key: cord-0712635-6raoo7j9
authors: Viola; Muhammad, Niaz; Khan, Ishaq N.; Ali, Zafar; Ibrahim, Mohammad; Shujah, Shaukat; Ali, Saqib; Ikram, Muhammad; Rehman, Sadia; Khan, Gul Shahzada; Wadood, Abdul; Noor, Awal; Schulzke, Carola
title: Synthesis, Characterization, Antioxidant, Antileishmanial, Anticancer, DNA and Theoretical SARS-CoV-2 Interaction Studies of Copper(II) Carboxylate Complexes
date: 2021-12-30
journal: J Mol Struct
DOI: 10.1016/j.molstruc.2021.132308
sha: 1325ea480eba6149c55a4f0fbfb3b1f2d1262aa0
doc_id: 712635
cord_uid: 6raoo7j9

Copper(II) carboxylate complexes [Cu(2)(OOCR)(4)L(2)] (1) and [Cu(2)(OOCR`)(4)OCO(R`)CuL(2)](n) (2), where L = 2-methyl pyridine, R = 2-chlorophenyl acetate and R` = 2-fluorophenyl acetate were synthesized and characterized by FT-IR spectroscopy and single crystal X-ray analysis. Complex 1 exhibits the typical paddlewheel array of a dinuclear copper(II) complex with carboxylate ligands. In complex 2, this scaffold is further extended into a polymeric arrangement based on alternate paddlewheel and square planar moieties with distinct coordination spheres. The complexes showed better 2,2-diphenyl-1-picrylhydrazyl (DPPH) and hydroxyl radical scavenging activities and have been found to be more potent antileishmanial agents than their corresponding free ligand acid species. UV-Vis absorption titrations revealed good DNA binding abilities {Kb = 9.8 × 10(4) M(–1) (1) and 9.9 × 10(4) M(–1) (2)} implying partial intercalation of the complexes into DNA base pairs along with groove binding. The complexes displayed in vitro cytotoxic activity against malignant glioma U-87 (MG U87) cell lines. Computational docking studies further support complex-DNA binding by intercalation. Molecular docking investigations revealed probable interactions of the complexes with spike protein, the nucleocapsid protein of SARS-CoV-2 and with the angiotensin converting enzyme of human cells.

Dinuclear copper(II) complexes with four bridging carboxylate ligands have been widely studied due to their facile preparation and diverse applications. The coordination chemistry of copper (II) carboxylates in general as well as in combination with additional donating ligands such as pyridine derived ligands is of particular interest to us in a medicinal-chemical context [1, 2] .

Such complexes have the general formula [Cu 2 (OOCR) 4 structurally characterized examples for dinuclear paddlewheel copper complexes and those with aromatic N-donor ligands complementing the respective coordination spheres are still well over 300 [3] . In the last two years (2018/2019) alone, almost thirty respective structures have been reported [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] . The apical nitrogen donor ligands normally saturate the coordination spheres of the copper centers and thereby result in the commonly observed discrete di-nuclear molecular units [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] . Less common polymeric paddlewheel structures may result in cases where carboxylates act as bridging bidentate ligands in the apical positions [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] . The formation of di-or polymeric structures may be attributed mainly to both steric as well as electronic effects of the employed ligands. Here we show that varying the electronic structures of the comparatively small phenyl acetic acids by halogen substituent based modulation together with appropriate pyridine co-ligands, can lead to the formation of either of such complex types selectively. The distinct combinations of carboxylates with N-donor ligands not only result in various complexes with a range of geometries but also (fine-)tunes their chemical thermal, magnetic and biological properties [2, 5, 35, 36] .

Two further copper(II) carboxylate complexes were synthesized and structurally characterized in course of our continued investigations into these species' redox active natures, their capability to interact with biomolecules and, hence, their biomedical relevance. The complexes were tested in vitro for their anticancer, antioxidant, antileishmanial activity and DNA interaction potential.

Since the outbreak of the coronavirus disease in 2019 (COVID- 19) , much attention has been paid to developing effective and safe antiviral agents [37] along with vaccination efforts. The potential of inorganic complexes being used as alternatives to conventional small organic molecules or antibody-based therapies is an area of immense recent interest [38, 39] . Well defined three-dimensional shapes of the coordination compounds which are used to bind the target protein(s) are prerequisite for an informed and detailed evaluation of their specific biological activities [40] . Thus, the synthesized complexes of this study were characterized structurally and then screened computationally for their interaction ability with the spike protein (protein that mediates the entry of coronavirus into host cells), with the Nucleocapsid protein of SARS-CoV-2 (a target for vaccine development), and with the angiotensin converting enzyme of human cells (host for SARS-CoV-2) as potential sites for successful corona inhibition.

Copper(II) chloride dihydrate, 2-chlorophenylacetic acid (HL 1 ), 2-florophenylacetic acid (HL 2 ), 2-methylpyridine, ascorbic acid, 2,2-diphenyl-1-picrylhydrazyl radical (DPPH), iron(II) sulfate, sulfuric acid and hydrogen peroxide were purchased from Sigma-Aldrich and used without further purification. All solvents were of analytical grade and used without further purification.

Distilled water was used after one time distillation. The melting points were measured using a Digital Electro-Thermal Melting Point Apparatus. The copper percentage was determined by using a Perkin Elmer Atomic Absorption Spectrometer A Analyst 700. The FT-IR Spectra (4000-400 cm -1 ) were recorded on a Nicolet-6700 FTIR spectrophotometer, Thermoscientific, USA using attenuated total reflectance (ATR) technique. The electronic spectra of the complexes for DNA interaction studies were recorded using a Perkin Elmer UV/VIS Spectrophotometer Lambda 25 in aqueous dimethyl sulfoxide (DMSO).

For the synthesis of complex 1, a solution of HL 1 (0.50 g, 2.93 mmol) in methanol (10 mL) was added drop wise to the constantly stirring solution of copper(II) chloride dihydrate (0.25 g, 1.46 mmol) in methanol (10 mL). The reaction mixture was refluxed for 3 hours followed by the addition of 2-methylpyridine (0.145 mL, 1.46 mmol). The resulting reaction mixture was refluxed for further 3 hours. The reaction was stopped and the mixture was then cooled to room temperature to afford dark green colored crystals of the product after a few days. These crystals were separated from the solution and analyzed by different analytical and spectroscopic techniques. 

For the X-ray single crystal analyses appropriate sized crystals of the synthesized complexes (1 and 2) were mounted on a glass fiber. The corresponding structural data were collected using a STOE-IPDS II diffractometer equipped with a normal-focus, 2.4 kW sealed-tube X-ray source with graphite-monochromated Mo K α radiation (λ= 0.71073 Å). The single crystal of 1 was measured at room temperature and that of 2 at low temperature (170 K). The program X-Area was used for the integration of diffraction profiles. Numerical absorption corrections were carried out with the programs X-Shape and X-Red32 (all from STOE ©). The structures were solved by direct methods with SHELXT-16 and refined by full-matrix least-squares methods using SHELXL-16 [41, 42] . All non-hydrogen atoms were refined anisotropically and hydrogen atoms were refined isotropically at calculated positions using a riding model (U iso values constrained to 1.5 U eq of their pivot atoms for methyl groups and 1.2 U eq of their pivot atoms for all other groups). The fluorine substituent of one carboxylic acid in the asymmetric unit of complex 2 is disordered over two positions (76% vs. 24%). The hydrogen atom in these two positions was not split but refined with full occupancy on the carbon atom (C4), to which the 24% fluorine atom is bound. The respective C-H distance was fixed (DFIX) and the displacement parameter constrained. Other than that, no constraints or restraints were applied for modelling this disorder. Crystallographic data were deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. These data can be obtained free of charge on quoting the depository numbers CCDC 2031623 (1) and 2031624

(2) by FAX (+44-1223-336-033), email (deposit@ccdc.cam.ac.uk) or their web interface (at http://www.ccdc.cam.ac.uk).

Salmon sperm DNA (SS-DNA) (10 mg) was dissolved in distilled water (pH=7.0) with continuous stirring for 24 hours. The absorbance ratio (A 260 /A 280 ) of this solution was found to be 1.8, indicating that the DNA solution was sufficiently free of protein impurities [43] . The SS-DNA {ε = 6600M -1 cm -1 (260nm)} concentration was determined via a UV absorption measurement [44] and was found to be 0. Equivalent solutions of SS-DNA were added to the test compound and reference solutions, to eliminate the absorbance of DNA itself. Test compound-DNA solutions were allowed to incubate for an hour at room temperature before absorption measurements. Spectra were recorded using 1cm path length cuvettes at room temperature (25±1°C).

Carboxylic acids (HL 1 and HL 2 ) and copper complexes (1 and 2) were assessed in vitro for their DPPH free radical scavenging potentials following a reported procedure [45] . Different concentrations (15, 30, 60, 

The hydroxyl radical scavenging activities of the acids (HL 1 and HL 2 ) and complexes (1 and 2)

were assessed according to a reported method [46] . Different concentrations (15, 30, 60, 

The antileishmanial activity test was performed with a slight modification of the previously reported method [47] . Leishmania tropica promastigotes (1 x 10 5 parasites/well) and test compounds were dispensed in a flat-bottom 96-well microtiter plate such that the final concentration of each test compound was 1000, 500 and 250 μg/mL in DMSO. Amphotericin-B was used as a positive control while growth media with DMSO were used as blanks. The plates were then incubated at 25 ±1°C for 72 hours. After the incubation period was over, 100 μL of 3-(4,5-methylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) solution was added to each well and the plate was re-incubated for 4 hours at 31°C. 40 μL of DMSO were then added to each well to dissolve the formazan crystals of MTT. The plates were stirred gently for 15 minutes. The optical absorbance of each plate was measured by the microplate reader (Bio

Tek TM ELx800) at 570 nm. Percentage inhibition was then calculated using the following formula.

*100 meanOD of sample meanOD ofblank PercentViablity meanOD of negativecontrol meanOD of blank

The malignant glioma U-87 (MG U87) cell lines were cultured as previously described [48] . The The growth inhibition assay was conducted as previously described [49] . The malignant glioma U87 cell lines were plated into 96-well plates, with each well containing 5000 cells per well. The cell count was done with a hemocytometer followed by culturing in DMEM:F12 medium for 24 hours in a CO 2 incubator with 5% CO 2 supply. Following 24 hours incubation, different concentrations of the test compounds (HL 1 , HL 2 , 1 and 2) were prepared in DMSO. The cell lines were treated with test compounds at different serial concentrations ranging from 0 to 250 μg (0 μg/mL (untreated control), 7.812 μg/mL, 15.625μg/mL, 31.25 μg/mL, 62.5 μg/mL, 125μg/mL and 250 μg/mL). All test compounds were analyzed at each concentration in triplicates. The plates' patterns were repeated three times, each with 24 hours of treatment.

Following the specified time of drug treatments, cells were fixed in 4% formalin, followed by staining with 0.1% crystal violet for 10 minutes [50, 51] . Each plate was washed three times using 1x phosphate buffered saline and air-dried, followed by addition of 200 μL of acetic acid into each well. The absorbance of each plate (well) was recorded at 630 nm using a 96-well plate spectrophotometer reader.

The three dimensional (3D) structures of the newly synthesized Cu-complexes were drawn with the Molecular Operating Environment (MOE-2016) software [52] . The hydrogen atoms were added to the synthesized compounds by 3D protonation followed by energy minimization with 

The complexes were synthesized by the reaction of copper(II) chloride dihydrate with 2chlorophenylacetic acid (HL 1 ) or 2-florophenylacetic acid (HL 2 ) in the presence of 2methylpyridine as co-ligand in methanol under reflux. The complexes are dark green crystalline solids and were obtained in good yields. Both complexes were soluble in methanol and DMSO.

The comparative analysis of FT-IR spectra of the synthesized complexes (1 and 2) with the corresponding free ligand acids (HL 1 and HL 2 ) spectra confirmed the complex formations. The most vivid difference was the absence of broader bands in the complex spectra in the range 3400-2400cm -1 which appear due to OH vibrations of the COOH moiety in the free ligand acids [2] . The presence of coordinated deprotonated carboxylate ligands in the complex spectra was also supported by the appearance of new medium intensity bands at 428 and 434 cm -1 for complexes 1 and 2, respectively. The coordination of 2-methyl pyridine to copper centers is reflected by the medium intensity bands at 592 and 593 cm -1 in the spectra of complexes 1 and 2, respectively, owing to the newly established Cu-N bonds and concomitant decrease in the vibration frequencies of the vC=N bond [2, 36] . The presence of 2-methyl pyridine was also supported by the strong pyridyl ring vibration at 740 and 750 cm -1 in complexes 1 and 2, respectively [2] . Such notable difference in the value for this vibration in the two complexes suggests a considerably distinct environment of the 2-methyl pyridine ring which was further confirmed by the X-ray single crystal analyses of both the complexes. The strong bands due to 

The two copper complexes reported here were synthesized by the reaction of HL 1 and HL 2 with [CuCl 2 ].2H 2 O in the presence of 2-methyl pyridine. Crystals of 1 and 2 suitable for X-ray analysis were grown from methanol and the molecular structures are shown in Figs. 1a and 2a, respectively. The crystal and refinement data for both complexes are summarized in Table 1 . Table 4 ). The observed distortions of the ideal coordination geometries are possibly also based to some extent on the strong intermolecular hydrogen bonds.

A DPPH free radical scavenging assay was performed for the free ligand acids (HL 1 and HL 2 ) and their copper complexes (1 and 2) and the results are shown in Table 5 . A dose dependent response was exhibited by the samples with an activity order 1 > HL 1 > 2 > HL 2 . Complexes 1 and 2, both have better radical scavenging activities than their corresponding free carboxylic acid ligands HL 1 and HL 2 , respectively. The observed activity enhancement after complexation reflects the importance of the metal center as well as the N-donor ligands for the reaction with the radical in this assay. The copper complexes acquire additional superoxide-dismuting centers leading to superior antioxidant activity [57] . Notably, the chloro substituted phenylacetic acid (HL 1 ) is a better DPPH radical scavenger than the fluoro substituted phenyl acetic acid (HL 2 ).

The higher activity of HL 1 could be attributed to the ease with which free radical formation can take place through homolytic fission in the presence of the chloro substituent for subsequent reaction with the DPPH. On the other hand the polarity induced by the fluorine probably hinders the process of free radical formation in HL 2 . Complex 1 has shown comparable activity to the standard drug ascorbic acid (vitamin C) therefore it might be utilized in the future as DPPH scavenger after some further evaluation.

The hydroxyl radical scavenging assay results of the tested compounds are shown in Table 6 . At lower concentrations essentially the same order of activity was observed as in the DPPH assay.

Complex 1 exhibits the highest hydroxyl scavenging ability throughout, followed by its corresponding acid ligand, HL 1 . Complex 2 was more active than its corresponding acid ligand, HL 2 , at lower concentrations. However, the activity order reverses at the two highest concentrations where HL 2 exhibits better activity than complex 2. Overall, the results point again at the role of the carbon halogen bond polarity in the radical scavenging activities.

The antileishmanial activity data, summarized in Table 7 , show dose dependent antileishmanial activities for all investigated species. In particular, the free carboxylic acid ligands give rise to an exponential increase at 1000 ppm concentration. The complexes exhibit higher activities than their free acid ligands (HL 1 and HL 2 ) at all concentrations. However, the difference was more pronounced at lower concentrations. The enhanced activities of the complexes (1 and 2) compared to the free acid ligands reflect the significance of the metal center as well as the Ndonor ligands for the antileishmanial activity and can be explained on the basis of their ability to interact with DNA of parasite through intercalation [58] . At lower concentrations the two complexes displayed significant differences in their activities. Comparable activities of the complexes were observed at higher concentration (1000 ppm). Among the tested compounds, complex 1 was found to be the most potent antileishmanial agent at all concentrations. In the applied concentration range, its activity was essentially dose independent within the error margin suggesting that even lower doses should be considered. Complex 1 is a strong candidate for further and more intense studies regarding its application as a potential antileishmanial agent.

Amphotericin-B was used as a positive control in this assay and reached 100% inhibition.

The acid ligands (HL 1 and HL 2 ) and synthesized complexes (1 and 2) were screened in vitro for their anticancer potential against malignant glioma U-87 (MG U87) cell lines and the results are summarized in Table 8 The calculated binding constant (K b ) values using the Benesi-Hildebrand equation were 9.8x10 4 and 9.9x10 4 M -1 for complexes 1 and 2, respectively [2] . The reported complexes are relatively better DNA binders compared to the previously reported copper(II) complexes with similar carboxylate ligands [2, 59, 60] . Since in a dinuclear complex the two adjacent Lewis acid metal sites are favorably poised to bind or interact with the donor sites of double-stranded DNA and by tuning the groups on the dimetal system one can even effectively control the nature of interaction with DNA [61] . The response of both complexes towards DNA in terms of interaction mode and binging strength was found to be very similar. This observation reflects that varying the halogen (F or Cl) on the coordinated ligand has no effect on the DNA binding ability of the complexes as long as the complexes have other structural or spacial geometric similarities. However, the position of the halogen substituent on the phenyl acetate ligand, considering previously reported studies, was found to affect the DNA binging ability, i.e. decreasing it in the order 2-Cl > 4-Cl > 3-Cl [2, 60] . Notably, the copper complexes with the paddlewheel arrangement are relatively more active DNA binders compared to the complexes with similar phenyl acetate ligands and bidentate nitrogen donor ligands in a different structural environment [2, 60] . (Fig. 6 ). An excellent arrangement was obtained as the best docked pose, which showed important binding features that are predominantly based on interactions of various interacting moieties of the complexes and phosphate derived functional groups of the DNA backbone.

Molecular docking investigations were performed to find any possible complex and SARS-CoV-2 interactions. For the docking studies a spike protein, the nucleocapsid protein of SARS-CoV-2 and the angiotensin converting enzyme of the human cell (host for SARS-CoV-2) were selected as target sites [62, 63] . The results of this theoretical study show that the synthesized complexes 1 and 2 do indeed bind to spike protein, nucleocapsid protein of SARS-CoV-2 and that they can also inhibit the angiotensin converting enzyme of the human cell. 

The successful synthesis of copper(II) carboxylate complexes by the condensation reaction of copper(II) chloride dihydrate with 2-chlorophenyl acetic acid (HL 1 ) and 2-fluorophenyl acetic acid (HL 2 ) in the presence of 2-methyl pyridine co-ligand was verified by FT-IR and X-ray single crystal analyses. While the reaction with the chloro-substituted acid led to a typical discrete paddlewheel structure, the fluoro-analog resulted in a coordination polymer with a dinuclear paddlewheel moiety plus a square planar mono-nuclear building block. The complexes exhibit better antioxidant and antileishmanial activity compared to the free acid ligands thus reflecting the importance of the synergistic effect of the metal ion center, carboxylate and nitrogen donor ligands for the activity. A mixed DNA interaction mode consisting of partial intercalation and groove binding was established by UV-Vis absorption spectroscopy. A dose dependent cytotoxic response against malignant glioma U-87 (MG U87) cell lines was observed at relatively low concentrations. The decrease in the cytotoxic effect at higher concentration suggests a detrimental influence of intermolecular H-bonding on the cytotoxic activity of the complexes or that they trigger defense mechanisms of the cell. A theoretical docking study puts the two complexes forward as potential corona virus inhibitors.

The author(s) declare(s) that there is no conflict of interest regarding the publication of this paper. 

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