Document Type : Research Paper
Authors
Department of Chemistry, College of Education, University of AL-Qadisiyah, Iraq
Abstract
Keywords
INTRODUCTION
Benzoxazole is an important class of heterocyclic compounds [1]. Due to different biological and pharmacological properties, benzoxazole and its derivatives are used as anti-inflammatory, antibacterial [2], antiviral, anti-tuberculosis [3], antispasmodic and antidepressant [4], cardiovascular stimulant [5], pharmaceutical applications, and medicinal chemical discovery programs [6]. These compounds are also used in the synthesis of bioactive substances such as lactams [7]. Benzoxazoles is also used to combat oxidative stress and to prevent many diseases such as types of cancer in many clinical processes [8-10]. Benzoxazole, known as 1,3-benzoxazole, is an aromatic organic heterocyclic compound consisting of an oxazole ring, one oxygen atom and one nitrogen atom attached to the benzene ring [11, 12]. It was first reported by chemist Haritzsch in 1887 AD. Molecular formula of C7H5NO, molar mass of 119.12 g/mol, melting point of 27-30°C, and boiling point of 182° [13-15] were reported for this compound. It has a white to light yellow color with a pyridine-like odor, insoluble in water, at the same time it dissolves completely in organic solvents like ethanol [16, 17]. In general, benzoxazole and its derivatives are employed in coordination chemistry for the production of complexes [18, 19], and other applications such as photoluminescence and bleaching and dyeing [20, 21]. Thiazole is a five-membered heterocyclic compound containing nitrogen and sulfur at positions 1 and 3, respectively. Thiazoles have diverse biological activities, such as anti-inflammatory [22, 23], anti-fungal, anti-cancer, anti-bacterial, anti-spasmodic, anti-viral, and anti-tumor [22, 24]. This diversified activity generated the investigations with preparation of new derivatives of thiazoles [25]. A thiazole is a pale yellow liquid with a pyridine-like odor, which its molecular formula is C3H3NS with a boiling point of 116 -118 °C, soluble in alcohol [22]. Thiazoles are part of many natural compounds, like vitamin B1 [26].
This study involved the preparation of the new ligand 2-(2-(benzoxazole-2-yl)-hydrazinyldine(-2,1-diphenyl-N-(thiazole-2-yl)ethane-1-amine (BOHPTEI)] Schiff base type and its complex with the Pd (II) to study the biological activity of the prepared ligand and its complex in anti-cancer fields.
MATERIALS AND METHODS
Chemicals and methods
Chemical reagents were supplied by Sigma-Aldrich, Merck, MSDS, and BDH companies. Uv-visible spectra were obtained in the wavelength from 200 to 1000 nm using a Shimadzu UV-165PCS spectrophotometer. NMR results were recorded on a Varian transform Fourier spectrometer, running at 300 MHz with tetramethyl silane as the internal reference standard in DMSO-d6 solvent. FTIR spectra were recorded in the range of 400-4000 cm-1 employing an FTIR 8400S Shimadzu spectrophotometer (Japan). The melting points of prepared samples were obtained employing the Stewart melting point. The magnetic susceptibilities at room temperature were measured with a Balance Magnetic Susceptibility Model MSB-MKI. The atomic absorption spectrophotometry was performed employing a Shimadzu AA-6300 instrument to determine the metal content. The FESEM images of the prepared compounds were recorded using the (MIRA3 TESCAN, Czech), while the XRD measurements were employed using the (D2Phaser Bruker AXS Gmbh) with an angular range of 2θ=20-80°. Conductivity were measured at a concentration of (1X10-3M) and at laboratory temperature, using the Digital Conductivity Series Ino device. Cond 3110 SET1 using ethanol as a solvent. Lastly, elemental analysis was performed using the EA-300.mth instrument.
Synthesis of the ligand (BOHPTEI)
The BOHPTEI ligand was synthesized in the following two stages:
Preparation of the 2-hydrazinylbenzoxazole (A) is the initial step. The 2-hydrazinylbenzooxazole (A) compound was made through the reaction between 1.5 g (0.01 mol) of the 2-mercaptobenzoxazole in 25 mL of absolute ethanol as a solvent in a round flask (250 mL), and then adding 5 mL (0.01 mol) aqueous hydrazine with 15 mL of absolute ethanol under stirring continuously. The solution was treated under a reflux process for 8 h. The precipitate was then collected and dried to produce yellow crystals, and the product was then recrystallized from chloroform to remove any unreacted substances. The precipitate was dried in the next step, as it gave a yield of (35%), with a melting point of (102 °C).
The second step is synthesis of the BOHPTEI ligand. 1.5 g (0.01 mol) of compound A was dissolved in 25 mL of absolute ethanol as a solvent under continuous stirring at a temperature of 40-50 oC. Next, a solution of 2.1 g (0.01 mol) of benzil dissolved in 25 mL of absolute ethanol, was acidified by adding 5-6 drops of glacial acetic acid. After that, a solution 1.0 g (0.01 mol) of 2-aminothiazole was added. The mixture was continually refluxed for 8 h. The solution was cooled, and a precipitate was seen. It was obtained by recrystallizing 99.9% ethanol, collecting the precipitate (yield: 73%), and measuring it at 95-98 oC. Fig. 1 showed the preparation of the BOHPTEI ligand.
Synthesis of the palladium (II) complex
0.423g (0.001 mol) of the BOHPTEI ligand was dissolved in 10 mL of absolute ethanol, followed by the addition of 0.177g (0.001mol) of palladium (II) chloride. After 2 h of refluxing under stirring, precipitates were produced, filtered, dried, and then recrystallized. Table 1 included some the physical properties of the prepared ligand (BOHPTEI) as well as palladium (II) complex.
Cytotoxic studies-MTT assay
By using an in vitro MTT cytotoxicity assay on breast cancer cell lines (MCF-7) grown in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% fetal bovine serum (FBS; Gibco) and 1% penicillin-streptomycin, the cytotoxicity of the BOHPTEI ligand and Pd (II)-complex was investigated. The MTT test was modified to determine cell viability by incubating at 37 °C in a humidified environment with 5% (v/v) CO [27]. For 24 h, cells were seeded at a density of 104 cells/well in 96 dishes). The medium was extracted after 24 h incubation at 37 °C, and cells were treated with different concentrations (0, 50, 100, 200, 400, 800 and 1600 µg/ mL) after a 72-h incubation period at 37 °C. The medium was then withdrawn from the board. Each became full of 100 µL of MTT reagent (1 mg/ mL) in serum-free medium. After 4 h of incubation, the medium was removed, and 200 µL of dimethyl sulfoxide (DMSO) were applied to each well. The absorbance of the MTT metabolite dissolved in DMSO was determined employing a microplate reader at a wavelength of 570 nm. There had three inspections for every concentration. % Inhibitory concentrations of 50% (IC50) were determined, cellular viability was estimated, and IC50 curves were generated using the x-dose response equation [28].
RESULTS AND DISCUSSION
The palladium (II) complex is dark brown, while the BOHPTEI ligand is represented as pale yellow crystals. The ligand and palladium (II) complex are soluble in several solvents, including ethanol, methanol, DMSO, and DMF, but they are insoluble in water and ether.
The ratio of ligand to metal was ascertained to be 1:2 employing the molar ratio method developed by the scientist (June) [29].
Molar conductivity Measurements
Ethanol was utilized as a solvent to measure molar conductivity at laboratory temperature and concentration (1×10-3 M). The molar conductivity value of palladium (II) complex is 78 ohm-1 cm2 mol-1). This value indicates that the complex is ionic with a ratio of 1:2 [30].
1H-NMR spectrum of the ligand (BOHPTEI)
Fig. 2 displays the proton NMR spectrum of the BOHPTEI ligand. Dimethyl sulfoxide (DMSO-d6) as solvent and tetramethyl silane (TMS) as a standard reference were used to measure the 1H-NMR spectrum of at laboratory temperature [31]. The 1H-NMR spectrum of the BOHPTEI ligand displayed a multiple signal at a chemical displacement of (M, 4H, δ = 7.37 – 7.49ppm) indicating the protons of the benzoxazole ring [32]. The two protons of the thiazole ring gave a doublet signal at (D, 2H, = 7.50 - 7.75 ppm) [33]. While the multiple signals at the chemical shift (M, 10H, δ = 7.80 - 7.97 ppm) belong to the protons of the two phenyl rings of benzil [34]. The singlet signal at (S, 1H, δ = 8.92ppm) belongs to the proton of the secondary amine group (–NH) [35]. A singlet signal at the chemical shift of 2.52ppm resulted from the protons of the solvent (DMSO-d6) [36].
FTIR spectra of the ligand BOHPTEI and palladium (II) complex
Potassium bromide was used to compare the FTIR spectra of the BOHPTEI ligand and its complex in Fig. 3 and Fig. 4. The FTIR spectrum of the free ligand displayed a clear absorption band at (3301 cm-1) indicating the secondary amine group υ(N-H) [37]. The band at 1666 cm-1 belongs to the azomethine group band (C=N) of the Schiff base [38, 39], while the carbonyl group disappears in the reactants. Other bands appeared at 1650 and 1604 cm-1 are due to the groups of azomethine of the thiazole and benzoxazole rings, respectively [40]. While, the two functional groups of C= C and C-H aromatic were concentrated at 1527, 1496 cm-1 and 3062 cm-1, respectively [41]. Additional bands appeared at 1172 and 1072 cm-1 are related to the functional groups of υ(C-N) and (C-O) of the thiazole and benzoxazole rings, respectively [41]. In the spectrum of the prepared complex, the azomethine group υ (C=N) of the Schiff base shifted towards a lower frequency and was reported as 1654 cm-1 [42]. Also, the frequency of the two azomethine groups belonging to the thiazole and benzoxazole rings shifted to a lower frequency, and they occurred at 1632 and 1594 cm-1 in the spectrum of the prepared complex. The interaction of the ligand with the palladium ion via the azomethine groups of nitrogen atoms is strongly supported by these shifts [43]. A new band at 516 cm-1 identifiesthe group of M-N [44, 45]. In Table 2, the spectral bands of both samples have been listed.
Electronic spectra
As illustrated by Fig. 5, three absorption peaks could be seen in the ligand’s electronic spectra. Two peaks at 204 nm (49020 cm-1) and 222 nm (45045 cm-1) belong to the transition (π-π*), while another peak at 263 nm (38023 cm-1) is due to the n-π* transition of the azomethine group (C=N) [46]. The spectrum of palladium (II) complex displayed many absorption peaks at 211 nm (47393 cm-1) and 268 nm (37313 cm-1). The absorption peaks at 546 nm (18315 cm-1), 602 nm (16611 cm-1) and 694 nm (14409 cm-1) indicate the electronic transitions of 1A1g→1Eg, 1A1g→1B1g and 1A1g→1A2g, respectively, which provide that the complex geometry is a square planar [37]. The magnetic sensitivity measurements proposed that the complex has diamagnetic properties [47]. Table 3 summarizes the electronic structure, magnetic and geometry of both samples.
XRD study
XRD measurement was employed to study the crystal structures and crystal size of the synthesized BOHPTEI ligand and its palladium (II) complex (Fig. 6). To determine the degree of purity and the crystalline nature of samples, microstrains, and dislocation density were also calculated. There are some diffraction peaks such as micro-strains caused by crystal distortions, an absence of lattice deformation and faulting, the crystal’s domain size, and the distribution of field size [48]. The XRD pattern revealed the existence of peaks, which is an indication of a crystal lattice, and broad peaks, which signify an amorphous structure. These peaks’ sharpness is determined by the crystal structure, the crystal lattice properties, the crystalline levels, and other factors [49]. The crystal sizes of the synthesized BOHPTEI ligand and palladium (II) complex was determined using the Debye-Scherer formula as below:
where, D is average crystal size, k represents shape factor (0.9), λ is the wavelength of the X-ray (CuKα = 1.54056 Å), β displays total width at half height FWHM, and ϴ is the deviation angle.
Also, the following equation was used to calculate the microcompliance:
where S is microstrains and β introduces total width at half-maximum height.
The density of dislocations is estimated using the formula that follows the equation.
where δ is density of dissolutions and D shows average crystal size of samples.
The difference in distance between the crystalline degrees d of the ligand and its synthesized metallic complex, as well as the density of dissolutions, was confirmed by Table 4. It was also found that there is an inverse relationship between the crystal size, the micro-compliantness, and the density of dissolutions [50]. XRD measurements of the BOHPTEI ligand showed a crystal size of 64.85 nm. Also, the palladium (II) complex illustrated a crystalline size of 42.03 nm. These values indicate that the prepared BOHPTEI ligand and its palladium (II) complex have a size of less than 100 nm, meaning that they are within the nanoscale range [51].
FE-SEM study
FESEM analysis was employed to analysis the surface properties of the BOHPTEI ligand and its metal complex prepared with palladium (II) ion, in terms of particle shape, size and distribution (Fig. 7). By analyzing the FE-SEM images, the BOHPTEI ligand showed in the form of large homogeneous crystals with a particle size of 237.79 nm, while the FE-SEM image analysis of palladium (II) complex appeared to contain irregular and inhomogeneous oval shapes with an average size of 43.93 nm. The results showed that the prepared ligand is introduced as particle size greater than 100 nm, and the prepared complex with size of less than 100 nm. These results enabled us to study the ligand and palladium (II) complex in the field of medicine and their ability to eliminate many types of cancer, including breast cancer [52].
Effect of BOHPTEI ligand and palladium (II) complex on the growth process of breast cancer cell line MCF7 and normal cells (HEK)
The effect of BOHPTEI ligand were investigated on the the development of breast cancer cell MCF7 and healthy cell HEK (Table 5 and Fig. 8). The highest rate of inhibition occurred at a concentration of 1600 µg/mL, while the lowest rate of inhibition of the growth of breast cancer cells was detected at a dosage of 50 µg/mL. In contrast, the lowest and highest rates of inhibition of normal cell development (HEK) were 50 and 1600 µg/mL, respectively. To assess the potential for employing these chemicals as a potential treatment, normal cells (HEK) were employed to compare with breast cancer cells. The greatest inhibition rate for breast cancer cells was discovered to be 89.8% at a dosage of 1600 µg/mL. The normal cell line’s inhibition rate in the same concentration was 85.8%, which is a reasonably decent result, but certain changes are required to raise this rate. According to these results, the half-maximal inhibitory concentration (IC50) of breast cancer cell line was 63.1 μg/mL, while for normal cells (HEK) the IC50was equal to 67.5 µg/mL. This also reinforces the good results obtained using the BOHPTEI ligand in the treatment of breast cancer [53]. On the other hand, palladium (II) complex inhibited the growth of cancer cells MCF7 by (95.5%) at a concentration of 1600 µg/mL, while it was less inhibiting on normal cells (HEK-293) at this concentration, as it gave an inhibition rate of 85.55% (Table 6 and Fig. 9). The values of IC50 for cancer cell line were IC50= 0.278 µg/mL, while it gave an equal value to IC50= 150 µg/mL with normal cell lines [54, 55].
CONCLUSION
In this research, the BOHPTEI ligand was prepared from 2-mercaptobenzooxazole and 2-aminothiazole. Also, Pd (II) complex was fabricated by reaction this ligand with divalent palladium chloride. These compounds were characterized by in a variety of spectroscopic and physical methods. The FTIR spectra confirmed the formation of the azomethine group. These spectra also demonstrated that the ligand coordinates with the palladium ion through the nitrogen atoms of the two azomethine groups of the Schiff base as well as the two azomethine groups of the benzoxazole and thiazole rings. When coordinated, the azomethine group shifted towards lower frequencies as compared to the free ligand. The electronic results of Pd (II) complex suggested that the complex has a square planar geometry. Molar conductivity measurements showed that Pd (II) has an ionic nature. XRD and FESEM analyses proved that the ligand and its complex have a nanoscale size. Also, MTT assay of both ligand and Pd (II) complex against breast cancer cell line (MCF-7) and normal cells (HEK) illustrated promising anticancer activity.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.
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