Mononuclear copper(i) complexes of triphenylphosphine and: N, N ′-disubstituted thioureas as potential DNA binding chemotherapeutics

Article abstract of DOI:10.1039/d0nj06182d

In this work, nine new mixed-ligand complexes with the general formula [CuBr(TPP)2Tu1-9] were synthesized. The copper(i) complexes of triphenylphosphine (TPP) and different N,N′-disubstituted thioureas (Tu) were characterized via spectroscopic techniques including Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (1H, 13C, and 31P NMR), and single-crystal X-ray diffraction (SC-XRD). The complexes were synthesized via the direct reaction of bromo(tris(triphenylphosphine)copper(i)) [BrCu(PPh3)3] precursor and thiourea ligand solution under ambient conditions. Complexes 1, 2 and 3 crystallized in a triclinic system with the P1 space group. Each complex is mononuclear, and the copper atom is tetrahedrally attached to two TPP groups through the phosphorous atom, one thiourea molecule through the sulfur atom and one bromine atom. The synthesized compounds were docked with a DNA macromolecule to predict their binding site and it was found that all molecules showed favorable binding to the DNA minor grooves. The DNA interaction studies of the representative complexes demonstrated their efficient DNA binding affinities. Based on the docking and DNA interaction results, complex 7 was found to be the best binder with a docking affinity of 382.2 kJ mol-1 and binding constant of 3.96 × 104 M-1. This compound tends to interact with the minor groove through the bromine atom positioning the side triphenylphosphine rings along the X-axis of the groove while keeping the 1-(2-chlorobenzyl)-3-(3-(trifluoromethyl)phenyl)thiourea ring on the outside.

Full text of DOI:10.1039/d0nj06182d

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Mononuclear copper(I) complexes of
triphenylphosphine and N,N0-disubstituted
thioureas as potential DNA binding
chemotherapeutics†
Cite this: DOI: 10.1039/d0nj06182d
Syed Ishtiaq Khan,*ab Sajjad Ahmad,cd Inayat Ali Khan,cd Amin Badshah,*a
a
Muhammad Khawar Rauf,a Jahangir Ali Putejo,
Muhammad Nasir Siddiq,b
f
Samia Kausare and Ataf Ali Altaf
*
In this work, nine new mixed-ligand complexes with the general formula [CuBr(TPP)2Tu1–9] were
synthesized. The copper(I) complexes of triphenylphosphine (TPP) and different N,N0-disubstituted
thioureas (Tu) were characterized via spectroscopic techniques including Fourier transform infrared
spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (1H, 13C, and 31P NMR), and single-
crystal X-ray diffraction (SC-XRD). The complexes were synthesized via the direct reaction of
bromo(tris(triphenylphosphine)copper(I)) [BrCu(PPh3)3] precursor and thiourea ligand solution under
%
ambient conditions. Complexes 1, 2 and 3 crystallized in a triclinic system with the P1 space group. Each
complex is mononuclear, and the copper atom is tetrahedrally attached to two TPP groups through the
phosphorous atom, one thiourea molecule through the sulfur atom and one bromine atom. The
synthesized compounds were docked with a DNA macromolecule to predict their binding site and it
was found that all molecules showed favorable binding to the DNA minor grooves. The DNA interaction
studies of the representative complexes demonstrated their efficient DNA binding affinities. Based on
the docking and DNA interaction results, complex 7 was found to be the best binder with a docking
Received 21st December 2020,
Accepted 12th April 2021
affinity of 382.2 kJ molꢀ1 and binding constant of 3.96 ꢁ 104
M
ꢀ1. This compound tends to interact with
the minor groove through the bromine atom positioning the side triphenylphosphine rings along the
X-axis of the groove while keeping the 1-(2-chlorobenzyl)-3-(3-(trifluoromethyl)phenyl)thiourea ring on
the outside.
DOI: 10.1039/d0nj06182d
rsc.li/njc
drug targets is an active research field in chemistry, molecular
biology, and medicine.2 In pharmacology, the significant
Introduction
Inorganic chemistry plays a vital role in the design of some feature is to understand the mechanism of the drug action,
therapeutic agents of medicinal importance, which are not which can be determined from the DNA drug interaction,
accessible by organic compounds.1 DNA interaction with inorganic playing a vital role in the design of efficient and targeted
drugs.3 The binding of metal ions with nucleic acids presents
a new promising methodology for cancer treatment, i.e., the
use of metal-based drugs.4–6
a Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan.
E-mail: aminbadshah@yahoo.com; Tel: +92 51 9064 21321
b Chemistry of Interfaces, Luleå University of Technology, SE-97187 Luleå, Sweden
c Department of Health and Biological Sciences, Abasyn University, Peshawar
25000, Pakistan
Copper is an essential trace metal and has been widely used
to disinfect liquids, solids, and human tissues for centuries
either as copper ions or in the form of complexes.7 Copper(I)
complexes have attracted considerable interest due to their
wide range of potential applications from solar energy devices,
luminescence materials, and supramolecular devices to biological
systems and catalysis owing to their unique structural features.8–10
Knowledge of the molecular structure is one of the fundamental
parameters for the manipulation of the chemical properties and
electronic configurations of the metal ion mainly influencing the
structure and stereochemistry of copper complexes. Copper (I) is a
d Geoscience Advance Research Laboratories, Geological Survey of Pakistan,
Islamabad, 45320, Pakistan. E-mail: sishtiaqk@gmail.com; Tel: +92 51 9255137
e Department of Chemistry, University of Gujrat, Hafiz Hayat Campus,
Gujrat 50700, Pakistan
f Department of Chemistry, University of Okara, Okara-56300, Pakistan.
E-mail: atafali_altaf@yahoo.com; Tel: +92 3325049532
† Electronic supplementary information (ESI) available. CCDC 2052822 is for
complex 1, 2052823 for complex 2 and 2075686 for complex 3. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0nj06182d
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soft acceptor center for soft ligands, having a d10 configuration and m = multiplet. The FT-IR absorption measurements were
and most probably a tetrahedral orientation with soft ligands. carried out on an Alpha Bruker Model 10018394 in the range of
Numerous studies have been reported on the synthesis and 4000–400 cmꢀ1 using the attenuated total reflection mode (ATR).
crystal structures of copper(I) halide complexes with TPP and A Fisons EA1108 CHNS analyzer was used for elemental analyses,
thiourea ligands.11–17 Thiourea ligands, which possess a thioamide and an atomic absorption spectrophotometer (AAS) PerkinElmer
moiety, have two coordination sites and can either coordinate A800 was used for the determination of the metal (copper)
through S or N due to their two tautomeric forms of thione contents.
(–NQC(–SH)) 2 thiol (–NH–C(QS)). TPP is a P donor ligand
A Bruker Smart APEX diffractometer was used for the single-
and is widely used in organic synthesis due to its nucleophilic crystal analysis of the synthesized thiourea phosphine copper(I)
character and reducing power. Herein, in this study, the thioureas bromide complexes. Suitable crystals, mostly yellow in color,
coordinated to Cu(I) in the thione form, which was confirmed by were analyzed on a Bruker Smart diffractometer fully equipped
the decrease in the stretching frequency of the CQS band due to with an APEX II CCD detector, an Incoatec IMuS source, and a
the coordination of S with Cu, while the other frequencies Quazar MX mirror. The distance between the crystal and
remained the same.18 Due to their significant relevance in detector was 4.0 cm, and the data was collected in a 512 ꢁ
biological systems, heterocyclic thiones have received considerable 512 pixel mode. For the determination of the initial unit cell
interest as ligands in metal complexes.19
parameters, a least-squares fit of the angular setting of strong
Thioureas are one of the most valuable classes of compounds reflections, collected by 1801 scan in 180 frames over three
with some broad spectrum anti-tumor activities and excellent different parts of the reciprocal space was used. Suitable
DNA binding affinities.20 Hence, the introduction of thiourea crystals of small sizes were selected and mounted on the
moieties in Cu metal complexes is expected to result in excellent Bruker Smart APEX diffractometer. During data collection, the
anti-cancer potential. In this study, we synthesized a series of temperature of the crystals was 100 K. Cu Ka radiation with a
Cu(I) complexes of thioureas having TPP and bromine in their wavelength of 1.54178 Å was used. By reduction of the data
coordination spheres. According to the SC-XRD analysis, the frames and using APEX2, the integrated intensity information
structures developed in a triclinic system with the P1 space for each reflection was obtained.22 Using Olex2, the structure
%
group and tetrahedral geometries. The complexes were screened was solved with the SHELXT23 structure solution program using
for DNA interaction via molecular docking, and among them, intrinsic phasing and refined with the XL (20) refinement
complex 7 was found to the best candidate with high docking package using full-matrix least-squares minimization on F2
scores for DNA grooves. It is worth mentioning that the Cu(I) and using all unique data.24,25 Atoms other than hydrogen were
complex with 1-(2-chlorobenzyl)-3-(3-(trifluoromethyl)phenyl) refined with anisotropic thermal parameters and the hydrogen
thiourea ligand has potential application in the chemotherapy atom positions were calculated and refined isotropically. The
of cancer cells.
additional symmetric absences and thermal voids were verified
with Platon.26 For the absorption corrections, SADABS was used.
Synthesis and characterization of [Cu(TPP)3]Br precursor.
The precursors were synthesized according to a modified
method reported in the literature.27 During the synthesis, an
acetonitrile solution of triphenylphosphine (TPP) was added to
a suspension of copper(I) bromide. The molar ratio was 1 : 3, as
shown in Scheme 1. The resulting mixture was stirred for at
least 1 h. A white crystalline solid was collected after the
reaction was completed on folded filter paper and washed with
ethanol several times. The white solid was dissolved in dichloro-
methane (DCM) and filtered to ensure the removal of the
insoluble salt. The resulting solution was kept for crystallization
and a transparent crystalline solid formed after 24 h in a very
high yield. The melting point was determined and compared
with the literature values. The melting point of the resulting
Experimental
Chemicals
The reagents, organic solvents, and chemicals were purchased
from Fluka, Sigma Aldrich, and E. Merck. CuBr, organic acids,
and triphenylphosphine (TPP) were used as received without
further purification. The organic solvents acetone, dichloro-
methane, chloroform, and alcohols were distilled, purified and
dried according to the reported methods,21 saturated with
nitrogen, stored over 4 Å molecular sieves, and degassed
before use.
Instrumentation
Melting points were determined using a Bio Cote SMP10-UK precursor was found to be in the range of 165–166 1C, which falls
melting point apparatus, and all values are uncorrected. NMR in the melting point range given in the literature (165 1C),
spectra were measured in deuterated solvents using a Bruker confirming the purity of the product.
AV-300 MHz spectrometer. 1H NMR (300 MHz): internal standard
The quantities of the reactants used for the synthesis of
solvent CDCl3 (7.28 ppm from TMS) (Si(CH3)4) and 13C NMR [Cu(TPP)3Br] were 0.143 g (1 mmol) of CuBr and 0.786 g
(75.47 MHz): internal standard solvent CDCl3 (77.0 ppm from (3 mmol) of the triphenylphosphine. Yield 71% M.P; 166 1C,
TMS). The 31P chemical shift was measured relative to 83% FT-IR, (ATR, n in cmꢀ1): 3047.5, (Aromatic C–H), 1584.8, 1479
orthophosphoric acid (H3PO4) in water at 121.49 MHz (operating (aromatic CQC), 1433.2, 1089, 740.6 (TPP), 1H NMR (300 MHz,
frequency). The splitting patterns of the proton resonance in the CDCl3, d in ppm, at 25 1C): 7.15–7.26 (m, 18H, Ar–H), 7.27–7.37
1H NMR spectra are defined as s = singlet, d = doublet, t = triplet, (m, 28H, Ar–H), 13C NMR (300 MHz, CDCl3, d in ppm, 25 1C):
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Scheme 1 General synthetic route for the synthesis [Cu(TPP)3Br], N,N0-disubstituted thiourea ligands (Tu1–9) and their Cu(I) complexes.
128.42–128.54 (d, due to splitting of C–P coupling), 129.51,
133.45–133.72 (d, due to splitting of C–P coupling), 133.84–
134.04 (d, due to splitting of C–P coupling), 31P NMR (300,
MHz,
d
in ppm at 25 1C): ꢀ4.07 (s). Anal. calcd for
C
55H45BrCuP3 (930.31): C, 69.72; H, 4.88; Cu, 6.83; Found, C,
68.65; H, 4.35; Cu, 6.05%.
Synthesis and characterization of N,N0-disubstituted thioureas.
The general synthetic route for the synthesis of ligands 1 to 9
is shown in Scheme 1, and their proposed chemical structures
are shown in Fig. 1. The ligands 1-(2-chlorobenzoyl)-3-phenyl-
thiourea (Tu1), 1-(3-chlorobenzoyl)-3-(3-chlorophenyl)thiourea
(Tu2), 1-(3-chlorobenzoyl)-3-(2,3-dichlorophenyl)thiourea (Tu3),
1-(3-chlorobenzoyl)-3-(2,6-dichlorophenyl)thiourea (Tu4), 1-(3-chloro-
benzoyl-3-(2,5-dibromophenyl)thiourea (Tu5)), 1-(3-chlorobenzoyl)-
3-cyclohexylthiourea (Tu6), 1-(2-chlorobenzoyl)-3-(3-fluorophenyl)
thiourea (Tu7), 1-(2-chlorobenzoyl)-3-(2,3-dichlorophenyl)thiourea (Tu8),
1-(benzoyl)-3-(2,3,4,6-tetrachlorophenyl)thiourea (Tu9) are shown in
Fig. 1, which were synthesized according to the method reported by
Rauf et al.24 The general synthetic route used for the complexes is
presented in Scheme 1.
Fig. 1 Chemical structure of the ligands (Tu1–9).
Synthesis and characterization of [CuBr(TPP)2Tu1–9] complexes.
In a 50 mL double-necked round-bottom flask, the appropriate
thiourea ligand (1 mmol) was dissolved in 20 mL of methanol
followed by the addition of [CuBr(TPP)3] (1 mmol) dissolved in yellow, and a solid precipitate was formed after few minutes. After
dichloromethane (DCM). The color of the mixed solution turned 1 h, the yellow solid was separated by filtration and washed twice
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with methanol. A small amount of product was dissolved in 25 1C): d 7.17–8.13 (m, 37H, Ar–H), 11.16 (s, 1H, NH proton), 12.24
dichloromethane, filtered, and left for natural solvent evaporation (s, 1H, NH proton), 13C NMR (75 MHz, CDCl3, d in ppm at 25 1C):
3
at 25 1C, during which prism-like light yellow crystals of complexes d 125.8, 126.9, 127.3, 128.0, 128.4–128.6 (d, J = 8.9 Hz, due to
1, 2, and 3 suitable for single crystal XRD analysis grew.
splitting of P–C coupling, Ar–C), 129.0, 129.3, 129.7, 129.9, 132.0,
Synthesis of bromo(1-(2-chlorobenzoyl)-3-phenylthiourea)bis 132.2, 132.7–133.1 (d, 1J = 31.5 Hz, due to splitting of P–C
(triphenylphosphine)copper(I) [CuBr(TPP)2Tu1] complex 1. For coupling, Ar–C), 133.6, 133.8–134.0 (d, 2J = 14.4 Hz, due to
the given reaction, the amount of reactants was 0.109 g splitting of P–C coupling, Ar–C), 134.8, 136.0, 167.6 (CQO),
(0.376 mmol) of the ligand (1) and 0.35 g (0.376 mmol) of 179.9 (CQS), 31P NMR (121.49 MHz, 83% aqueous H3PO4,
bromotris(triphenylphosphine)copper(I). The product was a d in ppm at 25 1C): d ꢀ3.66, anal. calcd for, C50H39CuBrCl3N2OP2S
yellow solid and the yield was 0.263 g (73%), M.P = 194– (1027.68): C, 58.44; H, 3.83; N, 2.73; S, 3.12; Cu, 6.18; found, C,
196 1C, FT-IR (ATR, n in cmꢀ1): 3195, 3050, 2980, 1672, 1588, 58.21; H, 3.59; N, 2.39; S, 2.93; Cu, 5.96%.
1522, 1663, 1477, 1431, 1328, 1278, 1249, 1154, 1091, 1046, 1026,
Bromo(1-(3-chlorobenzoyl)-3-(2,6-dichlorophenyl)thiourea)bis
997, 849, 740, 690, 652, 607, 504, 424, 1H NMR (300 MHz, CDCl3, (triphenylphosphine) copper(I) (B20) [CuBr(TPP)2Tu4] complex 4.
d in ppm at 25 1C): d 7.17–7.50 (m, 37H, Ar–H), 7.70–7.73 (d, 1J = For the given reaction, the amount of the reactants was 0.135 g
6.96 Hz, 2H, Ar–H), 11.15 (s, NH proton), 12.44 (s, NH proton), (0.376 mmol) of the ligand (4) and 0.35 g (0.376 mmol) of
13C NMR (75 MHz, CDCl3, d in ppm at 25 1C): d 124.9, 127.0, bromotris(triphenylphosphine)copper(I). The product was a yel-
127.2, 128.3–128.4 (d, 3J = 8.9 Hz, due to splitting of P–C low solid and the yield was 0.274 g (71%), M.P = 198–200 1C, FT-IR
coupling, Ar–C), 128.8, 129.5, 130.7, 131.9, 132.1, 132.9, 133.2– (ATR, n in cmꢀ1): 3180, 3051, 2974, 1672.7, 1521.3, 1477.6, 1432.9,
1
133.6 (d, J = 26.0 Hz, due to splitting of P–C coupling, Ar–C), 1325.6, 1253.2, 1160.7, 1091.9, 1063.9, 1026.3, 997.8, 873.5, 781.5,
2
133.9–134.1 (d, J = 14.7 Hz, due to splitting of P–C coupling, 740.9, 691.8, 569.2, 503, 426.6, 1H NMR (300 MHz, CDCl3,
Ar–C), 134.6, 137.0, 168.0 (CQO), 178.2 (CQS), 31P NMR d in ppm at 25 1C): d 7.12–7.46 (m, 34H, Ar–H), 7.50–7.59
1
(121.50 MHz, 83% aqueous H3PO4, d in ppm at 25 1C): (s, 1H, Ar–H), 8.03–8.05 (d, J = 7.5 Hz, 2H, Ar–H), 11.48 (s, 1H,
d ꢀ4.08, anal. calcd for, C50H41CuBrClN2OP2S (958.79): C, NH proton), 12.29 (s, 1H, NH proton), 13C NMR (75 MHz, CDCl3, d
62.63; H, 4.31; N, 2.92; S, 3.34; Cu, 6.63; found, C, 63.32; H, in ppm at 25 1C): d 127.5, 128.3–128.4 (d, 3J = 8.9 Hz due to
4.17; N, 2.66; S, 3.19; Cu, 6.40%.
splitting of P–C coupling, Ar–C), 128.5, 129.4, 129.4, 129.6, 129.7,
1
Bromo(1-(3-chlorobenzoyl)-3-(3-chlorophenyl)thiourea)bis 132.8, 132.9, 133.3–133.6 (d, J = 30.0 Hz, due to splitting of P–C
(triphenylphosphine) copper(I) [CuBr(TPP)2Tu2] complex 2. For coupling, Ar–C), 133.6, 133.9–134.1 (d, 2J = 14.5 Hz, due to
the given reaction, the amount of reactants was 0.122 g splitting of P–C coupling, Ar–C), 134.4, 134.7, 168.0 (CQO),
(0.376 mmol) of the ligand (2) and 0.35 g (0.376 mmol) of 180.7 (CQS), 31P NMR (121.50 MHz, 83% aqueous H3PO4,
bromotris(triphenylphosphine)copper(I). The product was a d in ppm at 25 1C): d ꢀ4.32, anal. calcd for, C50H39CuBrCl3N2OP2S
yellow solid and the yield was 0.265 g (71%) having (M.P) (1027.68): C, 58.44; H, 3.83; N, 2.73; S, 3.12; Cu, 6.18; found, C,
186–187.4 1C, FT-IR (ATR, n in cmꢀ1): 3137, 3046, 2961, 1671, 58.18; H, 3.52; N, 2.45; S, 2.98; Cu, 5.98%.
1573, 1521, 1479, 1433, 1338, 1306, 1261, 1163, 1092, 1070,
Bromo(1-(3-chlorobenzoyl)-3-(2,4-dibromophenyl)thiourea)bis
1
1026, 998, 859, 803, 777, 740, 691, 606, 543, 504, 425; H NMR (triphenylphosphine) copper(I) [CuBr(TPP)2Tu5] complex 5. For
(300 MHz, CDCl3, d in ppm at 25 1C): d 7.16–7.72 (m, 36 H, the given reaction, the amount of the reactants was 0.168 g
Ar–H), 8.05–8.16 (m, 2H, Ar–H), 10.61 (s, 1H, NH proton), 11.21 (0.376 mmol) of the ligand (5) and 0.35 g (0.376 mmol) of
(s, 1H, NH proton), 13C NMR (75 MHz, CDCl3, d in ppm, at bromotris(triphenylphosphine)copper(I). The product was
a
3
25 1C): d 123.1, 125.0, 127.2, 127.4, 128.4–128.5 (d, J = 8.9 Hz, yellow solid and the yield was 0.305 g (70%), M.P = 178–180 1C,
due to splitting of P–C coupling, Ar–C), 128.9, 129.2, 129.5, FT-IR (ATR, n in cmꢀ1): 3241, 3052, 2973, 1673, 1526, 1475, 1433,
129.8, 129.9, 132.8, 133.3–133.6 (d, 1J = 24.0 Hz, due to splitting 1326, 1250, 1166, 1092, 1062, 1024, 996, 871, 780, 744, 690, 570,
of P–C coupling, Ar–C), 133.8–134.0 (d, 2J = 14.7 Hz, due to 504, 425, H NMR (300 MHz, CDCl3, d in ppm at 25 1C): d 7.16–
1
1
splitting of P–C coupling, Ar–C), 134.3, 134.8, 138.1, 167.9 7.72 (m, 34H, Ar–H), d 7.8–7.8 (d, J = 8.2 Hz, 2H, Ar–H), 8.05 (s,
(CQO), 178.6 (CQS), 31P NMR (121.50 MHz, 83% aqueous 1H, Ar–H), 11.81 (s, NH proton), 12.74 (s, NH proton), 13C NMR
H3PO4, d in ppm at 25 1C): d ꢀ3.95, anal. calcd for, (75 MHz, CDCl3, d in ppm at 25 1C): 118.1, 124.6, 127.4, 128.7,
C
50H40CuBrCl2N2OP2S (993.23): C, 60.46; H, 4.06; N, 2.82; S, 128.7–128.8 (d, 3J = 9.0 Hz, due to splitting of P–C coupling, Ar–C),
3.23; Cu, 6.40; found, C, 60.21; H, 3.89; N, 2.59; S, 2.96; 129.3, 129.4, 131.2, 131.5, 131.5, 131.6–131.9 (d, 1J = 21.8 Hz, due
Cu, 6.11%. to splitting of P–C coupling, Ar–C), 132.0, 132.3, 133.4, 133.7–
2
Bromo(1-(3-chlorobenzoyl)-3-(2,3-dichlorophenyl)thiourea)bis 133.9 (d, J = 14.3 Hz, due to splitting of P–C coupling, Ar–C),
(triphenylphosphine) copper(I) [CuBr(TPP)2Tu3] complex 3. For 137.6, 166.7 (CQO), 179.7 (CQS), 31P NMR (121.49 MHz, 83%
the given reaction, the amount of the reactants was 0.135 g aqueous H3PO4, d in ppm at 25 1C): d ꢀ3.15, anal. calcd for,
(0.376 mmol) of the ligand (3) and 0.35 g (0.376 mmol) of C50H39CuBr3N2OP2S (1116.03): C, 53.78; H, 3.52; N, 2.51; S, 2.87;
bromotris(triphenylphosphine)copper(I). The product was
a
Cu, 5.69; Found, C, 53.41; H, 3.29; N, 2.31; S, 2.57; Cu, 5.38%.
yellow solid and the yield was 0.282 g (73%), M.P = 135–
Bromo(1-(3-chlorobenzoyl)-3-cyclohexylthiourea)bis(triphenyl-
137.6 1C, FT-IR (ATR, n in cmꢀ1): 3116, 3052, 2981, 1672, 1573, phosphine)copper(I) [CuBr(TPP)2Tu6] complex 6. For the
1520, 1479, 1433, 1321, 1250, 1160, 1107, 1091, 1026, 996, 971, given reaction, the amount of the reactants was 0.111 g
924, 848, 742, 720, 691, 616, 536; 1H NMR (300 MHz, CDCl3, (0.376 mmol) of the ligand (6) and 0.35 g (0.376 mmol) of
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bromotris(triphenylphosphine)copper(I). The product was a yel- coupling, Ar–C), 136.6, 169.3 (CQO), 179.4 (CQS), 31P NMR
low solid and the yield was 0.254 g (71%) having (M.P) 200– (121.50 MHz, 83% aqueous H3PO4, d in ppm at 25 1C): d ꢀ3.65,
203.5 1C, FT-IR (ATR, n in cmꢀ1): 3210, 3050, 2923, 1675, 1570, anal. calcd for, C50H39CuBrCl3N2OP2S (1027.68): C, 58.44; H, 3.83;
1531, 1478, 1433, 1318, 1283, 1245, 1168, 1119, 1091, 1026, 997, N, 2.73; S, 3.12; Cu, 6.18; found, C, 58.15; H, 3.54; N, 2.49; S, 2.88;
1
874, 786, 745, 642, 574, 506, 493, 419, 394, H NMR (300 MHz, Cu, 5.90%.
CDCl3, d in ppm at 25 1C): d 0.94 (s, 2H, cyclohexyl), 1.15 (s, 2H,
Bromo(1-benzoyl-3-(2,3,5,6-tetrachlorophenyl)thiourea)bis
cyclohexyl), 1.68–1.71 (m, 5H, cyclohexyl), 3.39 (m, 1H, cyclo- (triphenylphosphine)copper(I) [CuBr(TPP)2Tu9] complex 9. For
hexyl), 7.12–7.53 (m, 31H, Ar–H), 7.70–7.73 (d, 1J = 7.1 Hz, 1H, Ar– the given reaction, the amount of the reactants was 0.084 g
1
H), 7.81–7.88 (d, J = 8.3 Hz, 2H, Ar–H), 11.06 (s, NH proton), (0.21 mmol) of the ligand (9) and 0.20 g (0.21 mmol) of
12.64 (s, NH proton), 13C NMR (75 MHz, CDCl3, d in ppm at bromotris(triphenylphosphine)copper(I). The product was yel-
25 1C): d 25.3, 25.8, 32.7, 58.7, 125.3, 126.8, 126.9, 127.1, 128.4– low a solid and the yield was 0.153 g (67%), M.P = 210–212 1C,
128.5 (d, 3J = 8.9 Hz, due to splitting of P–C coupling, Ar–C), FT-IR (ATR, n in cmꢀ1): 3198, 3052, 1641, 1516, 1433, 1407,
129.6, 130.8, 133.0–133.4 (d, 1J = 26.7 Hz, due to splitting of P–C 1306, 1257, 1233, 1158, 1093, 1069, 1026, 998, 917, 863, 741,
coupling, Ar–C), 133.8–134.0 (d, 1J = 14.5 Hz, due to splitting of P– 692, 604, 505, 435, 1H NMR (300 MHz, CDCl3, at 25 1C): d 7.13–
C coupling, Ar–C), 136.2, 167.6 (CQO), 178.7 (CQS), 31P NMR 7.67 (m, 34H, Ar–H), 8.16–8.19 (d, 1J = 7.6 Hz, 2H, Ar–H), 11.29
(121.50 MHz, 83% aqueous H3PO4, d in ppm at 25 1C): d ꢀ3.94, (s, 1H, NH proton), 12.51 (s, 1H, NH proton), 13C NMR (75 MHz,
anal. calcd for, C50H47CuBrClN2OP2S (964.84): C, 62.24; H, 4.91; N, CDCl3, d in ppm, at 25 1C): d 128.3–128.4 (d, 3J = 8.8 Hz, due to
2.90; S, 3.32; Cu, 6.59; found, C, 61.86; H, 4.57; N, 2.58; S, 3.17; splitting of P–C coupling, Ar–C), 128.6, 129.3, 129.5, 130.5,
Cu, 6.27%.
130.8, 132.0, 132.1, 133.2–133.5 (d, 1J = 26.3 Hz, due to splitting
Bromo(1-(2-chlorobenzoyl)-3-(3-(trifluoromethyl)phenyl)thiourea) of P–C coupling, Ar–C), 133.9–134.1 (d, 2J = 14.4 Hz, due to
bis(triphenylphosphine)copper(I) [CuBr(TPP)2Tu7] complex 7. For splitting of P–C coupling, Ar–C), 134.6, 135.6, 169.2 (CQO),
the given reaction, the amount of the reactants was 0.122 g 180.7 (CQS), 31P NMR (121.50 MHz, 83% aqueous H3PO4,
(0.376 mmol) of the ligand (7) and 0.35 g (0.376 mmol) of d in ppm at 25 1C): d ꢀ4.15, anal. calcd for, C50H38CuBrCl4N2OP2S
bromotris(triphenylphosphine)copper(I). The product was a yellow (1062.12): C, 56.54; H, 3.61; N, 2.64; S, 3.02; Cu, 5.98; found, C, 56.21;
solid and the yield was 0.266 g (69%), M.P = 194–195.8 1C, FT-IR, H, 3.39; N, 2.48; S, 2.851; Cu, 5.59%.
(ATR, n in cmꢀ1): 3134, 3041, 1674, 1570, 1534, 1478, 1431, 1326,
1271, 1248, 1176, 1092, 1073, 1025, 990, 878, 782, 744, 682, 619, 506,
Molecular docking
491, 425, 1H NMR (300 MHz, CDCl3, d in ppm at 25 1C): d 7.23–7.86 Rigid molecular docking studies of the synthesized compounds
(m, 36H, Ar–H), 8.07–8.11 (d, 1J = 9.01 Hz, 2H, Ar–H), 11.38 (s, 1H, (complexes 1 to 9) with the crystal structure of B-DNA dodecamer
NH proton), 12.94 (s, 1H, NH proton), 13C NMR (75 MHz, CDCl3, (CGCGAATTCGCG)2 (PDB ID 1BNA),28 a breast cancer drug
3
d in ppm at 25 1C): d 121.3, 124.4–124.5 (d, J = 9.8 Hz, due to target, 17 beta-hydroxysteroid dehydrogenase type 1 (17beta-
splitting of P–C coupling, Ar–C), 124.1, 128.4–128.8 (t, 1J = 30.8 Hz, HSD1) (PDB ID 3hb5),28 human serum albumin (HSA) (PDB ID
due to splitting of F–C coupling), 128.9, 129.2, 129.6, 129.8, 130.5, 1AO6)29 and human glutathione S-transferase M1A-1A (GST)
1
131.4, 131.8, 132.2–132.5 (d, J = 21.0 Hz, due to splitting of P–C (PDB ID 1XW6)30 were performed to predict the binding affinity
coupling, Ar–C), 132.7, 133.3, 133.5, 134.4–134.6 (d, 2J = 15.0 Hz, due of the compounds associated with the best and preferred
to splitting of P–C coupling), 138.3, 169.6 (CQO), 178.7 (CQS), 31
P
binding conformation with respect to the receptor. The HEX
NMR (121.50 MHz, 83% aqueous H3PO4, d in ppm): d ꢀ4.79, 19F 8.0.0 software31 was used, which is an interactive molecular
NMR (282.23 MHz, CFCl3, d in ppm at 25 1C): d ꢀ62.27, anal. calcd graphics software often employed for calculating the binding
for, C51H40CuBrClF3N2OP2S (1026.79): C, 59.66; H, 3.93; N, 2.73; S, conformation of pairs of DNA and protein molecules and
3.12; Cu, 6.19; found, C, 59.29; H, 3.49; N, 2.48; S, 2.93; Cu, 5.86%. protein–ligand docking. The complexes were drawn in
Bromo(1-(2-chlorobenzoyl)-3-(2,3-dichlorophenyl)thiourea)bis ChemDraw Ultra 12.032 and converted to PDB format using the
(triphenylphosphine)copper(I) [CuBr(TPP)2Tu8] complex 8. For the OPENBABEL software.33 For each compound, 2000 poses were
given reaction, the amount of the reactants was 0.135 g generated and ranked based on the docking binding energy in
(0.376 mmol) of the ligand (8) and 0.20 g (0.376 mmol) of kJ molꢀ1. A negative docking score means the greater propensity
bromotris(triphenylphosphine)copper(I). The product a was yel- of the compound for the receptor macromolecule.34 The output
low solid and the yield was 0.269 g (70%), M.P = 184–186 1C, FT-IR complexes were visualized for binding poses and interaction
(ATR, n in cmꢀ1): 3189, 3049, 2928, 1671.8, 1572.7, 1530.6, 1491.3, through UCSF Chimera.35
1434.5, 1319, 1283.5, 1251.6, 1168, 1093.2, 1070.2, 1026.8, 996.8,
877.5, 787.3, 746.8, 681.2, 611.7, 506.2, 421.9, 1H NMR (300 MHz,
DNA binding studies
CDCl3, d in ppm at 25 1C): d 7.15–7.98 (m, 37H, Ar–H), 8.08–8.11 UV-vis spectroscopy was used for DNA interaction studies. The
(d, 1J = 8.7 Hz, 2H, Ar–H), 11.74 (s, NH proton), 12.31 (s, NH drug-DNA interaction study of the synthesized compounds was
proton), 13C NMR (75 MHz, CDCl3, d in ppm at 25 1C): d 123.0, carried out by preparing a stock solution of 10ꢀ4 M. Absorption
123.3, 126.5, 128.3, 128.5–128.6 (d, 3J = 9.0 Hz, due to splitting of spectra helped to explore the binding strength and mode of
P–C coupling, Ar–C), 129.2, 129.5, 130.6, 131.1, 131.2, 132.3–132.6 some of the representative complexes (Table 3) by observing the
1
(d, J = 27.0 Hz, due to splitting of P–C coupling, Ar–C), 132.7, change in the absorption peak of the compounds in both the
2
132.8, 133.5, 133.6–133.8 (d, J = 14.2 Hz, due to splitting of P–C presence and absence of DNA solution.
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Results and discussion
Spectroscopic characterization
in its thiourea ligand, and thus 19F NMR was performed for this
complex, exhibiting a singlet at d = ꢀ62.27 ppm for F.
The significant FT-IR bands of all the synthesized complexes
are presented in the Experimental section. The representative
bands of the complexes are located at (nmax/cmꢀ1) 3150–3350
for N–H, 2910–3050 for C–H in Ph, 1660–1685 for CQO, 1475–
1600 for CQC in Ph, 1433 and 1477 for C–P, 1320 and 1327 for
C–F, 1220–1270 for CQS, 1146 for C–O, 1035–1090 for C–N,
690–695 for C–Cl, 320–337 for Cu–S and 263–297 for Cu–X. The
FT-IR results clearly show that the addition of secondary
amines to the CQN bonds of the isothiocyanates resulted in
the formation of thiourea.24 The band shifting of the NH-group
Structural characterization
Recrystallization of the thiourea copper(I) complexes was car-
ried out in either chloroform or dichloromethane, and pure
suitable crystals were collected for solid-state X-ray studies after
24 h. The ORTEP structures of complexes 1, 2, and 3 are
presented in Fig. 2–4, respectively, the crystal data and refinement
parameters are presented in Table 1, specific bond-lengths,
selected bond angles and important torsion angles are presented
in Table S1 (ESI†), and the hydrogen bond lengths are presented
in Table S2 (ESI†).
(normal nmax/cmꢀ1: 3400) and carbonyl (CQO normal nmax/cmꢀ1
:
The X-ray analysis of complex 1 (Fig. 2) shows that the
triphenylphosphine, thiourea, and bromide ligands are coordinated
to copper(I) in a distorted tetrahedral way. The space group of the
1750) to lower wavelengths is attributed to the hydrogen bonding
between the H atom of one NH-group and an O atom of a
carbonyl on one side and between the H atom of another NH-
group and halogen attached to Cu on another side, as can be seen
in the single-crystal structures and will be discussed later. Accord-
ing to the FTIR analysis, the SH bands at about 2500–2600 cmꢀ1
are not observed in the spectra of all the complexes, confirming
the non-tautomeric orientation of the ligands.4,36,37 A comparison
of the FTIR bands of the complexes and ligands is provided in the
ESI† (Table S1). A comparison of the FT-IR data the free ligands38
and that for the complexes revealed that the binding of the
ligands to the metal did not modify the NH, CN, and CO
stretching frequencies probably due to the presence of hydrogen
bonding. All the FT-IR spectra of the complexes exhibit the
characteristic stretching and bending bands of the phosphines
%
complex is P1 and Z = 2, while the crystal system is triclinic
(Table 1). The bond length between Cu and S (Cu–S) is 2.395(4),
2.3956(6) and 2.4137(5) Å for complexes 1, 2 and 3, respectively.
The bond length for the Cu–P bond is in the range of 2.274(4)
to 2.276(4) Å for complex 1, 2.2645(6) to 2.2717(5) Å for
complex 2 and 2.4137(5) Å for complex 3. The Br–Cu bond length
is 2.459(2) Å for complex 1, 2.4567(4) Å for complex 2 and
2.4566(3) Å for complex 3 (Table 1). The average value of the
bond angles of the Br–Cu–S, P–Cu–Br, P–Cu–S, and P–Cu–P
bonds is close to a perfect tetrahedral geometry (109.5 Å). The
torsion angles show that the thiocarbonyl and the carbonyl
groups of the thiourea ligand are coplanar to each other. The
structural analysis further confirmed that there are two types of
intramolecular hydrogen-bonding. The first is between the O atom
of CQO and the H atom of the –NH group (O1ꢂꢂꢂHN2), and the
second is between the Br atom and the H atom of the –NH group
(Br1ꢂꢂꢂHN1), presenting a trans configuration of the thiourea
N protons for all the complexes (1–3) (Fig. 2–4, respectively).
at 1433 cmꢀ1, 1477 cmꢀ1, 1092 cmꢀ1, and 741–747 cmꢀ1
respectively.
,
NMR spectroscopy was conducted for the structural elucidation
1
of the synthesized copper(I) complexes. The H NMR data of the
complexes demonstrates signals for the aromatic protons in the
range of d = 7.14–8.20 ppm.39,40 In most cases, there are doublets
for the protons at the ortho positions having coupling constants in
the range of 6.0 to 9.6 Hz. The other signals at 10 and 12 ppm are
assigned to the N–H protons of CONH and CSNH, respectively.
1
These two separate signals for the NH protons in the H NMR
spectra confirm that none of the N atoms is bonded to copper. All
the 13C NMR signals for the aromatic rings of the synthesized
complexes are consistent with the reported values.41,42 In the 13C
NMR spectra, there are doublets for the ipso, ortho, and meta
carbon atoms of the phosphine groups due to 31P–13C coupling.
The ipso carbon atoms show higher coupling constants than the
1
2
meta carbon atoms in the range of J = 21.0–38.3 (for ipso), J =
3
12.8–15.8 Hz (ortho) and J = 7.5–10.5 Hz (for meta). The signals
for the CONH carbon atoms appeared in the range of d = 160–
168 ppm, while the signals for the CSNH carbons appeared in the
range of 173–183 ppm. In the 13C NMR spectrum of complex 7,
having fluorine in the thiourea ligands gives a triplet due to
19F–13C coupling with a coupling constant of about 30.8 Hz. In
the 31P NMR spectra, all the complexes having phosphine groups
give singlets in the range of d = ꢀ5.2 to ꢀ1.4 ppm with a shift
from the simple phosphines, indicating that phosphorous is
coordinated to the copper metal. Complex 7 possesses fluorine Fig. 2 Molecular structure of complex 1 with atomic labelling scheme.
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Table 1 Crystal structure refinement parameters and data for complexes 1–3
Crystal parameter
Complex 1
Complex 2
Complex 3
Chemical formula
Crystal colour
Fw; F (000)
C50H41BrClCuN2OP2S
Yellow
958.75; 980
100
C50H40BrCl2CuN2OP2S
Fluorescent light yellow
993.19; 1012
100
C50H39BrCl3CuN2OP2S,1(CHCl3)
Yellow
1147.00; 1160
100
T (K)
Radiation Cu Ka wavelength (Å) 1.54178
1.54178
Triclinic
%
P1
1.54178
Triclinic
P1
11.2132(2)
13.2839(2)
17.6392(2)
86.14
76.5080(10)
75.5120(10)
2
2473.58(7)
1.540
5.878
0.2 ꢁ 0.14 ꢁ 0.12
2.576–72.118; 0.979
5.152 to 144.236
16 722
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Triclinic
P1
%
%
10.643(4)
12.582(4)
17.869(6)
92.527(6)
92.308(6)
112.931(5)
2
2197.4(13)
1.449
3.769
0.08 ꢁ 0.08 ꢁ 0.06
2.480–37.394; 0.930
4.96 to 74.788
4155
11.1672(2)
12.7045(2)
17.4524(3)
79.4170(10)
79.5080(10)
67.4310(10)
2
2230.47(7)
1.479
4.273
0.42 ꢁ 0.24 ꢁ 0.18
2.595–72.111; 0.974
5.19 to 144.222
123 141
a (deg)
b (deg)
g (deg)
Z
V (Å3)
rcalcd (g cmꢀ3
)
m (mmꢀ1
)
Crystal size/mm3
y range (deg); completeness
2y range for data collection/1
Collected reflections; Rs
Unique reflections; Rint
Independent reflections
Data/restraints/parameters
Index ranges
4155; 0.0577
123 141; 0.0282
16 722; 0.0079
2094 [Rint = 0.0577, RSigma = 0.1003] 8513 [Rint = 0.0282, RSigma = 0.0099] 9490 [Rint = 0.0079, RSigma = 0.0101]
2094/0/280
8513/405/628
9490/258/654
ꢀ8 r h r 8, ꢀ9 r k r 9,
ꢀ12 r h r 13, ꢀ15 r k r 15,
ꢀ13 r h r 11, ꢀ16 r k r 16,
ꢀ14 r l r 13
ꢀ21 r l r 21
ꢀ21 rl r 21
Final R indexes [I Z 2s (i)]
Final R indexes [all data]
Goodness of fit on F2
R1 = 0.0551, wR2 = 0.1293
R1 = 0.0809, wR2 = 0.1477
0.988
R1 = 0.0344, wR2 = 0.0933
R1 = 0.0345, wR2 = 0.0934
1.071
R1 = 0.0298, wR2 = 0.0805
R1 = 0.0300, wR2 = 0.0806
1.028
Largest diff peak and hole/e Åꢀ3 0.33/ꢀ0.43
1.12/ꢀ0.96
1.00/ꢀ0.77
The crystal structure of complex 2 shows that there is there is disorder in the thiourea ligand of the molecule
disorder in the benzoyl part of the thiourea ligand and the starting at N1.
atoms have the occupancies of Cl1A (0.781(2)), Cl1B (0.219(2)),
The structural analysis of complex 3 shows that the solvent
N1A 0.781(2), N1B 0.219(2), O1A 0.781(2), O1B 0.219(2), C2A chloroform is also co-crystallized with the crystal and there is
0.781(2), C2B 0.219(2), C3A 0.781(2), C3B 0.219(2), C4A 0.781(2), disorder in the solvent molecule, in which the atoms have the
C4B 0.219(2), C5A 0.781(2), C5B 0.219(2), C6A 0.781(2), C6B occupancies of C1A (0.41), C1B (0.30), Cl1A (0.41), Cl1B (0.31),
0.219(2), C7A 0.781(2), C7B 0.219(2), C8A 0.781(2) and C8B Cl2A (0.41), Cl2B (0.31), Cl3A (0.41), and Cl3B (0.31).
0.219(2). The structure analysis also clearly shows that
Docking studies
The design and synthesis of molecules as anticancer drugs that
can recognize and bind to specific nucleic acid sequences and
structures are vital in understanding the molecular recognition
Fig. 3 Molecular structure of complex 2 with atomic labelling scheme.
Fig. 4 Molecular structure of complex 3 with atomic labelling scheme.
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of DNA and the development of novel chemotherapeutics.
In silico docking studies are an attractive scaffold to elucidate
the binding modes and interactions of chemical compounds to
the DNA macromolecule and are vital tools for substantiating
experimental findings and facilitating novel drug discovery.43
In the current investigation, all the synthesized compounds
were docked with a DNA macromolecule to predict the binding
site and the best binding orientation of the molecules inside
the DNA duplex. We found that all the used molecules (except
complex 4, which binds at the major groove) showed favorable
binding to the DNA minor groove. The docking scores of all the
compounds are presented in Table 2. Based on the docking
values, complex 7 was found to be the best binder with a
docking affinity of ꢀ382.2 kJ molꢀ1. This compound tends to
Fig. 5 Binding conformation of complex 7 in the minor groove of the
DNA duplex.
interact with the minor groove through the bromine atom with the bases through hydrophilic and hydrophobic interaction,
positioning the side triphenylphosphine rings along the X-axis while retaining the bis(triphenylphosphoranyl)copper(I) bromide
of the groove, while keeping the 1-(2-chlorobenzyl)-3-(3-(tri- on the outside of the groove. Complex 5 is seen in interactions via
fluoromethyl)phenyl)thiourea ring the outside. The central one side chain triphenylphosphine, which interacts mainly
phenyl phosphine of the one triphenylphosphine chain was through hydrophobic interactions to the DNA nitrogenous bases
seen bending to Guanosine 4 of chain A of the DNA duplex and 3-bromo-N-((2,4-dibromophenyl)carbamothioyl)benzamide
and involved in p–p interactions, whereas the sides phenyl that interacts with the sugar phosphate backbone of chain B.
phosphine did not interact with any of the chains. One phenyl The 3-chloro-N-methylbenzamide of complex 6 was found to
phosphine chain opposite to triphenylphosphine interacts with interact with the sugar phosphate backbone of chain A, keeping
a sugar-phosphate backbone of chain A. The bromine atom is the attached N-methylcyclohexanamine moiety away from
placed right on the top and interacts with Guanosine 22 from interactions and allowing one side triphenylphosphine bending
chain B. The binding pose of compound 7 at the minor groove downward for interaction with the nitrogenous bases. From
of the DNA duplex is shown in Fig. 5. Complex 1 was posed to complex 8, the 2-chloro-N-methylbenzamide is positioned down-
allow access to the DNA minor groove through the cyclohexa- ward towards Adenine 6 of chain A, whereas one triphenylpho-
1,3-dien-1-yldiphenylphosphine and 2-chloro-N-(phenylcarba- sphine is accommodated in the minor groove space and keeps the
mothioyl)benzamide moiety, while keeping (triphenylpho- opposite one outside the groove. Complex 9 is highly likely to fit in
sphoranyl)copper(I) bromide on the outside. Complex 2 also the minor groove through triphenylphosphine chains, while
favors binding to the minor groove mainly through one side keeping the N-((2,3,5,6-tetrachlorophenyl)carbamothioyl)benzamide
chain triphenylphosphine and N-(3-chlorophenyl)methane- to the outside.
thioamide of the 3-chloro-N-((3-chlorophenyl)carbamothioyl)
Molecular docking of the compounds with a breast cancer
benzamide. Similarly, complex 3 interacts with the minor protein was also performed in parallel to the DNA docking studies
groove bases and sugar phosphate backbone of chain A to understand the affinity of the compounds for the target proteins
through
3-chloro-N-((2,3-dichlorophenyl)carbamothioyl) as a test case. In this case, complex 5, followed by complex 7 were
benzamide and one side chain triphenylphosphine. The opposite revealed as the best docked molecules with a binding affinity of
triphenylphosphine side chain favors the interaction with the ꢀ429.4 kJ molꢀ1 and ꢀ418.1 kJ molꢀ1, respectively. Complex 7 was
sugar phosphate backbone of chain B. Complex 4 is the only found to be docked at the cavity formed by Helix 5 (Leu99 to
compound that favors a binding conformation at the DNA Ala101), Helix 12 (Asp208 to Ala229), Helix 9 (Asn152 to Phe176),
major groove. The binding positions the 3-chloro-N-((2,6- Helix 15 (Gly272 to Val283) and the loop region (Gly146 to Asn152)
dichlorophenyl)carbamothioyl)benzamide moiety in interactions that is involved in the interaction with the ligand molecules.
Mainly, the compound was seen in hydrophobic interactions with
the protein residues. The 2-chloro-N-((3-(trifluoromethyl)phenyl)-
carbamothioyl)benzamide chain was posed towards Helix 5 and
Helix 15 and was seen in hydrophobic interactions with Pro150
Table 2 Binding affinity of the compounds for the receptors used
DNA Receptor
17beta-HSD1
HSA Receptor
from the loop, Leu99 and Glu100 from Helix 5, and His280 and
Phe284 from Helix 15. One hydrogen bond having distance of
2.03 Å between Asp153 from Helix 9 and nitrogen atom of 2-chloro-
N-methylbenzamide was also observed. Both triphenylphosphine
chains were seen in hydrophobic interactions and positioned as
such that one interacts with the Helix 9 residues, whereas the
opposite favors interactions with the loop residues and Arg266
from Helix 14. The binding mode of complex 7 and its magnified
view are shown in Fig. 6.
Compound
(kJ molꢀ1
)
Receptor (kJ molꢀ1
)
(kJ molꢀ1
)
Complex 1
Complex 2
Complex 3
Complex 4
Complex 5
Complex 6
Complex 7
Complex 8
Complex 9
ꢀ364.6
ꢀ357.8
ꢀ378.2
ꢀ369.9
ꢀ363.4
ꢀ367.1
ꢀ382.2
ꢀ363.5
ꢀ353.9
ꢀ395.4
ꢀ390.2
ꢀ411.0
ꢀ405.3
ꢀ429.4
ꢀ400.0
ꢀ418.1
ꢀ413.9
ꢀ414.7
ꢀ401.71
ꢀ408.54
ꢀ384.15
ꢀ402.82
ꢀ399.47
ꢀ401.47
ꢀ482.67
ꢀ394.14
477.15
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non-toxicity and non-antigenicity, and is reproducible and
biodegradable, with many other advantages. Thus, HSA is an
attractive candidate for the transportation of the compounds to
the intended biological target.
For GST in contrast to the native substrate glutathione
(GSH), the compounds showed lower binding stability due to
their inappropriate binding poses with the protein. Thus, the
compounds can be assumed to be poor binders of GST and may
not affect GST functionality as an antioxidant.
Drug–DNA binding studies
Drug–DNA interactions can be studied by evaluating the
changes in spectral properties. Accordingly, absorption spectro-
scopy can be employed to determine the binding strength and
binding mode of drugs with DNA, which can be intercalative,
groove binding or electrostatic binding.44,45 Thus, UV spectro-
scopy was used to study the interaction of complexes 2–4, 6, 7
and 9 with DNA in the region of 220–260 nm by observing the
change in the absorption peak of the drug both in the presence
and absence of DNA solution. Fig. 9 shows the spectral behavior
of complex 7 and the others are provided in the ESI† (Fig. S1(a)–
S5(a), respectively). Complexes 2–4, 6 and 9 showed a hypso-
chromic effect with a slight blue shift for complexes 4, 6, 7 and
9, whereas no significant change was observed for complexes 2
and 3. These effects can be attributed to the electrostatic or
groove binding as these interactions are weak, and thus a
significant shift was not observed.26,27 For electrostatic binding
to be present, the species must be charged so that it can
interact with the negatively charged oxygen of the phosphate
group, thus causing changes in the spectral properties. One
criterion for groove binding is that the molecules must contain
unfused aromatic ring systems connected by torsional bonds,
and thus they can attain an appropriate conformation to
interact with the helical turns of the DNA grooves.46 All the
complexes showed electrostatic-type interactions.
Fig. 6 Binding mode of complex 7 at the docked side of the targeted
receptor protein and magnified view of the interactions between the
compound and protein residues within 3 Å of the compound.
Complex 5 was docked at the same cavity as that of complex
7; however, it preferred to interact mainly through its 3-bromo-
N-((2,4-dibromophenyl)carbamothioyl)benzamide, which bended
more towards the loop region (Gly146 to Asn152) and allowed
hydrophobic contacts with Leu149, Asp153, Cys156,Tyr275, and
Val276. A hydrogen bond between 3-bromo-N-methylbenzamide
oxygen and Gly148 at a distance of 2.83 Å was also observed. Both
triphenylphosphines were seen to be sandwiched between Helix 9
and loop (Asp269 to Gly272) of Helix 15 and Helix 14. In the case
of both complex 5 and 7, the copper and bromine atoms were not
involved in binding but could be fruitful in stabilizing the
attached chemical moieties at the docked sides. The binding
mode of complex 5 and its magnified view are shown in Fig. 7.
Complex 7 was again observed to form a stable conforma-
tion with respect to HSA with the lowest binding energy of
ꢀ482.67 kJ molꢀ1 (Table 2). This compound interacted with
several key residues of the proteins including Arg117, Arg145,
His146, Arg186, Lys190, Glu425, Ser517, Lys519, and Ile523
(Fig. 8). Within 3 Å, the compound formed a strong bond with
Arg173. Besides hydrogen bonding, the compound is engaged
via multiple van der Waals interactions, pi-cation, pi-sigma,
pi-alkyl and pi-anion bonding. HSA is considered a good
drug carrier in the pharmaceutical industry because it binds
well with drug molecules and exhibits enhanced stability,
The binding constants were evaluated by employing the
Benesi–Hildebrand equation as follows:
ꢀ
ꢁ
A0
eG
eG
1
¼
þ
ꢁ
(1)
A ꢀ A0 eHꢀG ꢀ eG eHꢀG ꢀ eG
K½DNAꢃ
Fig. 7 Binding mode of complex 7 at the docked side of the targeted
receptor protein and magnified view of the interactions between the
compound and protein residues within 3 Å of the compound.
Fig. 8 Binding mode of complex 7 at the docked side of HSA and
magnified view of the interactions between the compound and protein
residues within 3 Å of the compound.
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The binding constant and Gibbs free energy for representative
complexes are presented in Table 3. Complex 7 (Fig. 10) was found
to show the strongest electrostatic binding with DNA among the
complexes, which was also evaluated to be the best binder
through the molecular docking results.
Conclusions
In summary, nine new bromo(thiourea)bis(triphenylphosphine)
copper(I) complexes were synthesized and characterized using
different instrumental techniques. The single crystal X-ray analysis
revealed that the complexes are mononuclear tetrahedral, where
the copper atom is coordinated by two triphenylphosphines, one
thiourea ligand and one bromine atom. During the molecular
docking studies, all the synthesized complexes exhibited favorable
binding to the DNA minor grooves. Among the compounds,
complex 7 exhibited the best capability to bind with DNA and
produced a binding affinity value of 382.2 kJ molꢀ1. This com-
pound tends to interact with the minor groove through the
bromine atom positioning the side triphenylphosphine rings
along the X-axis of the groove, while keeping the 1-(2-
chlorobenzyl)-3-(3-(trifluoromethyl)phenyl)thiourea ring on the
outside. The DNA interaction studies of representative complexes
showed their efficient DNA binding affinities. Based on the
docking and DNA interaction results, complex 7 was found to be
the best binder with a docking affinity of 382.2 kJ molꢀ1 and
binding constant of 3.96 ꢁ 104 Mꢀ1. According to the theoretical
docking and experimental DNA interaction studies, it can be
concluded that the copper(I) thiourea complexes are potential
anti-cancer chemotherapeutics.
Fig. 9 UV-Vis absorption spectra of complex 7 in the absence of DNA (a)
and presence of increasing concentration of DNA (b–f) [10–50 mM].
Table 3 Binding constants and Gibbs free energies of molecules binding
to DNA
Binding constant
Gibbs free energies
Complex
values (K) (Mꢀ1
)
(kJ molꢀ1
)
2
3
4
6
7
9
1.01 ꢁ 104
1.10 ꢁ 104
2.72 ꢁ 104
4.29 ꢁ 103
3.96 ꢁ 104
2.87 ꢁ 104
ꢀ1.92
ꢀ1.94
ꢀ2.12
ꢀ1.74
ꢀ2.50
ꢀ2.13
where A0 and A are the absorbance of the free and DNA-bound
molecules, respectively. eG and eH–G are the absorption
coefficients of the molecule and its DNA adduct, respectively.
K is the binding constant, which can be obtained from the
slope to intercept ratio in the plot of A0/A0 ꢀ A versus 1/[DNA]
(Fig. S1(b)–S5(b), ESI†).47
Conflicts of interest
The authors declare no conflict of interest.
The Gibbs free energy from the binding constant of
complexes 2–4, 6, 7 and 9 was calculated using eqn (2), which
indicates the spontaneity of formation of the drug-DNA adduct.48,49
Acknowledgements
S. I. K. is highly thankful to Higher Education Commission of
Pakistan for Doctoral fellowship under International Support
Initiative Program (IRSIP) at University of Montreal Canada.
I am also thankful to Dr Michel Simard crystallographer for
giving me training on single crystal XRD and analyzing the
crystals at XRD lab at University of Montreal, Canada.
DG = ꢀRTln K
(2)
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