Spectral Characterization and Antimicrobial Activity of Chenodeoxycholic Acid Complexes with Zn(II), Mg(II), and Ca(II) Ions

Abstract

Chenodeoxycholic acid (CA) is a naturally occurring bile acid that is produced in the liver from cholesterol. Three CA complexes using Zn(II), Mg(II), and Ca(II) ions were synthesized to examine the chelation tendencies of CA towards these metal ions. The complexation reaction of CA with the metal ions under investigation was conducted with a 1:1 molar ratio (CA to metal) at 60–70 °C in neutralized media, which consisted of a binary solvent of MeOH and H₂O (1:1). The resulting CA complexes were characterized using elemental data (metal, H, C, and Cl analysis) and spectral data (UV–visible, FT-IR, and ¹H NMR). The results suggested that CA in anion form utilized oxygen atoms of the carboxylate group (-COO⁻) to capture Zn(II), Mg(II), and Ca(II) ions. This produced complexes with the general compositions of [Zn(CA)(H₂O)Cl], [Mg₂(CA)₂(H₂O)₄Cl₂], and [Ca₂(CA)₂(H₂O)₄Cl₂]·2H₂O, respectively. The Kirby–Bauer disc diffusion assay was then used to explore the bioactivity of the CA complexes toward three fungal species (Aspergillus niger, Candida albicans, and Penicillium sp.), three Gram-positive bacteria (Staphylococcus aureus, Streptococcus pneumoniae, and Bacillus subtilis), and two Gram-negative bacteria (Pseudomonas aeruginosa and Escherichia coli). The Ca(II) and Mg(II) complexes exhibited marked inhibitory effects on the cell growth of the fungal species Aspergillus niger with potency equal to 127 and 116% of the activity of the positive control, respectively. The Zn(II) and Ca(II) complexes strongly inhibited the growth of Penicillium sp., while the Zn(II) and Mg(II) complexes showed strong growth inhibition towards the Gram-negative species Pseudomonas aeruginosa.

1. Introduction

Bile acids (BAs) are essential factors in lipid metabolism and end products of cholesterol catabolism. Catabolism of cholesterol is the most important route for removing surplus cholesterol from the circulation [1]. The therapeutic uses of BAs have been recognized since the early 1970s. At that time, chenodeoxycholic acid (CA) and ursodeoxycholic acid were shown to be effective in solubilizing cholesterol stones. For almost two decades thereafter, they were extensively used in the treatment of gallbladder stones. In the middle of the 1990s, the clinical applications of CA and ursodeoxycholic acid declined because laparoscopic cholecystectomy was being used in therapies for gallstone dissolution. Thus, the clinical relevance of BAs in removing gallbladder stones has decreased [2]. CA or chenodiol (Figure 1) is a naturally occurring bile acid that is generated in the liver from cholesterol via several enzymatic steps. The first portion of CA’s name, “cheno”, comes from a Greek term meaning “goose” because CA was first isolated from the bile of the domestic goose. CA was used therapeutically in dissolving cholesterol stones in the gallbladder due to its ability to maintain cholesterol solubility in bile [3]. Nowadays, CA is used in investigations on its effects on the small-intestinal absorption of bile acids and as a long-term replacement therapy for cerebrotendinous xanthomatosis [4]. Considerable attention has been devoted to the chemistry of the interactions of metal ions with natural and biological active compounds in medicine, pharmacology, chemistry, and biology. Complexation data on the metal-based compounds resulting from these interactions are important for a wide range of applications in many fields, including pharmaceutical and medical applications, as well as improving and designing more biologically active drugs [5,6,7,8,9,10,11,12]. For example, several platinum-based compounds have been approved as broad-spectrum drugs for the treatment of solid tumors of the testicles, ovaries, and bladder [13,14,15,16]. Unfortunately, resistance can be acquired to some of these drugs, and some have severe side effects [17,18,19]. Therefore, researchers are investigating the interactions of metal ions with natural and vital compounds to discover bio-active metal-based compounds with anticancer, antifungal, or antibacterial properties, as well as lower toxicity, lower drug resistance, and higher efficacy. Metal complexes of Ca(II), Mg(II), and Zn(II) ions have been widely discussed and reported [20,21,22,23,24,25,26].

In our previous work [27], we reported the reaction between Fe(III), Zn(II), Ca(II), and Mg(II) ions with the quinolone antibiotic oxolinic acid (OA). The resulting complexes were [Fe(OA)(H₂O)₂Cl₂]·2H₂O, [Zn(OA)(H₂O)Cl]·2H₂O, [Ca(OA)(H₂O)Cl], and [Mg(OA)(H₂O)Cl]. In these complexes, the OA molecule captured the Fe(III), Zn(II), Ca(II), and Mg(II) ions using the two oxygen atoms of the carboxylate and pyridone C=O groups. The [Ca(OA)(H₂O)Cl] complex displayed remarkable antifungal and antibacterial activity against all tested microbial strains. The [Ca(OA)(H₂O)Cl] complex exhibited inhibitions of 23.0, 20.0, and 20.0 mm/mg against the fungal species C. albicans, Penicillium sp., and A. niger, respectively. The [Ca(OA)(H₂O)Cl] complex exhibited inhibition against the Gram-negative and Gram-positive microbes, measuring 27.0, 22.0, 20.0, 17.0, and 18.0 mm/mg for P. aeruginosa, E. coli, S. aureus, S. pneumoniae, and B. subtilis species, respectively.

In another work [28], we examined the chemical reaction between two widely used fluoroquinolones antibiotics lomefloxacin (F1) and pefloxacin (F2) with Fe(III), Zn(II), Ca(II), and Mg(II) ions. Complexes of F1 were formulated [FeF1(H₂O)₂Cl₂]·Cl·2H₂O, [ZnF1(H₂O)Cl], [CaF1(H₂O)Cl]·3H₂O, and MgF1(H₂O)Cl]·2H₂O. Complexes of F2 were formulated [FeF2(H₂O)₂Cl₂]·Cl·2H₂O, [ZnF2(H₂O)Cl], [CaF2(H₂O)Cl]·3H₂O, and [MgF2(H₂O)Cl]·2H₂O. In capturing the Fe(III) ion, the F1 and F2 ligands used the two nitrogen atoms of the piperazine ring, while in capturing the Zn(II), Ca(II), and Mg(II) ions, the ligands used the oxygen atoms of the carboxylate group and the pyridone C=O group. The screening data indicated that the [FeF1(H₂O)₂Cl₂]·Cl·2H₂O is the only complex that displayed remarkable activity against all examined fungi (A. niger, Penicillium sp., and C. albicans). The [FeF1(H₂O)₂Cl₂]·Cl·2H₂O complex displayed remarkable activity against S. aureus and S. pneumoniae strains, while the [MgF2(H₂O)Cl]·2H₂O complex displayed remarkable activity against S. pneumoniae and B. subtilis strains.

The aim of the current work was to examine the chelation tendency of CA towards Zn(II), Mg(II), and Ca(II) ions, as well as the resulting morphology and effects on fungi and bacteria species. The CA complexes with Zn(II), Mg(II), and Ca(II) metal ions were synthesized in a 1:1 molar ratio (metal to CA) in a binary solvent of H₂O and MeOH (1:1) at 60–70 °C. The elemental results and spectra obtained from Fourier-transform infrared (FT-IR) spectroscopy were used to characterize the resultant CA complexes. Outer surface-related information about the CA complexes were collected from images captured by scanning and transmission electron microscopies (SEM and TEM, respectively).

The antifungal and antibacterial activities of the resultant CA complexes were assessed in vitro against three fungi species and two types of bacteria (Gram-positive and Gram-negative) species. The inhabitation was examined using the Kirby–Bauer disc diffusion assay technique. Streptomycin and ketoconazole were used as standard drugs for comparison antibacterial and antifungal results, respectively.

2. Chemicals and Methods

2.1. Chemicals

All chemicals and solvents for the preparation of metal-based complexes were analytical or spectroscopic grade. They were obtained with the highest purity available (98–99.9%) and were used as received from commercial chemical sources (Fluka (Lausanne, Switzerland) and Sigma-Aldrich (St. Louis, MO, USA)). CA (C₂₄H₄₀O₄; 392.57 g/mol) was used as the ligand, while ZnCl₂ (136.30 g/mol), MgCl₂ (95.21 g/mol), and CaCl₂ (110.98 g/mol) were used as sources of metal ions.

2.2. Preparation and Characterization

The Zn(II), Mg(II), and Ca(II) complexes with CA were synthesized by a simple procedure involving dissolution of the ions (2 mmol) in deionized water (25 mL). A beaker containing an aqueous solution of a metal ion was placed on a hot plate with a magnetic stirrer and stirred for 2–3 min. A methanolic solution of CA (2 mmol in 25 mL MeOH) was introduced dropwise. The temperature of the mixture was adjusted to the range 60–70 °C, and the mixture was stirred for 5 min. The mixture was neutralized using drops of 5% NH₃ solution, and then yellow precipitates began to form. The mixture was stirred for an additional 10 min and left overnight for completion of the precipitation process. All of the precipitates were removed from the reaction vessels, and any unreacted starting materials were removed by washing with deionized water and organic solvents (methanol and diethyl ether). They were then rinsed with 10 mL of solvent to remove any unreacted starting materials. The resulting CA complexes were oven dried at 120°.

The purified CA complexes were then subjected to elemental analyses using a PerkinElmer 2400 Series II elemental analyzer to determine the contents of carbon and hydrogen. The chlorine content was determined using precipitation titration (Mohr’s Method). The water and metal contents were determined gravimetrically [29]. The purified CA complexes were also subjected to spectral analyses using a PerkinElmer Lambda 25 UV/Vis spectrophotometer, a Shimadzu FT-IR spectrophotometer, and a Bruker DRX-250 Digital FT-NMR spectrometer to obtain the complexes’ UV–visible, FT-IR, and ¹H NMR spectra, respectively. IR spectra were collected from 4000 to 400 cm⁻¹ at 2 cm⁻¹ resolution and 30 scans at room temperature. ¹H NMR spectra were collected at 600 MHz using DMSO-d6 as the solvent and TMS as the internal reference at room temperature.

2.3. XRD Spectra and Microscopic Images

The phase purity of the synthesized CA complexes was visualized using an X’Pert Philips X-ray powder diffractometer (XRD). The XRD spectra were collected within the diffraction-angle (2θ) range of 5 to 90° using a Cu Kα₁ radiation source (λ = 0.154056 nm). The texture and morphological features of the CA complexes were observed using a Quanta FEI 250 scanning electron microscope (SEM) and a JEOL JEM-1200 EX II transmission electron microscope (TEM). The accelerating voltage was 20 kV for the SEM analysis and 60–70 kV for the TEM analysis.

2.4. In Vitro Antimicrobial Experiments

In vitro antifungal screening of the CA complexes was performed using cultures of Penicillium sp., Aspergillus niger, and Candida albicans. In vitro antibacterial assays of the CA complexes were performed using cultures of two types of bacteria: (i) Gram-positive species (Streptococcus pneumoniae, Bacillus subtilis, and Staphylococcus aureus) and (ii) Gram-negative species (Pseudomonas aeruginosa and Escherichia coli).

All the tested microbes were clinical isolates. The inhibition was examined using the Kirby–Bauer disc diffusion assay technique [30,31,32]. Antibiotic discs containing ketoconazole (an antifungal agent) and streptomycin (an antibacterial agent) were employed as the positive controls for comparison of the antifungal and antibacterial results, respectively. The bacteria and fungi were grown in appropriate fresh medium to reach a desired volume (1 × 10⁸ cells/mL on average), and then the actively growing microbes were spread in a Petri dish containing nutrient agar medium. The dishes were left to solidify, and then a sterile cork borer was used to make four to six holes. A total of 0.1 mL of each complex at a concentration of 100 µg/mL in dimethyl sulfoxide (DMSO) was poured into these holes. The dishes were incubated for 48 h at 37 °C for bacteria and for 5 days at 28–30 °C for fungi [33,34,35]. Light inhibition zones were observed around the holes and used to measure the diameter of the inhibition zones in millimeters with a Vernier caliper. The screening was performed in triplicate, and each inhibition zone value represents the mean of three independent measurements.

3. Results and Discussion

3.1. Chemistry

3.1.1. Elemental Compositions

The metal-based complexes of CA were synthesized by dissolving CA in MeOH solvent and dissolving the metal chloride salts of Zn(II), Mg(II), and Ca(II) ions in deionized water. No precipitates were seen when mixing the solution of CA with the solutions of each metal ion until the mixture became neutralized. At this point, the CA molecule (C₂₄H₄₀O₄) loses the hydrogen atom of the -COOH group and becomes an anion (C₂₄H₃₉O₄⁻; CA⁻). The CA⁻ anion captures the metal ions and forms yellow crystals with the compositions described in the following:

• (i). Zn(II) complex:

Yellow crystals; C₂₄H₄₁O₅ZnCl; molecular weight, 510.40 g mol⁻¹; elemental contents (%): found (calculated) for C, 56.22 (56.43); H, 7.95 (8.03); Cl, 7.09 (6.95); Zn, 12.72 (12.81); Water, 3.65 (3.53).

• (ii). Mg(II) complex:

Yellow crystals; C₄₈H₈₆O₁₂Mg₂Cl₂; molecular weight, 974.65 g mol⁻¹; elemental contents (%): found (calculated) for C, 58.85 (59.10); H, 8.95 (8.82); Cl, 7.16 (7.27), Mg, 5.15 (4.99); Water, 7.30 (7.39).

• (iii). Ca(II) complex:

Yellow crystals; C₄₈H₉₀O₁₄Ca₂Cl₂; molecular weight, 1042.19 g mol⁻¹; elemental contents (%): found (calculated) for C, 55.40 (55.27); H, 8.53 (8.64); Cl, 6.95 (6.80), Ca, 7.86 (7.69); Water, 10.28 (10.36).

The elemental results suggest that the reaction stoichiometry is 1:1 (CA to metal) and comply with the suggested general compositions of the complexes obtained with Ca(II), Mg(II), and Zn(II) ions, which are [Ca₂(CA)₂(H₂O)₄Cl₂]·2H₂O, [Mg₂(CA)₂(H₂O)₄Cl₂], and [Zn(CA)(H₂O)Cl], respectively.

3.1.2. UV–Visible Spectra

The UV–visible spectra of free CA and its complexes were recorded over the wavelength range of 200–1100 nm using DMSO solutions at concentrations of 1 × 10⁻³ M. The obtained spectra are shown in Figure 2. The DMSO solution of free CA exhibited a strong absorption band at 313 nm. This band had a medium-intensity shoulder at 270 nm. The main absorption band was attributed to n→π* transitions, while the shoulder was associated with π→π* transitions. The intensity of the main absorption band was increased when the CA complexed with the metal ions and became much broader. The width of the main band of free CA was around 88 nm, but for the Ca(II), Mg(II), and Zn(II) complexes, the widths were ~200, ~250, and ~340 nm, respectively. The maximum wavelength (λ_max) of the main band was slightly shifted from 313 nm for free CA to ~315 nm for the complexes. The shoulder of the main band of free CA was still observed in the UV–visible spectra of the complexes at approximately the same position. However, new shoulder bands appeared in the spectra of the complexes. The complex of the Zn(II) ion had two shoulder bands at 395 and 413 nm. The Mg(II) complex displayed a shoulder band at 375 nm. The Ca(II) complex presented two shoulder bands at 370 and 412 nm. All of these shoulder absorptions may be assignable to the metal-to-ligand charge transfer band (MLCT) [36,37].

3.1.3. FT-IR and ¹H NMR Spectra

The FT-IR spectra of free CA and its complexes were recorded at wavenumbers of 400–4000 cm⁻¹, as shown in Figure 3. Free CA contains one carboxylic group (-COOH), two hydroxyl groups (-OH), ten methylene moieties (-CH₂), eight -CH moieties, and three methyl groups (-CH₃). The hydroxyl groups and the carboxylic acid O–H were responsible for the strong, broad absorption bands observed in the FT-IR spectrum of free CA. The bands were centered at ~3590 and 3255 cm⁻¹ and could be attributed to ν(O–H) vibrations of alcoholic and carboxylic groups, respectively. The carboxyl C=O of the -COOH group was responsible for the medium absorption bands located at 1735 and 1830 cm⁻¹ and could be attributed to ν(C=O)_COOH vibrations. Methylene (-CH₂) scissoring (δ_scissCH₂), rocking (δ_rockCH₂), wagging (δ_wagCH₂), and twisting (δ_twistCH₂) vibrations occurred at 1377, 1267, 920, and 660 cm⁻¹, respectively. Methyl (-CH₃) scissoring (δ_scissCH₃), rocking (δ_rockCH₃), wagging (δ_wagCH₃), and twisting (δ_twistCH₃) vibrations appeared at 1438, 1312, 1000, and 820 cm⁻¹, respectively. ν(C–O) vibration occurred at 1173 cm⁻¹, and ν(C–C) vibration occurred at 1050 cm⁻¹ [38,39]. In the FT-IR spectra of the Zn(II), Mg(II), and Ca(II) complexes, the absorption bands due to the to ν(C=O)_COOH vibrations, and the ν(O–H) vibrations of carboxylic group were no longer found. The absence of these characteristic absorption bands resulted from the deprotonation of the -COOH group. The CA⁻ anion utilized the carboxylate group (-COO⁻) to capture the metal ions, which generated two new absorption bands in the FT-IR spectra of the complexes. These were attributed to the characteristic carboxylate bands. The first band appeared at 1555, 1552, and 1550 cm⁻¹ in the spectra of Zn(II), Mg(II), and Ca(II) complexes, respectively, and could be attributed to ν_asym(COO⁻) vibrations of the carboxylate group. The second band was located at 1410, 1414, and 1415 cm⁻¹ in the spectra of Zn(II), Mg(II), and Ca(II) complexes, respectively, and could be attributed to ν_sym(COO⁻) vibrations of the carboxylate group. The band-frequency differences between the ν_asym(COO⁻) and ν_sym(COO⁻) values [∆ν= ν _asym − ν _sym] for Ca(II), Mg(II), and Zn(II) complexes were 135, 138, and 145 cm⁻¹, respectively. These ∆ν values were lower than that of the free CA⁻ anion. According to the Nakamoto criterion [40], the coordination mode of the CA carboxylate group is bidentate chelating. The multiple -CH, -CH₂, and -CH₃ moieties generated strong absorption bands at (2930 and 2865 cm⁻¹) for the Zn(II) complex, (2926 and 2864 cm⁻¹) for the Mg(II) complex, and (2932 and 2868 cm⁻¹) for the Ca(II) complex, which could be attributed to ν_asym(C–H) and ν_sym(C–H) vibrations. The bands at 495 cm⁻¹ for the Zn(II) complex, 485 cm⁻¹ for the Mg(II) complex, and 490 cm⁻¹ for the Ca(II) complex were due to ν(M–O) vibrations [40]. The disappearance of the characteristic FT-IR band associated with the ν(C=O)_COOH vibrations, and the appearance of the characteristic carboxylate FT-IR bands (ν_asymCOO⁻ and ν_symCOO⁻) suggest the deprotonation of the -COOH group. The CA⁻ anion utilized the oxygen atoms of the resulting carboxylate group (-COO⁻) to capture the Zn(II), Mg(II), and Ca(II) ions.

Figure 4 shows the ¹H NMR spectrum of the CA complex with Zn(II) ions. In this spectrum, the signals at 3.48–3.75 ppm could be attributed to the protons of the -CH₃ and -CH moieties, while the signals at 6.62 to 7.58 ppm could be assigned to the protons of the -CH₂ moieties. The ¹H NMR spectrum of free CA contained a single sharp signal resonating at δ ~11.9 ppm due to the proton of the -COOH group [39]. This signal was no longer found in the spectrum of the Zn(II) complex. The disappearance of the ν(C=O)_COOH characteristic FT-IR band, the absence of the characteristic ¹H NMR -COOH proton signal, and the appearance of the characteristic carboxylate FT-IR bands (ν_asymCOO⁻ and ν_symCOO⁻) suggest the deprotonation of the -COOH group. The CA⁻ anion utilized the oxygen atoms of the resulting carboxylate group (-COO⁻) to capture the investigated metal ions. Based on the elemental and spectral observations, chemical structures for the synthesized CA complexes are proposed, as shown in Figure 5. An octahedral configuration for Mg(II) and Ca(II) complexes was proposed in a dimeric structure with chloride as bridge.

3.2. Morphology

XRD spectra and microscopic images obtained by SEM and TEM were used to observe the phase purity, texture, and morphological features of CA complexes, and the results are presented in Figure 6, Figure 7 and Figure 8. The synthesized CA complexes displayed a single, narrow, and very strong diffraction line in their XRD spectra. This prominent diffraction line was precisely located at 2θ values of 32.647, 8.598, and 16.621° in the XRD spectra of Zn(II), Mg(II), and Ca(II) complex, respectively. These XRD patterns indicate the formation of well-crystallized complexes. The XRD profile of Zn(II) complex displayed five low-intensity lines that occurred in a wide range (approximately 20 to 60°), with the most intense lines appearing at 22.736 and 58.051°. The XRD spectrum of Mg(II) complex exhibited multiple low-intensity lines in the range of 10 to 40°, and the most intense lines in this range were located at 12.066, 18.078 and 21.607°. The complex of Ca(II) ion exhibited fewer low-intensity lines compared to the Mg(II) complex, with the most intense lines appearing at 28.461 and 37.715°. Generally, the XRD patterns of Zn(II) and Ca(II) complexes show diffraction lines at high 2θ angles. This might indicate the presence of some impurities.

Both SEM and TEM images revealed that all complexes had well-crystallized structure. The complexation of CA with the investigated ions led to particles with long rod-like shapes. The heads of these rods were sharp and resembled a knife. A few rods were seen individually, but most of them accumulated together and formed individual units. Several broken rods were seen in Ca(II) and Mg(II) complexes in particular. Some of the Mg(II) complex particles showed no complete development into long or short rods.

3.3. Biological Effects

In vitro antifungal screening of the free CA and its complexes was performed using cultures of three fungi species. In vitro antibacterial assays of the free CA and its complexes were performed using cultures of two types of bacteria: (i) Gram-positive species, and (ii) Gram-negative species. The antibiotic drugs ketoconazole and streptomycin (positive controls) were used to compare the antifungal and antibacterial results, respectively. Table 1 indicates the inhibition zones created by the positive controls, free CA and its complexes towards the tested microbes are listed in, and a diagram is shown in Figure 9.

The zone of inhibitions for the free CA against all the tested microbes were in the range of 5–9 mm/mg. The synthesized CA complexes were inactive or displayed a very low inhibitory effect against the fungal species Candida albicans. Zn(II) and Ca(II) complexes showed strong activity against Penicillium sp. with slightly lower inhibition than that of the positive control (17.0 for Zn(II) complex; 18.0 for Ca(II) complex; 21.0 mm/mg for the positive control). Mg(II) and Ca(II) complexes exhibited excellent activity against A. niger. Their zones of inhibition of were 23.0 and 21.0 mm/mg, indicating 127% and 116% of the activity of the positive control, respectively. The synthesized Zn(II), Mg(II), and Ca(II) complexes exhibited low to moderate activity against all the investigated Gram-positive bacterial species (B. subtilis, S. pneumoniae, and S. aureus). The zones of inhibition observed for the positive control against them were 18.0, 17.0, and 20.0 mm/mg, respectively. The zone of inhibitions for the synthesized complexes against these microbes were in the range of 4–12 mm/mg. All complexes showed moderate lethality against the Gram-negative species E. coli with zones of inhibition in the range of 9–15 mm/mg. Mg(II) and Zn(II) complexes strongly inhibited the growth of the Gram-negative species P. aeruginosa with zones of inhibition of 22.0 and 20.0 mm/mg, respectively. The zone of inhibition of the positive control against this species was 27.0 mm/mg. In general, the complexes are more potent than the free CA against all the investigated microbes.

The augmented antimicrobial activity of the synthesized CA complexes compared with the free CA can be interpreted based on Overtone’s and Tweedy’s chelation theories [41,42]. Cells are surrounded by a lipid membrane, and these lipid layers allow the passage of only lipid-soluble components. Complexation of metal ion with a ligand reduced the polarity of the metal ion to a greater extent due to the partial sharing of the metal positive charge with the donation sites in the ligand and the overlap of the ligand orbital. This increases the lipophilicity of the formed metal complex and enhances the penetration of the metal complex via lipid membranes and blocking of the metal binding sites on the enzymes of the microorganism [43,44,45,46].

4. Conclusions

Complexes of one of the naturally occurring bile acids, CA, with metal ions (Ca(II), Mg(II), and Zn(II)) were synthesized. Elemental and spectral results showed that the complexation stoichiometry was 1:1 (metal to CA), CA coordinated towards the metal ions at the carboxylate-group oxygen atoms, and the complexes were formulated as [Ca₂(CA)₂(H₂O)₄Cl₂]·2H₂O, [Mg₂(CA)₂(H₂O)₄Cl₂], and [Zn(CA)(H₂O)Cl]. The Mg(II) and Ca(II) complexes had excellent inhibitory effects on the cell growth of A. niger with a potency exceeding that of the positive control. The Zn(II) and Ca(II) complexes showed strong inhibitory effects on the cell growth on Penicillium sp. species, while the Zn(II) and Mg(II) complexes displayed strong inhibitory effects on the cell growth of P. aeruginosa. In future works, we plan to examine the cytotoxic potency of CA complexes toward various human cancer-cell lines to provide wide-spectrum data on the pharmacological effects of these complexes.

Author Contributions

Conceptualization, M.A.A.-O. and M.A.B.; methodology, M.A.A.-O., M.A.B. and M.S.R.; software, M.A.A.-O. and M.A.B.; validation, F.S.A. and F.A.A.; formal analysis, F.S.A. and F.A.A.; investigation, F.S.A. and F.A.A.; resources, A.S.A.-W. and A.M.A.A.; data curation, A.S.A.-W. and A.M.A.A.; writing—original draft, A.S.A.-W. and A.M.A.A.; writing—review and editing, A.M.A.A.; visualization, M.S.R.; supervision, A.A.A., A.M.N. and M.S.R.; project administration, A.A.A. and A.M.N.; funding acquisition, A.A.A. and A.M.N. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors extend their appreciation to the Deputyship for Research and Innovation, Ministry of Education in Saudi Arabia for funding this research. (IFKSURC-1-0129).

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

Figure 1. Chemical structure of chenodeoxycholic acid (CA).

Figure 2. The UV–visible spectra of free ligand and the synthesized complexes.

Figure 3. The FT-IR spectra of the synthesized CA complexes: (A) Zn(II) complex, (B) Mg(II) complex, and (C) Ca(II) complex.

Figure 3. The FT-IR spectra of the synthesized CA complexes: (A) Zn(II) complex, (B) Mg(II) complex, and (C) Ca(II) complex.

Figure 4. The 1H NMR spectrum of Zn(II) complex.

Figure 5. Proposed formulations of the CA complexes [M: Mg(II) and Ca(II) ions].

Figure 6. XRD spectrum, SEM and TEM images of Zn(II) complex.

Figure 7. XRD spectrum, SEM and TEM images of Mg(II) complex.

Figure 8. XRD spectrum, SEM and TEM images of Ca(II) complex.

Figure 8. XRD spectrum, SEM and TEM images of Ca(II) complex.

Figure 9. Diagram of the zone-of-inhibition diameter results for the positive controls, and the synthesized CA complexes: (A) Fungi species, (B) Gram-positive species, and (C) Gram-negative species.

Figure 9. Diagram of the zone-of-inhibition diameter results for the positive controls, and the synthesized CA complexes: (A) Fungi species, (B) Gram-positive species, and (C) Gram-negative species.

Table 1

Screening results (mm/mg sample) for antimicrobial assays for the positive controls, free CA, and the synthesized complexes.

Microbe	Positive Controls		Free CA	Complexes
	Ketoconazole	Streptomycin		Zn(II)	Mg(II)	Ca(II)
Fungi species:
Aspergillus niger	18.0	-	9.0	16.0	23.0	21.0
Penicillium sp.	21.0	-	7.0	17.0	5.0	18.0
Candida albicans	21.0	-	5.0	9.0	2.0	3.0
Gram-positive species:
Bacillus subtilis	-	18.0	6.0	10.0	7.0	12.0
Streptococcus pneumoniae	-	17.0	5.0	9.0	4.0	8.0
Staphylococcus aureus	-	20.0	6.0	11.0	10.0	12.0
Gram-negative species:
Escherichia coli	-	22.0	7.0	12.0	9.0	15.0
Pseudomonas aeruginosa	-	27.0	8.0	12.0	22.0	20.0

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