Introduction

Schiff bases and their transition metal complexes are essential compounds with many exciting properties and extensive medicinal, agricultural, pharmaceutical, and material science applications. The Schiff base and its complexes have been reported for various biological activities such as antioxidant, anti-bacterial, anti-fungal, anti-tumor, etc. [1]. When Schiff base ligands combine with transition metal ions, they show different electronic, geometrical, and biological properties. This is because the ligand moiety is made up of different groups of atoms. Over time, ligands with nitrogen-sulfur donor sites and their derivatives have garnered significant attention because of the possibility of their donor site being available. For metal chelation, these donor atoms are readily accessible [2,3,4,5,6]. Owing to the site-specific binding properties of metal ions, DNA binding with metal complexes provides a new manifesto for developing new chemotherapeutic drugs [7]. The schiff base and its complexes, which are synthesized from a sequence of NS donor atoms, have demonstrated an amazing level of activity against some antifungal and antibacterial agents [8, 9]. Various analogs are found to be capable of binding to DNA, hilting its replication and apoptosis [10, 11].

The study shows that when a ligand is coupled to metal ions present in biological systems (Zn2 + , Cu2 + , Co2 + , Mn2 + , Ni2 + , Fe2 + , VO2 +), its biological activity is significantly enhanced [12]. Zinc is an essential element that plays a very important role in various biological processes. Their complexes bind strongly with DNA and also exhibit predominant anti-tumor activities. Due to its pivotal role in numerous physiological and pathological processes, it is responsible for cell proliferation, differentiation, and viability, including apoptosis [13].

Apart from choosing the transition metals, the design of the ligand framework is regularly crucial as it can modulate the oral or systemic bioavailability of metal ions, secure the selective target DNA or enzyme, and protect and deliver a metal ion at the target site [14, 15]. A slight change in the ligand framework can drastically improve the therapeutic potential of the metal complex via the uptake of pharmacologically active metal ions. In this case, Schiff base ligands work as a platform because they can bind to metals in a flexible way, have biological properties that can't be changed, and can change their structure to get the desired molecule. Therefore, the design of Schiff base-based metal complexes tethered with a heterocyclic moiety shows a multi-fold increase in biological properties [16, 17]. Computational simulation also helps in this regard to understand the stable geometry of the complex with the calculations of different electronic parameters.

Materials, Reagents and Methods

The chemicals thiophene-2-carbaldehyde and 4-nitro-o-phenylenediamine, CT-DNA, and Ethidium Bromide were purchased from Sigma Aldrich, Germany. Zinc(II) sulphate heptahydrate and solvents like methanol, dimethylsulphoxide were purchased from SD Fine Chemicals, India. Solvents were purified prior to their use following standard literature procedures.

Instruments

A Vario Micro Cube elemental analyzer instrument was used for the elemental analysis of ligand and complex. The 1H and 13C NMR spectra were recorded using a Bruker 400 MHz FT-NMR instrument (DMSO-d6 as solvent). A Jasco V-530 UV–Vis spectrophotometer was used to record the UV–Visible spectra of the ligand and its metal complex. A Perkin Elmer RX-1 FT-IR Spectrometer with IR range of 400–4000 cm−1 was used to record the IR spectra using KBr pellets. A Jasco V-530 Spectrophotometer was used to carry out fluorescence emission experiments at room temperature. A Jasco V-530 double beam UV–Vis spectrophotometer was used to study the thermal denaturation of DNA. Viscosity was studied using an Ostwald viscometer maintained at constant room temperature. A Systronics India conductivity meter TDS-308 was used to measure specific conductance of synthesized complex in DMSO solvent at room temperature. Thin-Layer Chromatography was used to affirm the purity of ligand and its complex. Finally, an open-capillary technique was used to determine the melting point of the ligand and its complex.

Synthesis of 2-Amino-5-Nitro-N-[(E)-Thiophen-2-yl-Methylidene]Aniline Schiff Base

4-nitro-o-phenylenediamine (1 mmol, 0.153 g) and thiophene-2-carbadehyde (1 mmol, 0.112 g) were dissolved in ethanol and refluxed for 2 h with constant stirring. The yellow color precipitate was obtained after 2 h of reaction, was filtered using Whatman 41 filter paper, and was washed several times with cold ethanol. The product was kept in vacuum desiccators for 24 h.

Synthesis of [{(2-Amino-5-Nitro-N-[(E)-Thiophen-2-yl-Methylidene]Aniline-κ3N1:N4:S)(Sulphato-κ2O1:O3)}Zinc(II)]

A Zn (II) Sulphate Heptahydrate (0.1 mmol, 0.287 g) was dissolved in ethanol and was added dropwise to the equimolar hot ethanolic solution of the ligand (0.1 mmol, 0.247 g) in a two neck round bottom flask. The reaction was kept under refluxed condition for 10 h at 45–50 °C. The change of the color from yellow to dark yellow was noticeable during the reaction and consequently it resulted in the dark yellow precipitate. The precipitate was filtered using whatman 41 filter paper and was washed thoroughly using cold ethanol three times. The product was collected and kept in vacuum desiccators for 36 h.

Computational Study

Computational simulation using density functional theory (DFT) of the synthesized complex was performed in an aqueous medium with Gaussian 09 software [18, 19]. B3LYP hybrid functional [20] with 6-31 g(d,p) basis set [21] was used in an ethanol medium. The frequency calculations of the optimized geometry of the complex indicated that the geometry lies on the minima to the potential energy surface [22, 23] as no imaginary frequencies were obtained.Different electrochemical parameters like Mulliken Charge Distribution, HOMO–LUMO, and Molecular Electrostatic Potential (ESP) maps were also analysed to understand the characteristics of the synthesized complex [24, 25].

DNA Binding Experiment

Absorption Study

The absorption study is the best technique to study the interaction of the complex with the CT-DNA. As per the literature, the Tris–HCl buffer was prepared and used during the study. The purity of the CT-DNA was confirmed as it showed absorbance peaks at 260 nm, which further indicated that the CT-DNA was free of any contamination [26]. During the absorbance study, the concentration of the complex was fixed at 10 µM, and to prevent the absorption of free CT-DNA, an equal amount of CT-DNA was added to both the reference and solution chamber. Therefore, the spectra obtained were only due to the interaction of CT-DNA with the complex. The Wolfe-Shimer equation was used to calculate the intrinsic binding constant (kb) [27,28,29].

$$[{\text{DNA}}]/(\mathrm{\varepsilon a}-\mathrm{\varepsilon f})\hspace{0.17em}=\hspace{0.17em}[{\text{DNA}}]/(\mathrm{\varepsilon b}-\mathrm{\varepsilon f})\hspace{0.17em}+\hspace{0.17em}1/{\text{Kb}}(\mathrm{\varepsilon a}-\mathrm{\varepsilon f})$$

The data obtained was used for plotting the [DNA]/(\(\upvarepsilon\)a-\(\upvarepsilon\)f) versus [DNA] plot and calculating the intrinsic binding constant. From the above equation \(\upvarepsilon\)a, \(\upvarepsilon\)f and \(\upvarepsilon\)b denotes the extinction coefficients observed for charge transfer, complex free in solution, and complex fully bound to DNA, respectively. The intrinsic binding is calculated as per the literature, where kb is obtained as the ratio of slope to the intercept [30].

Fluorescence Emission Spectroscopy

The relative binding of the complex to CT-DNA is investigated using an Ethidium bromide CT-DNA solution. Ethidium bromide is widely used as a fluorescent DNA structural probe for studying the mode of binding of complexes to DNA. A 3 ml solution of 2.0 µM DNA and 1 × 10–5 M EB was titrated by 5–30 × 10–5 M compound (λex = 540 nm, λem = 540 – 680 nm). The obtained spectra were solved using the Stern–Volmer equation, I0/I = 1 + Ksvr, and the quenching constant Ksv was calculated. Here, I and I0 denote the fluorescence intensities in the presence and absence of the complex, respectively and ‘r’ is the concentration of the complex [31].

Thermal Denaturation Study

A thermal denaturation investigation of DNA was studied using a temperature-control programmer spectrometer, where the temperature was set to elevate by 5 °C min−1. The absorbance of DNA in both the absence and presence of complex was observed [28, 30, 31].

Viscosity Measurements

The viscosity measurements of the complex were studied by gradually raising the concentration of complex from 10 to 100 µM keeping the concentration of CT-DNA fixed at 100 µM at a temperature of 25 ± 1 °C. The flow time for each measurement was recorded three times with the help of a stopwatch, and the average time was computed. The values were calculated using the relation ƞ = (t-to)/to, where t and to are the flow times of CT-DNA containing complex and buffer respectively. The data were plotted as (ƞ/ƞo)1/3 verus 1/R, where ƞ and ƞo are the viscosity of DNA in absence and presence of complex and R is equal to the ratio of concentration of DNA to the concentration of complex (R = [DNA]/[complex]) [31,32,33].

DNA Cleavage Study

Agar gel electrophoresis was used to detect the ability of the ligand and its metal complex to cleave supercoiled pBR322 plasmid DNA in the presence and absence of the oxidant H2O2. DNA (20 µM) and H2O2 (200 µM) concentrations were held constant in each reaction mixture, while the concentration of the complex (5–15 µM) was varied [30]. After letting the samples sit for two hours at 37 °C, 1 µL of loading buffer (Bromophenol Blue in H2O) was added to each one. The samples were then carefully loaded onto the 0.8% agarose gel electrophoresis chamber wells, along with a control sample that was just DNA. In a dark chamber for two hours at 50 V, a tris–acetic acid-EDTA buffer with a pH of 8.3 was used for the electrophoresis. The gel was photographed under UV light (280 nm) following electrophoresis [34].

Antimicrobial Activity

Using the disk diffusion technique, the antimicrobial activity of the ligand and its corresponding metal complex was evaluated (in vitro) for antibacterial activity in accordance with a previously published methodology [35]. During the course of the study, two Gram ( +) (Bacillus subtilis and Staphylococcus aureus) and two Gram (-) (Escherichia coli and Klebsiella pneumoniae) bacterial strains were examined. From Hi-media laboratory Pvt Ltd, Mumbai, India, the Nutritional Agar (NA) medium was procured. 100 ml of distilled water were used to suspend 2.8 g of NA, which was then brought to a full boil. Poured into a sterile Petri dish and allowed to set, it was autoclaved for 15 min at 15 lbs of pressure (121 ºC) to sterilize it. Sterility check was done after the agar was solidified [36]. Each bacterium's pure cultures were prepared in nutrient broth and incubated for the entire night at 37 ºC. Following incubation, a non-toxic, sterile cotton swab was dipped into the microbial growth to inoculate each of the necessary plates. By firmly pressing and rotating the swab against the tube's wall above the liquid's level, extra inoculate was eliminated. The swab was equally streaked across the plate's whole surface in three different directions to inoculate the media. The ligand and complex test solutions were prepared in DMSO at concentrations of 5, 7.5, and 10 mg/mL. The ligand and complex test solutions were prepared in DMSO at concentrations of 5, 7.5, and 10 mg/mL. The agar well diffusion method was used to conduct the assay for antibacterial activity. After the test organisms were inoculated into sterile Petri dishes, sterile nutritional agar (NA) was added. Each Petri plate was thoroughly prepared (9 mm) using a sterile cork borer. Using a sterile pipette, about 200 μl of a synthetic chemical (10 mg/ml) was added to each well (microorganism-inoculated plates), and the mixture was then incubated for 24 h at 37 ºC. Following incubation, the widths of the inhibitory zones were measured and all tested plates were inspected. Every experiment was run in three duplicates. There was a control, DMSO. The standard deviation of the mean (SD) is used to express the zone of inhibitions as mean values [37].

Molecular Docking

Auto Dock Vina programe (version 1.1.2) [38] was used to perform the molecular docking of complex with the selected protein PDB ID (https://www.rcsb.org). The receptor structure was defined as rigid and the grid dimensions varied from being 44, 50 and 80 for X,Y and Z dimensions for protein having PDB ID’s 1BNA with 1 Å spacing. Similarly, the dimensions for X, Y and Z for 5ZH8 was 62, 58 and 60, 2GCX was 40, 44 and 40, 2BH0 was 56, 54 and 40, 1XFF was 40, 46 and 48, respectively with 1 Å spacing.

Results and Discussion

The ligand and Zn (II) complex are characterized by different analytical and spectroscopic techniques (FT-IR, 1H and 13C NMR, UV–Vis, Molar conductance, SC-XRD, PXRD, and Mass Spectrometry) and their formation is depicted in scheme 1.

Scheme 1
scheme 1

Synthesis of Ligand and Zn(II) complex

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The analytical data of compounds (both ligand and complex) and physical properties are listed in Table 1. Both compounds are soluble in Methanol and DMSO. All the analytical data are consistent with the calculated value and refer to forming a mononuclear Zn (II) complex with a 1:1 molar ratio [30]. The molar conductance of the complex was measured, and the value indicates the non-electrolytic nature (Table 1) [39].

Table 1 Analytical and physical data of Ligand and Zn(II) complex
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It was possible to obtain the Ligand's Single Crystal appropriate for Single Crystal XRD. The CCDC/Deposit no. assigned to the CIF file upon deposit was 2280507. Regretfully, the crystal results agreed with Geiger et al.'s earlier research [40] which they have only published with the crystal structure. Thus, this work has covered every detail, explanation, and application pertaining to the ligand and its Zn (II) complex.

Spectroscopic Characterization

Infrared Spectroscopy

The ligand and complex's IR spectra were acquired using the KBr pellet method in the 4000–400 cm−1 spectral region, as shown in Fig. S1. This information gave structural details necessary for the complex's formation.

The schiff base ligand's azomethine v(C = N) group has a distinctively strong band at 1621 cm−1 [41]. The band shifts to 1603 cm−1 in the case of the Zn (II) complex. This suggested that azomethine N had a role in the formation of complex [39]. Furthermore, the ligand displayed a distinctive band of N–H stretching (-NH2 group) at 3453 cm−1 and 3339 cm−1 [42], which are present in the schiff base ligand. The presence of the sulfato group in the metal complex is what causes the emergence of a new medium band at 1141–1096 cm–1 [43]. The M–N bond is responsible for the 471 cm−1 in the ML complex, which is absent from the spectra of the free ligand. The new M-S bond in the synthesized complex ML may be responsible for a second band at 630 cm−1.

ESI-mass Spectrometry

The mass spectra of the ligand and complex are shown in Fig. S2a and b respectively, where the peak [M +] at m/z = 247.30 precisely matches the predicted ligand's computed value, which has the chemical formula C11H9N3O2S. The mass spectrum of the complex is also shown in Fig. 2, where the molecular weight of the Zn (II) complex C11H9N3O6S2Zn is confirmed by the peak [M +] at m/z = 408.45. The other peaks, which correspond to the other segments—C11H11N3O4SZn, C11H9N3O2SZn, and C11H9N3O2S—are located at m/z = 342.33, 312.46, and 247.30, respectively. Therefore, we may infer that the expected ligand and complex have been synthesized based on the total mass spectrum of both the ligand and the complex.

1H and 13C NMR Spectral Analysis

The 1H and 13C NMR of the ligand and complex are shown in figs. S3a, b and S4a , b respectively. The Schiff base ligand's azomethine proton (-CH = N-) displayed a peak at 8.938 ppm [44, 45]. However, this azomethine proton peak is displaced to 8.792 ppm in the case of the Zn (II) complex. This could suggest that the azomethine N and Zn (II) ion are coordinated. The ligand also had a peak at 6.526 ppm, which might be the result of the ligand's –NH2 proton. But in the synthesized Zn (II) complex, this peak shifted to 5.971 ppm, suggesting that the -NH2 group was involved in complexation [46]. In the 13C NMR Spectra of the ligand, we observed a characteristic peak at 154.168 ppm and 151.164 ppm for the C atom of the azomethine group (-CH = N-) and –NH2 group respectively. While the complex shows a clear shift in the spectra (149.778 ppm and 148.561 ppm).

Electronic Absorption Study

The UV–Vis absorption spectra of the ligand and its associated complex are shown in Fig. S5. It was measured in DMSO solvent (1 × 10–5 M) at ambient temperature. Two distinct bands were visible in the ligand's electronic absorption spectra at 296 nm and 354 nm. The phenyl ring's π-π* transitions are responsible for the higher energy band at 296 nm, while the Schiff base's azomethine (-CH = N) group's n-π* transitions are responsible for the other lower energy band at 354 nm [46, 47]. The complex's spectra similarly exhibits same absorption bands, however they experience a blue shift (hypochromic shift). This change in the spectra is explained by the complex that forms with the Zn (II) ion. The spectra lack the d-d transition due to its d10 configuration. Consequently, it suggests that the compound has a diamagnetic nature.

Powder X-Ray Diffraction

The phase purity of the ligand and complex is confirmed using powder X-ray diffraction technique. PXRD of the ligand and complex was recorded at 2θ = 5–70° range (at wavelength 1.54 Å), as shown in Fig. S6. The sharp peaks in the case of ligand indicate the crystalline nature which was missing in the case of complex.

Study of the Electronic Parameters from Computational Simulation

Figure 1 represents the optimized structure of the synthesized complex in the ethanol medium. Bond distance measurements clearly represents that Zn is stabilized by the two ‘O’ and two ‘N’ atoms. Interaction of ‘S’ with Zn (3.5434 Å) is comparatively weaker. DFT study clearly indicated that the complex is stable enough.

Fig. 1
figure 1

Optimized structure of the synthesized Complex wherein Bond distances are represented in Å

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Mulliken Charges Analysis

Mulliken charge distribution analysis helps to understand the charge accumulation on each and every atom during complex formation. Charge distribution in the synthesized complex is represented by Fig. 2(a) where the red end indicates high negative charge accumulation and the light green end indicates high positive charge accumulation. Mulliken charge analysis indicates that the positive charge accumulated on the Zn metal and it is well attached with negative charge accumulated O. Charge distribution reflects major interaction present in between ‘Zn’ and ‘O’ atom and ‘S’ atom did not contribute much more here, also reflected from bond distance values of Fig. 1.HOMO–LUMO energy gap [Fig. 2(b)] of the complex is quite small (3.36 eV) [21]. ESP map [Fig. 2(c)] indicated the surface charge of the complex. The combination of negative surface (reddish) and positive surface (bluish) well supports the Mulliken charge analysis study.

Fig. 2
figure 2

a Mulliken charge distribution; b HOMO–LUMO calculation; c Molecular Electrostatic Potential maps of the synthesized complex

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HOMO–LUMO and ESP Analysis

HOMO–LUMO energy gap of the complex is represented by Fig. 2(b). As per the calculation, HOMO–LUMO energy gap of the complex is quite small (3.36 eV) [21]. A low band gap of the complex can increase its application in the field of light sensitivity, catalysis, and optoelectronics. ESP map [Fig. 2(c)] indicates the surface charge of the complex. Sulphate ends reflects the negative surface (reddish) which combined with the Positive surface (bluish) of Zn. Combination of surface charges indicates the stability of the complex and this observation well supports the Mulliken charge analysis study.

DNA Binding study

Absorption Study

All metal complexes with DNA have their binding properties determined using electronic absorption spectroscopy. Variations in wavelength and absorbance are frequently used to study how DNA binds to metal complexes. When an incremental concentration of CT-DNA solution was added to a fixed concentration of metal complex solutions, a decrease in absorption intensity (hypochromism) at 354 nm was observed. Hypochromism, which denotes a strong contact between the electronic states of the chromophore and the DNA bases, is commonly observed when a complex binds to DNA by intercalation because of a strong bond between an aromatic chromophore and a base pair of DNA [48, 49]. The Wolfe-Shimer equation was used to calculate the intrinsic binding constant Kb, and the value of Kb was 2.7 × 104 M−1 for complex (Fig. 3).

Fig. 3
figure 3

Complex's absorption spectra in Tris–HCl buffer with added CT-DNA concentration during the titration. Inset: [DNA]/(\(\upvarepsilon\)a-\(\upvarepsilon\)f) versus [DNA] plot for binding constant (Kb)

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EB-Competitive Study with Fluorescence Emission Spectroscopy

The Zn (II) complex's DNA binding mechanism with CT-DNA was investigated using ethidium bromide (EB) displacement. The use of the planar cationic dye EB as a susceptible fluorescence molecule for inherent DNA is well known [7]. The planar phenanthridine ring of EB intercalates between neighboring DNA base pairs, resulting in the EB-DNA conjugate having a prominent emission band at 559 nm [30]. DNA intercalating probes are compounds that change the EB-DNA system's fluorescence intensity. Regardless of the binding modes, they provide a precise tool for assessing the complex's affinity for binding DNA [7]. The synthesized Zn (II) complex does not show an emission band in solution, CT-DNA, or EB when stimulated at 540 nm. Therefore, it is essential to examine the observed changes in the emission spectra of the EB-DNA solution with the introduction of complex in order to determine the EB displacement capability of the synthesized complex [49]. Figure 4 shows the anticipated emission band at 559 nm upon the addition of increasing quantities of complex to CT-DNA that had previously been treated with EB solution. The calculated quenching constant (Ksv) value for the synthesized compound is 4.5(± 0.2) × 103 M−1 (Fig. 4).

Fig. 4
figure 4

Emission spectra of EB bound to DNA both without and with increasing complex concentration during the titration. The intensity of the emission varies as the complex concentration increases (5 µM, 10 µM, 15 µM, 20 µM, 25 µM, and 30 µM). Inset: Io/I versus r plot for quenching constant (Ksv)

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Thermal Denaturation Study

Thermal denaturation studies of DNA provide additional insight into the mode of binding and binding strength of complexes into the DNA helix by providing information on the conformational alteration as the temperature is increased [50]. The thermal denaturation profile of the CT-DNA solution with and without the complex is shown in Fig. 5. The melting temperature (Tm) of the CT-DNA solution was 80 °C; however, the Tm value rose sharply to 85 °C upon the addition of complex. When complex is present, CT-DNA melts at a higher temperature (Tm = 5 °C), which supports complex intercalative binding to DNA [51].

Fig. 5
figure 5

An absorption vs temperature (°C) plot for the melting of 1) CT-DNA alone 2) CT-DNA + complex

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Viscosity Measurement

Viscosity measurements were used to investigate the complex's interaction with DNA. The spacing between the DNA base pairs existing at the intercalation site to host the binding molecule is frequently increased when DNA is intercalated with metal complexes. Consequently, the DNA helix lengthens, raising the viscosity of the DNA [52, 53]. The DNA helix lengthened and became more viscous as base pairs were split apart to create space for the binding complex in a classical interaction mode. Conversely, a partial or unconventional intercalation might bind to the DNA helix and shorten it, reducing its effective length and viscosity [54, 55]. Figure 6 illustrates how the relative viscosity of the DNA solution continuously increases as the complex concentration in the CT-DNA solution rises. The UV–Vis and fluorescence spectrum data are consistent with the increased viscosity of the complex solution, suggesting that the synthesized complex may bind to DNA via intercalation mode.

Fig. 6
figure 6

Effect of increasing amounts of a EB b Complex on the relative viscosity of CT-DNA in Tris–HCl buffer

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DNA Cleavage Study

DNA endonucleolytic cleavage processes that are initiated by metal ions are of great and increasing significance [56, 57]. The cleavage reaction on plasmid DNA (pBR322 DNA) can be seen via agarose gel electrophoresis. DNA can be broken down by the complex into its linear and nicked forms, as well as back into its super-coiled form. The intact supercoiled form of circular plasmid DNA will migrate quite quickly when electrophoresed. If one strand is scissile (nicking), the super coil will relax creating an open circular form that moves more slowly. If both strands split, a linear form that moves between Forms 1 and 2 will be produced [58] as shown in Fig. 7. The cleavage effect upon irradiation of the plasmid pBR322 DNA in the presence of different concentration of complex. In figure Lane 1 DNA alone, Lane 2 DNA + H2O2, Lane 3 DNA + ligand + H2O2 and Lane 4–6 DNA + Zn(II) complex + H2O2 with increasing concentration of complex. As the complex's concentration rises, Form 1 progressively declines and Form 2 steadily increases. The results point to concentration-dependent single-strand cleavage of the super-coiled Form1 to the nicked Form 2, according to the study. The varied binding affininty of the complex to DNA may be the cause of the varying cleaving efficacy.

Fig. 7
figure 7

Changes in the agarose gel electrophoretic pattern of pBR322 plasmid DNA induced by H2O2 for ligand and Complex. Lane (1): DNA control, Lane (2): DNA + H2O2, Lane (3): DNA + 10 μM ligand + H2O2, Lane (4–6): DNA + Zn (II) complex + H2O2, [complex] = 5, 10, 15 μM respectively

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Antibacterial Activity

Using two gram positive (Staphylococcus aureus, Bacillus subtilis) and gram negative (Escherichia coli, Klebsiella pneumonia) bacteria, the antibacterial activity of the ligand and complex was evaluated. It was highly anticipated that the ligand and complex would exhibit good action against bacteria, as seen in Fig. 8 and Table 2, given the presence of biologically active donor sites (N and S). It was discovered that the metal complex and ligand were very effective against Escherichia coli, with high inhibition zone diameters of 14 mm and 11 mm, respectively. Regarding other bacterial strains, Klebsiella pneumonia, Bacillus subtilis, and Staphylococcus aureus exhibit descending patterns in the sequence of activity of the ligand and complex. A prior article examined the effectiveness of standard drugs against several strains of bacteria. The findings indicated that minor modifications in ligand and complex could potentially improve the antibacterial activity [59, 60].

Fig. 8
figure 8

Antibacterial Activity of Ligand and complex (denoted as 2 and 2A respectively) against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Klebsiella pneumonia

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Table 2 Antibacterial screening results
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Molecular Docking

Using the Auto Dock Vina program, the complex was tested for interaction with various bacteria and DNA receptors (Fig. 9 and Table 3). Our earlier statements are further confirmed by the complex's binding affinity of -7.7 kcal/mol with protein (1BNA), which suggests that the complex interacts with DNA. Further molecular docking of the complex was done using the receptor proteins of Escherichia coli (1XFF), Bacillus subtilis (2BH0), Klebsiella pneumoniae (2GCX), and Staphylococcus aureus (5ZH8). The binding affinities were reported to be -8.0, -6.3, -6.7, and -7.6 correspondingly. Thus, the total finding suggests that the complex exhibits robust interaction with many bacterial protein receptors, claiming the previously mentioned antibacterial activity.

Fig. 9
figure 9

Complex's docking image with DNA (PDB ID: 1BNA), Staphylococcus aureus (PDB ID: 5ZH8), Bacillus subtilis (PDB ID: 2BH0), Escherichia coli (PDB ID: 1XFF), and Klebsiella pneumoniae (PDB ID: 2GCX)

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Table 3 Docking score with interaction profile
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Pharmacokinetic Properties

An in-silico theoretical ADME prediction study has been conducted on a Schiff base and its complex. For this, the web application Swiss ADME was utilized. The bioavailability score was calculated by estimating several factors using Swiss ADME software. Important pharmacokinetic factors for any chemical include its molecular weight, hydrogen donors, hydrogen acceptors, rotatable bonds, lipophilicity (Log PO/W), gastrointestinal absorption (GI), water soluble capacity (Log S), CYP1A2 inhibitor, and Blood–Brain Barrier (BBB) [61]. From Table 4, it is evident that the compounds (Ligand and complex) with bioavailability 55% have consensus liphophilicity (Log PO/W) value 1.79 and 1.24 for ligand andcomplex respectively. It is evident that the Ligand and the complex meet the requirements for druglikness and do not violate the Lipinski rule, making them viable candidates for use as drugs.

Table 4 Lipinski’s properties and pharmacokinetic properties of the ligand and complex
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Conclusion

This study reports the synthesis and characterization of a Zn (II) complex with Schiff base ligand obtained from 4-nitro-o-phenylenediamine and thiophene-2-carbadehyde. The physical measurements' result shows that the Zn (II) metal ion is coordinated by two oxygen of the sulphate ion, one azo methine nitrogen, and one sulfur atom of the thiophene group; as a result, it may adopts a distorted square pyramidal geometry. The synthesized complex has no electrolytic properties and is stable. The experiment's outcome shows that the complex exhibits remarkable interactions with CT-DNA and validates an intercalative method of binding. DFT studies clearly reflected the stabile geometry of the complex from the calculations of different electronic parameters. The pharmacokinetics study has revealed that the Ligand and Complex could act as potential drug candidate. The antibacterial activity and molecular docking reports reveals that the complex has high activity over selected bacteria.