Facile synthesis, antimicrobial activity, and molecular docking analysis of 8-hydroxyquinoline-4-thiazolidinone hybrids

ABSTRACT

Background

8-Hydroxyquinoline and 4-thiazolidinone derivatives are promising antimicrobial agents, recognized for their activity against resistant pathogens.

Aim

The aim of this study is to develop 8-hydroxyquinoline-4-thiazolidinone derivatives as potential antimicrobial agents.

Methods

Using a one-pot reaction with sodium tetrafluoroborate as an efficient and eco-friendly catalyst, compounds 6a – l were synthesized and subsequently screened for antibacterial and antifungal activity. Additionally, molecular docking and molecular dynamic simulations were performed to evaluate the active compounds and gain deeper insights into their potential as antimicrobial agents.

Results

Compounds 6f and 6 g showed superior antibacterial activity to ciprofloxacin, particularly against Gram-negative bacteria, while 6b, 6 g, and 6 h demonstrated strong antifungal effects. Molecular docking, molecular dynamics simulations, and MM-GBSA calculations highlighted strong binding interactions and stable conformations of the active compounds within binding pocket of the FabZ enzyme. The ADMET analyses further indicated that these compounds possess favorable drug-like properties.

Conclusion

The synthesized 8-hydroxyquinoline-4-thiazolidinone hybrids exhibit strong potential as broad-spectrum antimicrobial agents and merit further investigation as drug candidates.

1. Introduction

The global rise of antimicrobial and antibiotic resistance has become a critical public health challenge, threatening the effectiveness of current antibiotics and other antimicrobial therapies [1,2]. Pathogenic microorganisms, including bacteria, fungi, and viruses, are increasingly acquiring resistance to conventional drugs, leading to prolonged infections, higher mortality rates, and greater healthcare costs [3,4]. This crisis has prompted urgent efforts to discover novel classes of antimicrobial agents with mechanisms of action that can overcome existing resistance.

Among the various strategies being explored, the development of small molecules that target essential biological processes in microorganisms remains a promising approach. In particular, molecules containing the 8-hydroxyquinoline and 4-thiazolidinone moieties have gained attention due to their unique chemical structures and demonstrated biological activities [5,6]. The 8-hydroxyquinoline group is known for its metal-chelating properties, which play a key role in disrupting microbial enzymatic functions [7], while the 4-thiazolidinone ring has shown promise in inhibiting bacterial cell growth and biofilm formation [8,9]. These characteristics make them valuable scaffolds for the design of novel antimicrobial agents. Patel et al. studied the metal complexes of 1-((8-hydroxyquinolin-5-yl)methyl)-3-(thiazol-2-ylimino)indolin-2-one. Both the ligand and metal complexes were screened for antibacterial activity against Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Proteus vulgaris, and Pseudomonas aeruginosa. Metal complexes were found to be more potent bactericides than ligands. The lead 2-hydroxyquinoline derivative 1 also demonstrated excellent inhibition of both Gram-positive and Gram-negative strains (Figure 1) [10]. Rbaa et al. developed new heterocyclic derivatives of 8-hydroxyquinoline as antimicrobial agents. The compounds studied exhibited remarkable antibacterial activity, surpassing that of the standard antibiotic penicillin G. Compound 2 demonstrated particularly strong antibacterial activity against V. parahaemolyticus and S. aureus [11]. Eleni Pitta et al. reported 4-thiazolidinone derivatives as potent antimicrobial agents. Lead compound 3 demonstrated remarkable growth inhibition across a broad spectrum of Gram-positive and Gram-negative bacteria, surpassing the activity of reference drugs ampicillin and streptomycin. In addition, its antifungal efficacy was significantly higher than that of the reference drugs bifonazole and ketoconazole [12]. Desai et al. reported 4-thiazolidinone derivatives containing pyridine and quinazoline moieties act as antimicrobial agents. The compound 4 exhibited good to excellent inhibitory potency against Gram-positive bacteria compared to Gram-negative strains. Additionally, this compound showed excellent activity against S. aureus, surpassing the effectiveness of chloramphenicol, ciprofloxacin, and norfloxacin [13].

Figure 1.

Representative structures of reported 8-hydroxyquinoline and 4-thiazolidinone derivatives as antimicrobial agents, and design of novel 8-hydroxyquinoline-4-thiazolidinone hybrids (6a-1).

Recent studies have demonstrated that hybrids incorporating heterocyclic scaffolds exhibit enhanced antimicrobial activity, often surpassing that of traditional antibiotics [14,15]. The combination of two pharmacophores offers a synergistic effect, potentially targeting multiple pathways in pathogens and reducing the likelihood of resistance development [16]. Moreover, the versatility of hybrid molecules allows for easy modification to optimize their potency, selectivity, and pharmacokinetic properties, making them ideal candidates for the next generation of antimicrobial therapies [16].

The rational design of the synthesized 8-hydroxyquinoline-4-thiazolidinone hybrids is rooted in the complementary biological activities of their individual scaffolds. The 8-hydroxyquinoline moiety is well documented for its metal-chelating properties, which disrupt microbial metalloenzyme activity and interfere with essential cellular processes [7,10]. Similarly, the 4-thiazolidinone ring has demonstrated potent antimicrobial activity through mechanisms, such as inhibiting bacterial biofilm formation and impairing cell growth [8,9]. By combining these two pharmacophores into a single hybrid structure, we aim to achieve a synergistic effect that enhances antimicrobial potency while reducing the risk of resistance development. This strategy is supported by recent studies highlighting the success of hybrid molecules in targeting multiple microbial pathways and overcoming the limitations of traditional antibiotics [14,16]. Furthermore, the ease of chemical modification within these scaffolds allows for fine-tuning of physicochemical properties, paving the way for the development of optimized antimicrobial agents.

In light of the selective nature of 8-hydroxyquinolines and 4-thiazolidinone scaffolds toward antimicrobial activities, we synthesized a series of 8-hydroxyquinoline-4-thiazolidinone derivatives (Figure 1), which have not been previously reported as antimicrobial agents. Although 8-hydroxyquinoline-4-thiazolidinone derivatives have been identified as novel inhibitors of leishmanial methionine aminopeptidase 1 [17], their potential antimicrobial activity remains unexplored. Therefore, the newly synthesized compounds were screened for antibacterial and antifungal activities, using ciprofloxacin and voriconazole as reference standards. To support the antimicrobial screening, computational studies, such as molecular docking and molecular dynamics simulations, were performed to gain insights into the potential mechanisms of action of the active compounds. These studies focused on evaluating the stable interactions of the synthesized 8-hydroxyquinoline-4-thiazolidinone derivatives with the targeted protein FabZ, a critical enzyme in the fatty acid biosynthesis pathway and a validated therapeutic target in pathogens such as Pseudomonas aeruginosa and Escherichia coli. These computational analyses complemented the experimental findings, offering a comprehensive approach to understanding the antimicrobial potential of the synthesized compounds.

2. Experimental section

2.1. Materials & methods

All chemicals and solvents were of analytical grade and used without further purification, as received from the suppliers. Reactions were monitored by TLC on silica gel, using Polygram® precoated silica gel TLC sheets SIL G/UV254. Melting points were obtained on a Stuart digital melting-point apparatus (SMP 30) and are uncorrected. ¹H NMR spectra were recorded on an Avance NMR instrument operating at 500 MHz, while ¹³C NMR spectra were recorded at 125 MHz. Chemical shift values are reported in ppm with TMS as an internal reference, and J values are given in Hertz. The following abbreviations were used for ¹H NMR spectra to indicate signal multiplicity: s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). High-resolution mass spectra (HRMS) were measured on an Agilent 6520 LC/MS electrospray ionization mass spectrometer, with a TOF mass analyzer. A Power sonic 405 ultrasonic bath (Hwashin Technology, Seoul, Korea; frequency 50 hz, power 350 W) was used for ultrasonication.

2.2. General procedure for the synthesis of simple 4-thiazolidinones (3a-e)

An equimolar mixture of the amine (1.0 equiv.), thioglycolic acid (1.0 equiv.), and aldehyde (1.0 equiv.) was combined with sodium tetrafluoroborate (30 mol%) as a catalyst in acetonitrile and subjected to ultrasonic irradiation for 30 min. Upon completion of the reaction, acetonitrile evaporated under reduced pressure. An aqueous solution of sodium bicarbonate was added to neutralize any excess thioglycolic acid. The resulting precipitate was collected by vacuum filtration and recrystallized from ethanol to afford compounds 3a-e. The characterization data for all the synthesized compounds (3a – e) is provided in the supporting information.

2.3. Synthesis of 8-hydroxy-2-quinolinecarbaldehyde (4)

8-Hydroxy-2-quinolinecarbaldehyde was prepared by the oxidation of 8-hydroxy-2-methylquinoline (1.0 equiv., 31.4 mmol) with selenium dioxide (1.2 equiv., 37.6 mmol) in a mixture of dioxane (100 mL) and water (2 mL) under reflux for 24 h [17,18]. After completion of the reaction, the mixture was concentrated under vacuum and purified by column chromatography on silica gel (100–200 mesh) using 5% ethyl acetate in hexane as the eluent to give the product (4) in 70% yield.

2.4. General procedure for the synthesis of 8-hydroxyquinoline-4-thiazolidinones (6a-l)

To a stirring solution of 8-hydroxy-2-quinolinecarbaldehyde (4) (0.003 mol), substituted anilines (5a – l) (0.003 mol), and thioglycolic acid (0.003 mol) in acetonitrile (15 mL), sodium tetrafluoroborate (30 mol%) was added as a catalyst, and the mixture was subjected to ultrasonic irradiation for 30 min. After completion of the reaction (monitored by TLC), the solvent was removed under reduced pressure. The resulting residue was dissolved in ethyl acetate, and an aqueous solution of Na₂S₂O₃ was added. The organic layer was separated, washed with water, and dried over anhydrous Na₂SO₄. The solvent was then removed under reduced pressure. The residue was purified by column chromatography on silica gel (eluent: hexane/ethyl acetate, 7:3) to yield the pure title compounds (6a – l) in 58–78% yield. The characterization data for all synthesized compounds (6a – l) is provided in the supporting information.

2.5. Antibacterial and antifungal activity

The disk diffusion technique is commonly used to assess the effectiveness of antimicrobial agents, particularly for infections caused by drug-resistant pathogens. Laboratory sensitivity testing via the disk diffusion technique is an effective method for determining the susceptibility of both bacteria and fungi to various antimicrobial agents [19].

2.6. Procedure of disk diffusion technique

The disk diffusion technique, also known as the Kirby–Bauer method, is widely employed for evaluating the antibacterial and antifungal activities of compounds. For bacterial testing, an actively growing culture is first isolated in pure form and adjusted to a standardized turbidity, typically using an overnight culture in nutrient broth. This suspension is then evenly swabbed across the surface of a Mueller–Hinton agar plate to ensure a consistent bacterial lawn. After allowing the plate to dry for a few minutes, antibiotic discs are placed at equal distances on the agar surface using sterile forceps and pressed gently for full contact. The plates are incubated at 37°C for 24 h, after which zones of inhibition are observed and measured. The diameter of each clear zone around the disc is recorded in millimeters (mm) and then converted to centimeters (cm) [20]. These values are compared with a standardized chart to determine bacterial susceptibility to antibiotic tested.

For antifungal testing, the disk diffusion technique follows a similar approach, but with modifications tailored to fungal growth requirements. The pure culture of the test fungus is prepared in a suitable medium and standardized to a specific turbidity. The fungal suspension is then evenly swabbed onto Mueller–Hinton agar supplemented with Sabouraud dextrose agar. After placing antifungal discs on the agar, the plates are incubated at a lower temperature, typically 25–30°C, for 24–48 hours. Following incubation, the zones of inhibition around the discs are measured and recorded in millimeters, and then converted to centimeters [20]. This adaptation of the disk diffusion technique is effective for screening antifungal compounds and determining their activity against various fungal pathogens.

2.7. Molecular docking studies

Modeling calculations were performed using Intel (R) Xenon(R) 2 Duo CPU E7600 with the LINUX operating system [21,22]. Schrodinger software module-22–1 was used for protein and, ligand preparation, high-throughput virtual screening and molecular docking analysis [23,24]. The 3D crystal coordinates of (3 R)-hydroxyacyl-acyl carrier protein dehydratase (FabZ) from Pseudomonas aeruginosa were retrieved from the Protein Data Bank (PDB ID: 1U1Z). The protein was processed for any missing loops and added missing hydrogens and generated the lowest energy complex at pH 7.0 ± 2. The hydrogen bong optimization was performed, and the protein restrained minimization was done to get the lowest energy state using the OPLS_2005 force field. The receptor grid was generated using the centroid of the residues GLY58, ARG98, MSE56, and GLU63 of the binding pocket interacting with the flavonoid molecules as reported. The ligands for the docking were prepared from using the ligprep module. The molecules were converted from 2D to 3D and then the ionizations were generated at pH 7.0 ± 2. And the chirality was retained from the structures, and then the minimizations were performed using OPLS_2005 force field. Docking was performed with the Glide docking module using the extra precision (XP) docking protocol, and the results were analyzed. The most active molecules were further investigated for their mmGBSA binding energy calculations using the prime module with solvent model VGSB and the flexibility of protein residues at 3 Å around the ligands.

2.8. Molecular dynamic simulations and simulation MM-GBSA

Molecular dynamics (MD) simulations were conducted for the best binding poses of compounds 6f and 6 g against the FabZ binding site over 100 ns using Desmond software [24]. The system was constructed using an orthorhombic box with an SPC solvent model, ensuring a 15 Å buffer between the protein and box edges. The system was neutralized with sodium and chloride ions.

The simulation protocol began with a relaxation phase consisting of five stages: stage 1: A 100 ps simulation at 10 K with harmonic restraints (force constant of 50.0 kcal mol–1 Å–2) on solute heavy atoms, using the NPT ensemble and Brownian dynamics; stage 2: A 12 ps simulation at 10 K with harmonic restraints, using the NVT ensemble and a Berendsen thermostat; stage 3: A 12 ps simulation at 10 K and 1 atm, retaining harmonic restraints, using the NPT ensemble with a Berendsen thermostat and barostat; stage 4: A 12 ps simulation at 300 K and 1 atm, retaining harmonic restraints, using the NPT ensemble with a Berendsen thermostat and barostat. Stage 5: A 24 ps simulation at 300 K and 1 atm without harmonic restraints, using the NPT ensemble with a Berendsen thermostat and barostat. Following the relaxation, a 100 ns production phase was performed using the NPT ensemble at 300 K with a time step of 2 fs. Hydrogen bond lengths were constrained using the MSHAKE algorithm, and atomic coordinates were saved every 100 ps throughout the trajectory [24]. Analysis of the MD simulation included computation of protein and ligand RMSD values, ligand torsion evolution, and the occupancy of intermolecular hydrogen bonds and hydrophobic contacts. These analyses were performed using the Simulation Interaction Diagram tools implemented in Maestro.

The MM-GBSA throughout the simulations were performed by taking the snapshots of the simulations at specific time intervals like 1,5,15,20 … 100 and the MM-GBSA calculations were performed using the prime module, and the graphs were plotted accordingly.

2.9. ADMET parameters

The prepared ligands were used to generate the ADMET properties using the Qikprop module of the Schrödinger suite, and the generated properties were tabulated accordingly [25].

3. Results and discussion

3.1. Chemistry

In this study, we initially synthesized simple 4-thiazolidinone derivatives (3a-e) by optimizing various reaction conditions, including solvent type, temperature, and catalyst amount. These optimized conditions were then successfully applied to synthesize a series of 8-hydroxyquinoline-incorporated 4-thiazolidinone hybrids (6a-l), with consistent yields and high efficiency. The incorporation of the 8-hydroxyquinoline moiety into the 4-thiazolidinone framework was achieved through the same multicomponent approach, demonstrating the versatility of the methodology. The synthesis of simple 4-thiazolidinone derivatives using sodium tetrafluoroborate (NaBF₄) as a catalyst is shown in Scheme-1 (Figure 2(a)). Several protocols have been reported for synthesizing 4-thiazolidinones [26–29]. To address the limitations of previous methods, such as the use of hygroscopic catalysts and complicated purification, we developed a simple and efficient protocol using NaBF₄, an inexpensive, water-soluble, and recyclable catalyst [30,31], via a multicomponent reaction (MCR) strategy. We have previously reported the synthesis of 4-thiazolidinone derivatives at room temperature in dichloromethane over approximately 12 h [17], a process that is highly time-consuming. We have also developed a different class of 4-thiazolidinone derivatives using a similar MCR approach under microwave irradiation without any catalyst, completing the reaction in just 10 min [32]. Building on these advancements, we explored the use of ultrasonication, a green and efficient technique along with an eco-friendly catalyst, to further accelerate the reaction. Ultrasonication significantly reduced the reaction time to just 30 min by enhancing mass transfer and increasing the efficiency of molecular collisions, demonstrating its potential as a rapid and eco-friendly alternative for synthesizing 4-thiazolidinone derivatives.

Figure 2.

(a) Synthesis of 4-thiazolidinones using NaBF₄ as a catalyst, which is translated to the synthesis of target compounds; 8-hydroxyquinoline-incorporated 4-thiazolidinone hybrids. (b) Synthesis of 8-hydroxyquinoline-4-thiazolidinones (6a-l).

The model reaction was initially conducted using an equimolar mixture of 3,4,5-trimethoxybenzaldehyde (1a), p-anisidine (2a), and thioglycolic acid with sodium tetrafluoroborate (100 mol%) as a catalyst for THF at room temperature for 20 h. However, the desired product was obtained with low yield. To improve reaction kinetics and yield, the reaction was then carried out under ultrasonic irradiation. Under these modified conditions, the reaction completed within 30 min, and the yield significantly increased. To identify the optimal solvent, several solvent systems were screened. Although the desired product was obtained in all tested solvents, acetonitrile was found to be the most effective, providing a yield of 71% with 50 mol% catalyst loading. Further optimization of the catalyst load revealed that at 30 mol%, the yield increased to 82%, while at 20% and 10%, the yields were 67% and 52%, respectively (Table 1) Therefore, 30 mol% catalyst loading in acetonitrile under ultrasonic irradiation was selected as the optimal condition. Using these conditions, a series of 4-thiazolidinones were successfully synthesized from aromatic amines, various substituted benzaldehydes, and thioglycolic acid, and the methodology was subsequently applied to synthesize a library of 8-hydroxyquinoline-4-thiazolidinone hybrids.

Table 1.

Optimization of reaction conditions for the synthesis of simple 4-thiazolidinone derivatives.

Entry	NaBF4 (mol %)	Solvent	Condition	Yield (%)
1	100	tetrahydrofuran	rt (20 h)	30
2	100	acetonitrile	’’	35
3	100	methanol	’’	25
4	100	dimethylformamide	80°C (10 h)	<2
5	100	toluene	reflux (10 h)	15
6	100	tetrahydrofuran	ultrasonication	48
7	100	methanol	’’	40
8	100	acetonitrile	’’	64
9	100	dioxane	’’	25
10	100	water	’’	35
11	10	acetonitrile	’’	52
12	20	acetonitrile	’’	67
13	30	acetonitrile	’’	82
14	50	acetonitrile	’’	71

Conditions: amine (1.0 equiv.), aldehyde (1.0 equiv.), thioglycolic acid (1.0 equiv.).

All the synthesized simple 4-thiazolidinones were characterized by using various spectroscopic techniques, such as FT-IR, HRMS, and NMR. In the ¹H NMR spectra, chemical shifts of all the final compounds appeared in the region of 3.90–4.15 δ as two doublets (4-thiazolidinone, -CH₂ group), 5.9–6.20 δ as singlet (4-thiazolidinone, -CH), 6.80–8.50 δ as multiple peaks (aromatic protons). In ¹³C NMR spectra, chemical shifts of the final compounds appeared in the region of 32.0–33.9 δ (4-thiazolidinone, -CH₂ group), 60.0–66.8 δ (4-thiazolidinone, -CH group), 105.4–167.2 δ (aromatic carbons) and 170.1–171.6 δ (carbonyl group).

Encouraged by the results obtained from this newly developed method, the protocol was further extended to synthesize a series of novel 8-hydroxyquinoline-4-thiazolidinone hybrids (6a-l) from 8-hydroxy-2-quinolinecarbaldehyde (4). The synthesis of the target molecules (6a-l) was carried out in two steps using optimized reaction conditions, as shown in Scheme 2 (Figure 2(b)). In the first step, 8-hydroxy-2-quinolinecarbaldehyde (4) was prepared by the oxidation of 8-hydroxy-2-methylquinoline (1a) with selenium dioxide [17]. In the second step, 8-hydroxy-2-quinolinecarbaldehyde (4) underwent cyclization with substituted anilines (5a-l) and thioglycolic acid in the presence of NaBF₄ (30 mol% catalyst) in acetonitrile under ultrasonic irradiation, which was identified as the optimal condition to obtain the final 8-hydroxyquinoline-substituted 4-thiazolidinone derivatives (6a-l) in good to excellent yields.

All the synthesized target 8-hydroxyquinoline-4-thiazolidinone (6a-l) hybrids were confirmed with their melting points, IR, ¹H-NMR, ¹³C-NMR and HRMS analysis. ¹H-NMR spectra of all the compounds are showing the characteristic peaks as two doublets in the region of 3.8–4.2 δ (4-thiazolidinone, -CH₂ group), one singlet in the region of 6.2–6.5 δ corresponds to the bridge proton between quinoline moiety and thiazolidine ring, and a hydroxy proton between 7 and 8 δ region represents the 8-hydroxy quinoline group. ¹³C-NMR spectra of all compounds are showing the characteristic peaks around 33 and 66 δ (4-thiazolidine ring carbons), 170–180 δ regions (carbonyl group). The IR spectra of all the compounds are showing a broad band in the region of 3300 to 3500 corresponds to OH group, the tertiary amine showing a broad band or sometimes splatted band in the region of 2700 to 3000. The carbonyl group is showing strong stretch in the region of 1670 to 1820. The spectral data of synthesized compounds are presented in the supporting information.

3.2. Antibacterial activity

The newly synthesized compounds (6a-l) were examined for their in vitro antibacterial activity against two Gram-positive bacteria (Bacillus subtilis and S. aureus) and two Gram-negative bacteria (E. coli and P. aeruginosa). Ciprofloxacin was used as a standard control drug, and DMSO served as the diluent, showing no effect on microbial growth. The antimicrobial activities were tested at concentrations of 25 µg/µL, 50 µg/µL, and 100 µg/µL with ciprofloxacin at a concentration of 100 µg/µL as the standard. Among all the synthesized compounds, the compounds 6b, 6e, 6f, and 6 g showed greater inhibition effect at 25 µg/µL, and 50 µg/µL as shown in Figure 3(a,b), and Table-S1 in supporting information. The results reveal that compound 6f is the most potent, exhibiting activity against all four bacterial strains at 100 µg/µL. Specifically, compound 6f exhibited superior activity against E. coli, with a 4.4 cm zone of inhibition, and against P. aeruginosa, with a 4.7 cm zone, compared to the standard drug ciprofloxacin, which shows a 3.7 cm zone of inhibition for both strains. For Bacillus subtilis, 6f shows a 2.2 cm zone of inhibition, while for S. aureus, the inhibition zone is 1.4 cm. Thus, compound 6f was considered as more potent candidate than ciprofloxacin. Additionally, compound 6 g was also found to be more active than ciprofloxacin. Compound 6 g demonstrated higher activity against E. coli with a 4.1 cm inhibition zone and against P. aeruginosa with a 4.3 cm zone, both exceeding the standard ciprofloxacin with 3.7 cm zone of inhibition for both strains. However, for Bacillus subtilis, compound 6 g showed a 1.5 cm zone of inhibition, while for S. aureus, it displayed a 1.1 cm inhibition zone. Although 6f and 6 g are more potent than ciprofloxacin, they are highly selective for Gram-negative bacteria compared to Gram-positive (Figure 3(c)). In addition, compounds 6b, 6c, and 6e showed excellent inhibition activity, comparable to the standard drug, while the remaining compounds exhibited low to moderate antibacterial activity across all bacterial strains.

Figure 3.

(Continued).

3.3. Antifungal activity

The newly synthesized compounds (6a-l) were tested for their antifungal activity against Aspergillus niger and Candida albicans at concentrations of 100 µg/µL and 200 µg/µL, with voriconazole used as the reference standard. Among these, compounds 6b, 6 g, and 6 h showed excellent inhibition effects at both concentrations, as shown in Figure 3(d,e), and Table S2 in supporting information. The results indicate that compounds 6b, 6 g, and 6 h exhibited antifungal activity comparable to voriconazole against both fungal strains. Notably, at 100 µg/µL, compound 6 h demonstrated higher activity against Candida albicans with a 2.68 cm inhibition zone, compared to the standard drug voriconazole, which shows a 2.53 cm inhibition zone. However, compound 6 h also showed good activity against Aspergillus niger with a 2.58 cm inhibition zone, comparable to the standard drug voriconazole, which shows a 2.82 cm zone of inhibition. In addition, the compound 6 g exhibited antifungal activity against Aspergillus niger with a 2.78 cm inhibition zone, and against Candida albicans, with a 2.44 cm inhibition zone, comparable to the standard drug voriconazole, which shows a 2.82, and 2.53 cm zone of inhibition, respectively, for both fungal strains. Compound 6b also showed good inhibition activity against Aspergillus niger at 2.75 cm and against Candida albicans with a 2.64 cm, both are comparable to voriconazole. The remaining compounds exhibited low to moderate antifungal activity across all fungal strains.

Figure 3.

In vitro antibacterial activity of synthesized 8-hydroxyquinoline-4-thiazolidinone hybrids (6a-l) against Gram-positive bacteria (Bacillus subtilis and S. aureus) and Gram-negative bacteria (E. coli and P. aeruginosa). Antimicrobial activity was assessed at concentrations of 25 µg/µl (a), 50 µg/µl (b), and 100 µg/µl (c), with ciprofloxacin (100 µg/µl) as the standard control drug. In vitro antifungal activity was assessed against Aspergillus niger and Candida at concentrations of 100 µg/µl (d), 200 µg/µl (e), with Voriconazole (100 µg/µl, and 200 µg/µl) as the standard control drug.

3.4. Structural activity relationship (SAR)

The compounds 6f (2-methoxyphenyl) and 6 g (4-chlorophenyl) stand out as the most potent, particularly against Gram-negative bacteria (E. coli and P. aeruginosa). Compounds 6f and 6 g exhibit high potency against Gram-negative bacteria, with inhibition zones surpassing ciprofloxacin (3.7 cm inhibition zone for E. coli and P. aeruginosa). Compound 6f achieved a 4.4 cm inhibition zone against E. coli and 4.7 cm against P. aeruginosa, while 6 g showed a 4.1 cm and 4.3 cm inhibition zone, respectively. Compounds 6f (2-methoxyphenyl) and 6e (3-methoxyphenyl) displayed good antimicrobial activity, which suggests that the methoxy group (electron-donating group), especially in the ortho and meta positions, may enhance Gram-negative bacterial inhibition. This effect might be due to increased lipophilicity, allowing these compounds to interact more effectively with the outer membrane of Gram-negative bacteria. Compound 6 g (4-chlorophenyl derivative) showed high activity, likely due to the electron-withdrawing effect of chlorine. This may increase the compound overall polarity, enhancing binding interactions with bacterial targets and improving permeability in the bacterial cell wall. Compounds 6f and 6 g demonstrated antibacterial activity that exceeded ciprofloxacin against Gram-negative strains, making them promising candidates for further development as Gram-negative antibacterial agents. Whereas compound 6b (4-methylphenyl), 6c (3-methylphenyl), and 6e (3-methoxyphenyl) also exhibited antibacterial effects comparable to ciprofloxacin but did not reach the high potency as compounds 6f and 6 g, possibly due to less favorable electronic or steric configurations.

Compounds 6 h (4-bromophenyl) and 6 g (4-chlorophenyl) exhibited strong antifungal activity, with inhibition zones comparable to voriconazole. This suggests that halogen substituents in the para position enhance antifungal potency, potentially by increasing lipophilicity, which aids in cell wall penetration of fungal cells, especially for Candida albicans and Aspergillus niger. The compound 6 h shows slightly higher activity against Candida albicans (2.68 cm inhibition zone) compared to voriconazole (2.53 cm). The antifungal activity of 6b suggests that electron-donating groups (e.g., methyl) in the para position can still promote substantial inhibition, although not quite as strong as electron-withdrawing halogen groups in compounds 6 h and 6 g. Although compound 6b is less potent, the diagram shows that it exhibits a broad-spectrum antimicrobial activity profile (Figure S1).

4. Molecular docking

Type II fatty acid biosynthesis represents a promising pathway for drug development targeting microbial pathogens. The third step in this biosynthesis process involves the enzyme β-hydroxyacyl-(acyl carrier protein) (ACP) dehydratase (FabZ), which catalyzes the elongation cycle. FabZ plays a key role in the metabolism of both saturated and unsaturated long-chain acyl-ACPs in bacteria and parasites. Due to its critical function, FabZ has been identified as a potential therapeutic target in several pathogens, including P. aeruginosa, and E. coli. Given its broad inhibitory effects across different bacterial species, FabZ is regarded as an attractive target for developing antimicrobial drugs. Molecular docking studies were performed to understand the interactions of compounds 6f and 6 g with the binding site of FabZ using Schrodinger software module [21]. The 3D crystal coordinates of (3 R)-hydroxyacyl-acyl carrier protein dehydratase (FabZ) from Pseudomonas aeruginosa were retrieved from the Protein Data Bank (PDB ID: 1U1Z). The 3D ligand interaction diagrams for compounds 6f and 6 g are presented in Figure S2 and S3. The lowest energy docked poses of 6f and 6 g revealed that these compounds can easily fit into the binding site of FabZ with high binding affinities (docking score: −4.983 and −5.573, respectively). Compound 6f (Figure S2) showed three hydrogen bonding interactions between Quinoline-OH and the carbonyl O of Mse56 (selenomethionine residue), another hydrogen bonding interaction between quinoline-N and NH of Gly58 and one more H-bond between thiazolidinone carbonyl oxygen and NH of Val59. Additionally, several other hydrophobic interactions are observed stabilizing the binding pose within the active site. Compound 6 g (Figure S3) showed a hydrogen bonding interaction between the NH group of His49 acting as a donor and the hydroxy group (O) of the 8-hydroxyquinoline ring acting as an acceptor. π-π stacking interaction was observed between the Phe97 and the phenyl ring and the quinoline rings of 6 g and hydrophobic interactions of the residue established the binding pose within the active site. The other compounds were also showing similar binding mode as shown in Table-S3.

5. Molecular dynamic simulations

Molecular dynamics simulations demonstrated that the molecules remained stable within the binding pocket throughout the 100 ns simulation. The root mean square deviation (RMSD) plots of the ligand 6f and 6 g (red) and protein (blue), shown in Figures 4(a,b) and 5(a,b) illustrate the changes in RMSD and ligand conformations during the simulation within the binding site of the FabZ enzyme (PDB ID: 1U1Z) along with changes in the MM-GBSA binding energy throughout the 100 ns simulation. Compounds 6f and 6 g achieved stable conformations in the binding pocket, with fluctuations in RMSD around 1.5 Å during perturbations, while retaining key interactions with surrounding amino acids. The analysis of the protein–ligand interactions revealed key interacting residues and their interaction types, as shown in the plots for compound 6f and 6 g (Figures 4(c,d) and 5(c,d)). Additionally, the changes in the root mean square fluctuation (RMSF) of the protein upon ligand binding were evaluated, with green lines indicating specific residues involved in interactions with the ligand (Figures 4(e) and 5(e)). The ligand properties, including RMSD, radius of gyration, intramolecular hydrogen bonding, molecular surface area (MolSA), solvent-accessible surface area (SASA), and polar surface area (PSA), were also assessed, providing insights into the stability and conformational behavior of the selected compounds (Figures 4(f) and 5(f)).

Figure 4.

(a) Plot shows the changes in RMSD of ligand (6f) (red), and protein (blue) in the 100 ns MD simulation against FabZ (PDB ID: 1U1Z). (b) Plot showing the changes in the mmGBSA binding energy throughout the 100 ns simulation. (c,d) Plot shows interacting residues and type of interaction with the ligand (6f). (e) Plot showing the changes in the RMSF of the protein with the green lines indicating the interactions with the ligand. (f) Plot showing Ligand properties, including RMSD, the radius of gyration, intramolecular hydrogen bonding, molecular surface area (MolSA), solvent accessible surface area (SASA), and polar surface area (PSA), are presented for the selected compounds.

Figure 5.

(a) Plot shows the changes in RMSD of ligand (6 g) (red), and protein (blue) in the 100 ns MD simulation against FabZ (PDB ID: 1U1Z). (b) Plot showing the changes in the mmGBSA binding energy throughout the 100 ns simulation. (c,d) Plot shows interacting residues and type of interaction with the ligand (6 g). (e) Plot showing the changes in the RMSF of the protein with the green lines indicating the interactions with the ligand. (f) Plot showing Ligand properties, including RMSD, the radius of gyration, intramolecular hydrogen bonding, molecular surface area (MolSA), solvent accessible surface area (SASA), and polar surface area (PSA), are presented for the selected compounds.

6. ADMET Properties

The physicochemical properties of the synthesized molecules using Qikprop were determined as shown in Table 2 [25], to evaluate the absorption, distribution, metabolism, and excretion (ADME) properties, as well as toxicity (Tox) characteristics, of the active compounds 6f and 6 g. Both compounds demonstrate drug-likeness, as indicated by their favorable water and lipid solubility coefficients. They also show optimal water solubility, promising gastrointestinal absorption, and high bioavailability scores, suggesting their potential for effective oral administration and systemic availability.

Table 2.

Physicochemical properties of the synthesized molecules using qikprop.

Title	MW	SASA	donorHB	Accept HB	QPlogPo/w	QPlog HERG	#metab	% Human Oral Absorption	Rule Of Five
6a	322.381	552.342	1	5.25	3.324	−5.474	3	100	0
6b	336.408	591.095	1	5.25	3.662	−5.61	4	100	0
6c	336.408	584.044	1	5.25	3.63	−5.442	4	100	0
6d	352.407	593.273	1	6	3.268	−5.437	4	100	0
6e	352.407	590.911	1	6	3.425	−5.467	4	100	0
6f	352.407	604.685	1	6	3.348	−5.724	4	100	0
6 g	356.826	576.182	1	5.25	3.813	−5.433	3	100	0
6 h	401.277	581.279	1	5.25	3.888	−5.471	3	100	0
6i	401.277	581.129	1	5.25	3.887	−5.469	3	100	0
6j	364.461	638.855	1	5.25	4.292	−5.646	4	100	0
6k	414.478	676.276	1	5.75	4.842	−6.738	3	100	0
6 l	340.371	561.579	1	5.25	3.557	−5.374	3	100	0
Cipro	331.346	576.612	1	6	0.28	−3.251	0	48.734	0

7. Conclusion

In summary, this study presents a rapid, one-pot synthesis of 8-hydroxyquinoline-incorporated 4-thiazolidinone hybrids using sodium tetrafluoroborate as an efficient and eco-friendly catalyst. The synthesized compounds demonstrated significant antimicrobial potential, with several compounds like 6f and 6 g showing superior antibacterial activity compared to the standard ciprofloxacin, particularly against Gram-negative bacteria. Additionally, compounds 6b, 6 g, and 6 h exhibited promising antifungal properties, underscoring the broad-spectrum potential of these hybrids. Molecular docking analysis confirmed the strong binding affinity of compounds 6f and 6 g within the active site of the FabZ enzyme, offering valuable insights into their potential mechanism of action. Molecular dynamics simulations and MM-GBSA calculations further demonstrated the stable conformations of these active compounds within the enzyme binding pocket. Additionally, ADMET profiling revealed favorable drug-likeness characteristics, highlighting these hybrids as promising candidates for further development as antimicrobial agents.

Funding Statement

The authors did not receive any funding for this study.

Article highlights

• A one-pot multicomponent reaction was employed to synthesize 8-hydroxyquinoline-4-thiazolidinone hybrids using an ultrasonication process.

• Ultrasonication, combined with an eco-friendly catalyst, was employed as a green and efficient technique to significantly accelerate the reaction process.

• The inhibitory potential of all synthesized compounds was evaluated against Gram-positive, Gram-negative bacteria, and fungi.

• Compounds 6f, and 6 g showed excellent inhibition against Gram-negative bacteria superior to standard Ciprofloxacin.

• Compounds 6 g, and 6 h showed good inhibition against fungi, which is comparable to the standard voriconazole.

• Structure–activity relationship analysis showed that compounds 6f (2-methoxyphenyl) and 6e (3-methoxyphenyl) displayed good antimicrobial activity, which suggests that the methoxy group (electron-donating group), especially in the ortho and meta positions, may enhance Gram-negative bacterial inhibition.

• Molecular docking analysis was also conducted to understand ligands 6f and 6 g interaction with the active site of the FabZ enzyme. According to their ADMET profiles, the active compounds demonstrate good drug-likeness.

• Furthermore, molecular dynamics simulations confirmed that compounds 6f and 6 g achieved stable conformations within the binding pocket of the FabZ enzyme.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/17568919.2025.2463876