 4.64 (d, J = 4.90 Hz, 2H, H-1′′),
4.96 (dd, J = 10.0, 1.8 Hz, 1H, H-3′b), 4.96 (d, J =
10.0, 1.6 Hz, 1H, H-3′′b), 5.04 (dd, J = 17.1, 1.8 Hz,
1H, H-3′a), 5.44 (dd, J = 17.1, 1.6 Hz, 1H, H-3′′ ),
5.88-6.13 (m, 2H, H-2′), 5.88-6.13 (m, 2H, H-2′a′),
6.42 (s, 1H, H-4), 7.35 (t, J = 7.5 Hz, 1H, H-7), 7.41
(d, J = 7.5 Hz, 1H, H-5), 7.68 (ddd, J =7.5,1.2 Hz,
1H, H-6), 8.24 (dd, J = 7.9, 1.2 Hz, 1H, H-8), 12.95
(s, 1H, 1-OH), HR-TOF-MS (ESI, +ve) m/z: 309.1129
[M+H]+ (calcd. 309.1121 for C19H16O4+H).
Claisen rearrangement of 1,3-diallyloxyxanthone
analogues 3a and 5a
2-allyl-3-allyloxy-1-hydroxythioxanthone
1,3-diallyloxyxanthone analogues (5 mmol)
were dissolved in N,N-Dimethylformamide (DMF, 8 mL)
and refluxed for 12 h in an. sand-bath. The reaction
mixture was cooled at room temperature and
quenched with dil. HCl (10%, 15 mL). The crude
product was extracted with EtOAc (3 x 15 mL). The
organic solution was dried, evaporated and
chromatographed on silica gel by eluting with
hexane-acetone (15% acetone-hexane) to give
rearrangement products 3b, 5b, 3c and 5c in about
45-60% yield.
(5c), yellow solid, m.p.: 109-100 °C Rf: 0.50 (30%
acetone-hexane), H NMR [CDCl3] δ: 3.48 (d, J =
1
6.1 Hz, 1H, H-1′), 4.62 (d, J =4. 7 Hz, 1H, H-1′′),
4.96 (d, J = 9.5 Hz, 1H, H-3′b), 5.05 (d, J = 17.1 Hz,
1H, H-3′a), 5.31 (d, J = 10.5 Hz, 1H, H-3′′b), 5.44 (d,
J = 17.2 Hz, 1H, H-3′′a), 6.00 (m, 1H, H-2′), 6.00 (m,
1H, H-2′′), 6.51 (s, 1H, H-4), 7.45 (m, 1H, H-5), 7.45
(m, 1H, H-7), 7.56 (t, J =7.3 Hz, 1H, H-6), 8.44 (d, J
= 7.7 Hz, 1H, H-8), 14.15 (s, 1H, 1-OH), HR-TOF-
MS (ESI, +ve) m/z: 325.0894 [M+H]+ (calcd.
325.0892 for C19H16O3S+H).

41
natural ligand receptor on the active site. The
KHAMMEE et al., Orient. J. Chem., Vol. 33(6), 38-44 (2017)
In vitro xanthine oxidase inhibitory activity
The inhibitory activity of each compound docking validation method was evaluated based
was determined using a slight modification of the on the value root-mean-square deviation (RMSD)
reference methods12. The assay mixture consisted and declared valid if the value of RMSD smaller
of 10 mL of the tested compound (18.75, 37.5, 75.0, than 3.5 Å16. In this study, the RMSD was 0.88
150.0, 300.0 and 600.0 μg/mL), 180.0 mL of indicating this protocol and calculation parameter
phosphate buffer (pH 7.5) and 10 mL of 0.06 mM settings meet the criteria of validity docking methods.
xanthine solution (in phosphate buffer, pH 7.5). The
assay reaction was started by the addition of 10 mL
of xanthine oxidase enzyme solution (0.4 units/mL
in phosphate buffer, pH 7.5), which was prepared Chemistry
RESULTS AND DISCUSSION
immediately before use. The assay mixture was
1,3-Dihydroxyxanthone (3), was prepared
incubated at 37° C for 30 minute. The reaction was in 71% yield by heating a mixture of salicylic acid
stopped by adding 10 mL of 0.5 M HCl. For blank (1) and phloroglucinol (2) using a freshly prepared
assays, the protocol is similar but adding 0.5 M HCl mixture of methane sulfonic acid and P2O5 under
before enzyme. The absorbance at 295 nm was
O
9
OH
1
OH
A
8
9a
8a
COOH
7
2
recorded using a microplate reader (BioTek Synergy
HTX). The test was performed in triplicate and allopurinol
(5−100 μg/mL) was used as the standard.
i.
A
B
+
4a
6
10a
HO
OH
X
OH
3
OH
5
4
10
1
2
3
OH
O
OH
B
COOH
SH
ii.
+
Computational studies
S
OH
HO
OH
2
5
4
The three-dimensional structure of
Scheme 1 Reaction conditions: i. P2O5/ MsOH, 80
°C, 30-35 min; ii. MsOH, Al2O3, 110°C.
xanthine oxidase from bovine milk was obtained
from Protein Databank (www.pdb.org, PDB code
1FIQ) from 2.50 Å resolved crystal structure13. Before
molecular docking calculations, the crystallographic
water molecules in the structure were removed, and
the galantamine inhibitor was separated. The
ligands were docked into the salicylic acid binding
site of XO using AutoDock 4.214. AutoDock 4.2 in
Linux version was used because its algorithm allows
full flexibility of small ligands. The best conformers
were searched by the Lamarckian genetic algorithm
(LGA). During the docking process, a maximum of
100 conformers was considered. The population
size was set to 150. A maximum number of energy
evaluation was set to 2500000. A grid box size of
100 Å x 100 Å x 100 Å with a spacing of 0.375 Å
between the grid points and centered at 33.836 (x),
8.227 (y) and 30.991 (z) was used covering the
active site of the 1FIQ. The lowest energy cluster
returned by AutoDock for the compound was used
for further analysis. All other parameters were
maintained at their default settings. Before docking,
the ligands were optimized at the HF/3-21G level of
calculation using Gaussian 03 packages15. All
molecular graphics material was prepared using
the Discovery Studio 3.1 Client (ver. 3.1.1.11157,
Accelrys Software Inc., San Diego, CA, USA).
Docking method validation was done by redocking
3'
2'
3'
1'
O
OH
1
2'
8
O
OH
O
O
1
O
OH
1
1'
9a
4a
8
8
3'
8a
9a
4a
9a
4a
9
10
X
8a
8a
7
6
ii.
1'
2
2
9
i.
9
7
6
7
6
2'
2
1''
1''
+
3''
3''
10
10
X
10a
3
OH
4
1''
5
10a
3
X
OH
10a
3
O
X
O
5
2''
4
5
2''
4
2''
3;
5
X=O
; X=S
3b;
X=O
3c;
X=O
3a;
X=O
5b
; X=S
5c
; X=S
3''
5a
; X=S
Scheme 2 Reaction conditions: i. Allyl bromide,
K2CO3, DMF, RT, 24 h.; ii. DMF, reflux, 24 hours.
heating at 80 o C (Scheme 1A). In a likewise manner,
1,3-dihydroxythioxanthone (5) were obtained from
treating thiosalicylic acid (4) and phloroglucinol (2)
in alumina acidic type-methanesulfonic acid which
served as a catalyst and also as a coupling agent to
give the thioxanthone building block (Scheme 1B).
The synthesis of di-O-allyl xanthone derivatives 3a
and 5a were carried out by the interaction of allyl
bromide with corresponding hydroxyxanthone
derivatives in the presence of K2CO3. After the
completion of etherification, compound 3a and 5a
were subjected to Claisen rearrangement under
thermal conditions smoothly provided 3b-3c and
5b-5c. The structures of synthesis compounds were
confirmed mainly using NMR (1D- and 2D-NMR)
and HR-MS techniques. Spectroscopic data of 3-5
were in agreement with the structures and were
consistent with the literature values.

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KHAMMEE et al., Orient. J. Chem., Vol. 33(6), 38-44 (2017)
inhibitory activity and their IC50 values are shown in
Biological activity
The inhibition of xanthine oxidase results
Table.1. Among the compounds tested
hydroxylated xanthone series, 2,4-diallyl-1,3-
dihydroxythioxanthone (5b, IC50 = 0.69 0.02 mM)
was most active which was comparable to that of
in a decreased production of uric acid was
measured spectrophotometrically at 295 nm. All
synthesized compounds were evaluated for XO
Table. 1:The XO inhibitory activity and their IC50 values (µM) for xanthones derivatives
Structure
IC50 SD (mM)
Structure
IC50 SD (mM)
O
OH
O
OH
2.48 0.03
2.19 0.03
O
OH
S
OH
3
5
O
O
O
O
inactive
0.76 0.03
1.17 0.01
0.12 0.01
inactive
0.69 0.02
1.11 0.02
S
O
O
O
5a
3a
O
OH
O
OH
O
O
O
OH
3c
3b
O
OH
O
OH
S
O
S
OH
5c
5b
allopurinol
inhibitor and XO. To further investigate
the interaction mode of 2,4-diallyl-1,3-
dihydroxythioxanthone (5b) to XO (1FIQ). The
2,4-diallyl-1,3-dihydroxyxanthone (3b, IC50
=
0.76 0.03 mM). It was less active than the standard
gout drug allopurinol (6, IC50 = 0.12 0.01 mM). A
drop-in activity was noted for 3c and 5c with their
IC50 1.17 0.01 and 1.11 0.02 mM, respectively. The
parent compounds, 3 and 5, displayed less active
than their derivatives with IC50 2.48 0.03 mM and
2.19 0.03 mM, respectively. However, the loss of
inhibitory potency was observed in di-O-allyl
derivatives 3a and 5a. It should be noted that the
hydroxyl groups at C-1 and C-3 seemed crucial as
was evident from the activity of 3b and 5b. Based on
potent XO inhibitory activity of 5b, the compound 5b was
selected for further molecular modeling study.
Molecular modeling
Molecular docking study was performed
to comprehend the binding mode of synthesized
(a)

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KHAMMEE et al., Orient. J. Chem., Vol. 33(6), 38-44 (2017)
model displayed hydrogen bond interactions
occurred with amino acids residues. Six hydrogen
bond interactions of the highest active compound
with Glu 960, Phe1153, Pro1248, Asn1249 and
Lys1250 residues of the 1FIQ were observed. The
first hydrogen bond was observed between the 1-
OH of 5b and Glu960 residue (4.061 Å). The second
and the third hydrogen bond were found between
the C-1 hydroxyl group and the C-9 carbonyl group
of 5b and Phe1153 distance 3.658 and 3.184 Å,
respectively. The forth hydrogen bond were detected
between the C-3 hydroxyl group of 5b and Pro1248
(3.066 Å). Also, two H-bond interactions between
C-3 hydroxyl group and Asn1249 (3.066 Å) and
Lys1250 (3.092 Å) were detected. The allyl moieties
of 5b interacted with Lys1250 (3.150 Å) and
Lys1251 (3.214 Å) residues via hydrophobic
Fig. 1 (a) Molecular surface of XO with 2,4-diallyl-
1,3-dihydroxythioxanthone (5b).
(b) Selected molecular interaction of compound 5b
in the binding sites of XO.
conformation with the least binding energy, having
maximum cluster size at the active site of the interactions. Compound 5b formed O-S interaction
respective protein was considered for interaction
studies. As shown in Table 2 and Fig.1, 2,4-diallyl-
1,3-dihydroxythioxanthone (5b) showed binding
energy -9.50 kcal/mol. The predicted binding mode
of the highest active compound against the XO
with the residues Glu 960 (3.404 Å). Furthermore,
two O-C interactions were found with the residues
Asp1181 and Cys1247 distance 3.610 and 3.020
Å, respectively. All these interactions helped 5b to
hold in the binding site of the XO.
Table. 2:The interactions data for compound 5b docked into binding site of XO (1FIQ)
Ligand
Residue
Type of interaction
Distance
(Å)
Ligand interacting
moiety
5b (binding
energy -9.50
kcal/mol)
Glu 960
Tyr 1152
Phe 1153
Hydrogen bond
4.061
3.404
3.256
3.658
3.184
3.560
3.610
3.020
3.066
3.066
3.092
3.150
3.214
1-OH
S
O-S interaction
C-C interaction hydrophobic
Hydrogen bond
C-8
1-OH
C=O
C-8
Hydrogen bond
C-C interaction hydrophobic
O-C interaction
Asp 1181
Cys 1247
Pro 1248
Asn 1249
Lys 1250
3′b
3′b
O-C interaction
Hydrogen bond
3-OH
3-OH
3-OH
C-2′′b
C-2′′′b
Hydrogen bond
Hydrogen bond
C-C interaction hydrophobic
C-C interaction hydrophobic
Lys1251
CONCLUSIONS
and evaluated for the inhibition of XO
activity. Among the tested, 2,4-diallyl-1,3-
In this study, a series of hydroxyxanthones dihydroxythioxanthone (5b) had a highly inhibitory
and hydroxythioxanthones have been synthesized effect on XO with IC50 = 0.69 0.02 mM as compared

44
KHAMMEE et al., Orient. J. Chem., Vol. 33(6), 38-44 (2017)
to standard allopurinol (IC50 = 0.12 0.01 mM).
Molecular docking study was fulfilled to understand
the molecular interaction of compounds with the
active site of XO. This study has identified a new
series of lead compounds which can be used for
the development of new XO inhibitors for the
prevention and treatment of gout.
ACKNOWLEDGMENTS
The authors are grateful to Faculty of
Science and Technology, Phranakhon Rajabhat
University for providing necessary research facilities
and we would also like to thank Buker daltonic LLC
for providing ESI-HR-TOF-MS.
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