The synthesis itself began with a modified Pictet–Spengler cyclization, in which L-tryptophan was reacted with excess formalin in dilute sulfuric acid, stirred at room temperature and then heated at 50 degrees Celsius to drive the formation of the tricyclic tetrahydro-beta-carboline ring system through a Schiff base intermediate. The light-yellow ligand was isolated in roughly 85 percent yield and recrystallized from ethanol and water. Complexation proceeded by dissolving the ligand with sodium hydroxide to generate the sodium carboxylate in situ, followed by dropwise addition of aqueous nickel or copper acetate. Thick precipitates of the metal carboxylates formed, and slow evaporation from water afforded crystalline products in yields of 78 percent for NZ2 and 82 percent for NZ3. Characterization by FTIR spectroscopy revealed the diagnostic separation between asymmetric and symmetric carboxylate stretches—116 wavenumbers for the nickel complex and 121 for the copper complex—confirming bidentate coordination, while new metal–oxygen stretches appeared near 416 and 418 wavenumbers. Elemental analysis, ICP-AES, atomic absorption spectroscopy and EDX measurements corroborated the metal content of each complex.

Structural and thermal studies added further depth to the picture. X-ray diffraction showed the free ligand to be relatively crystalline, whereas the metal complexes displayed broad, semi-crystalline patterns typical of aqua complexes in which hydrogen bonding between coordinated water and carboxylate oxygens produces only partial ordering. Scanning electron microscopy revealed a dramatic morphological shift: the coarse, irregular bulk particles of the free ligand gave way to compact nanoscale granules of roughly 150 to 200 nanometers for the nickel complex and larger agglomerated particles for the copper complex, evidence that chelation alters nucleation and growth. Thermogravimetric analysis demonstrated that both metals raise the thermal stability of the ligand substantially, delaying major decomposition and leaving thermally stable metal oxide residues—nickel oxide in the case of NZ2 and copper oxide for NZ3—as the final products, with the nickel complex proving slightly more thermally robust than its copper counterpart.

The centerpiece of the study was a systematic interrogation of how the three compounds engage double-stranded DNA isolated from chicken blood. Under physiological conditions of pH 7.4 and 310 kelvin, UV–visible titration revealed large hyperchromic effects of 40.75 percent for NZ1, 53.58 percent for NZ2 and 29.66 percent for NZ3, all without any meaningful shift in the absorption maxima. That combination—intensified absorbance with unshifted peaks—is the spectroscopic fingerprint of groove binding rather than classical intercalation, which would normally produce hypochromism and a red shift. Benesi–Hildebrand analysis of the titration data yielded intrinsic binding constants on the order of 10 to the fourth per molar, with the metal complexes outperforming the free ligand, and the corresponding Gibbs free energies were negative, confirming that binding is spontaneous under physiological conditions. Fluorescence experiments told a consistent story: adding DNA progressively quenched the intrinsic emission of all three compounds without shifting emission maxima, and Stern–Volmer analysis at multiple temperatures identified static quenching with constants near 10 to the fourth per molar, indicating ground-state compound–DNA association. Thermodynamic signatures distinguished the complexes mechanistically—NZ1 and NZ2 showed enthalpically favorable, entropy-opposed profiles consistent with hydrogen bonding and van der Waals contacts, while NZ3 displayed large positive enthalpy and entropy changes, the hallmark of hydrophobic interactions accompanied by displacement of water molecules from the groove.

Viscosity measurements provided an independent hydrodynamic check. Classical intercalators lengthen the DNA helix and sharply increase solution viscosity, but all three compounds produced only modest, gradual rises in relative viscosity, with NZ3 showing the largest yet still restrained effect. This confirmed that the compounds nestle into the grooves of the helix with limited structural distortion rather than prying the base pairs apart. Molecular docking against the canonical B-DNA duplex (PDB ID 3BNA) using the Molecular Operating Environment software, with Gaussian-optimized ligand geometries and AMBER-minimized receptor structures, reproduced the experimental picture at atomic resolution. The lowest-energy poses placed every compound in the minor groove, and metal coordination emerged as the key geometric driver: attaching the metal transformed the non-planar ligand into a semi-circular, crescent-shaped entity that is sterically and electrostatically complementary to the groove. The nickel complex added a pi–pi stacking interaction with a cytosine base, while the copper complex assembled the richest interaction network of all—hydrogen bonds donated by a coordinated water molecule and by the indole N–H group, plus pi–pi contacts with thymine bases—rationalizing its highest measured DNA affinity.

Biological assays then revealed just how consequential these molecular differences are. In DPPH and ABTS radical scavenging tests at 200 micrograms per milliliter, the free ligand and the nickel complex showed moderate antioxidant activity—roughly 18 percent DPPH inhibition and 31 to 32 percent ABTS inhibition—while the copper complex was markedly weaker as a direct radical scavenger. Antibacterial testing by agar well diffusion against Klebsiella pneumoniae, Escherichia coli, Bacillus subtilis and methicillin-resistant Staphylococcus aureus told a different story: the copper complex delivered consistent 20-millimeter inhibition zones across all four strains, outperforming the nickel complex and even exceeding the levofloxacin control, which produced zones of only 14 to 15 millimeters. The ligand itself was moderately active, and the nickel complex proved the weakest, failing entirely against Klebsiella pneumoniae.

The most dramatic results came from the anticancer screen against MDA-MB-231 triple-negative breast cancer cells, an aggressive subtype that lacks estrogen receptors, progesterone receptors and HER2 expression and remains one of the hardest breast cancers to treat. Using the MTT assay across concentrations from 0.75 to 200 micrograms per milliliter, the researchers observed clear dose-dependent killing. The free ligand was only weakly cytotoxic, reducing viability to about 61 percent at the highest dose with an IC50 above 200 micrograms per milliliter. The nickel complex fared better, dropping viability to roughly 40 percent with an IC50 of 123.46 micrograms per milliliter. The copper complex was in a different league altogether: viability collapsed to a mere 7.46 plus or minus 3.15 percent at 200 micrograms per milliliter, and the IC50 came out at approximately 10.50 micrograms per milliliter—more than an order of magnitude more potent than the nickel analogue and vastly better than the uncomplexed alkaloid.

The authors argue that this striking copper advantage cannot be explained by DNA binding strength alone, since all three compounds bind with comparable, moderate affinities. Instead, they point to the redox chemistry of copper. Intracellular cycling between Cu(II) and Cu(I) can catalyze the generation of reactive oxygen species through Fenton-like and Haber–Weiss reactions, and cancer cells—already running at elevated basal oxidative stress—are uniquely vulnerable to further ROS amplification beyond their antioxidant capacity. In this mechanistic model, minor groove binding serves a targeting function, parking the copper complex in intimate proximity to DNA so that locally generated radicals inflict site-specific oxidative damage and trigger apoptosis through intrinsic pathways. The same redox-driven oxidative stress, the researchers suggest, underlies the copper complex’s broad-spectrum antibacterial performance, where it disrupts bacterial membranes, proteins and nucleic acids across both Gram-positive and Gram-negative species.

The study represents the first report of mononuclear Ni(II) and Cu(II) metallocarboxylates of this tetrahydro-beta-carboline ligand, and it delivers a clear structure–activity message: metal coordination converts a moderately bioactive natural metabolite into a far more thermally stable, DNA-avid and cytotoxic scaffold, with copper providing the most potent therapeutic profile. The team cautions that rigorous validation remains ahead—direct quantification and localization of intracellular reactive oxygen species, comparative cytotoxicity against normal cell lines, hemocompatibility testing to establish a therapeutic index, and in vivo tumor model studies of biodistribution, pharmacokinetics and efficacy are all required before these complexes can advance toward clinical relevance. Structural optimization of the beta-carboline scaffold and exploration of alternative transition metals may further sharpen selectivity, positioning these DNA-targeting redox-active metallocarboxylates as promising candidates in the ongoing search for treatments against triple-negative breast cancer and drug-resistant bacterial infections.

Subject of Research: Synthesis of mononuclear Ni(II) and Cu(II) metallocarboxylate complexes of a tetrahydro-beta-carboline carboxylic acid ligand and evaluation of their DNA binding, antioxidant, antibacterial and anticancer activities against MDA-MB-231 triple-negative breast cancer cells.

Article Title: DNA targeting mononuclear Ni(II) and Cu(II) metallocarboxylates of 2,3,4,9-tetrahydro-β-carboline-3-carboxylic acid: a combined in silico to in vitro approach with antibacterial and anticancer assays against MDA-MB-231 cell lines

Article References: Abbas, N., Arfan, M., Iqbal, M., Iqbal, Y., Aftab, U., Gatasheh, M. K., Alharbi, M. G., & Ehsan, M. F. (2026). DNA targeting mononuclear Ni(II) and Cu(II) metallocarboxylates of 2,3,4,9-tetrahydro-β-carboline-3-carboxylic acid: a combined in silico to in vitro approach with antibacterial and anticancer assays against MDA-MB-231 cell lines. Journal of Saudi Chemical Society, 30(4), Article 59. https://doi.org/10.1007/s44442-026-00101-w

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00101-w

Keywords: beta-carboline, metallocarboxylate complexes, DNA groove binding, copper complex, nickel complex, triple-negative breast cancer, MDA-MB-231, molecular docking, reactive oxygen species, antibacterial activity, antioxidant activity, coordination chemistry

Schiff bases are compounds formed when a primary amine condenses with an aldehyde or ketone, producing a characteristic imine, or C=N, group. They are prized in coordination chemistry because they are easy to make, structurally versatile, and able to grip metal ions through nitrogen, oxygen or sulfur donor atoms. The two ligands in this study were built from naphthalene-derived starting materials: LQ-CFNA was produced by condensing 2-hydroxy-1,4-naphthoquinone with 3-chloro-6-fluoro-2-nitroaniline, while HN-MPD came from 2-hydroxy-1-naphthaldehyde and 4-methylpiperidin-3-one. The naphthoquinone unit is particularly interesting because it is redox-active and appears in many biologically active natural products, giving the resulting complexes a built-in electronic handle for reactivity.

The researchers refluxed ethanolic solutions of the precursors at 50 degrees Celsius for several hours with a drop of acid catalyst, collecting yellow precipitates that were recrystallized and dried. The ligands were then combined with copper, manganese, iron and zinc salts in a 1:1:1 molar ratio to give the mixed-ligand complexes [Cu(LCH)], [Mn(LCH)], [Fe(LCH)] and [Zn(LCH)]. Confirmation of coordination came from infrared spectroscopy: the azomethine C=N stretching bands of the free ligands, seen at 1677 and 1615 wavenumbers per centimeter, shifted to lower frequencies of 1599, 1644, 1532 and 1539 for the copper, manganese, iron and zinc complexes respectively, indicating that the imine nitrogen binds the metal. New low-frequency bands between roughly 400 and 600 wavenumbers, absent in the free ligands, were assigned to metal-nitrogen and metal-oxygen vibrations, sealing the case for coordination through both donor types.

Other physical measurements filled in the picture. Molar conductance values of 8.77, 23.58 and 29.17 S cm2 mol-1 for the copper, manganese and zinc complexes pointed to non-electrolytic behavior in dimethyl sulfoxide solution, while the iron complex, with a strikingly high value of 279 S cm2 mol-1, behaved as a strong electrolyte, suggesting counter ions outside its coordination sphere. Magnetic susceptibility measurements at room temperature revealed paramagnetic behavior across the series. The electronic spectra and magnetic moments supported predominantly octahedral geometries, with the manganese complex showing a high-spin moment of 6.6 Bohr magnetons consistent with five unpaired electrons, while the copper complex, with a moment of 2.2 Bohr magnetons and a single low-energy d-d transition, was assigned a square planar geometry possibly with some tetrahedral distortion. The iron complex sat between high-spin and low-spin limits, suggesting an equilibrium of spin states in an octahedral field.

Nuclear magnetic resonance spectroscopy of the free ligands revealed subtle tautomeric differences. For LQ-CFNA, a downfield imine proton at 8.06 parts per million and an imine carbon signal at 184.02 parts per million pointed to the keto form, with no enol peaks in the 5 to 7 parts per million window. HN-MPD, by contrast, showed an imine proton singlet at 3.46 parts per million, consistent with the enol tautomer. The complexes also melted at far higher temperatures, between 280 and 325 degrees Celsius, than the free ligands, which the authors attribute to strong metal-ligand bonding, increased lattice energy and the rigidity that chelation imposes on the molecular framework.

The biological results were among the most eye-catching findings. In disk diffusion assays against seven bacterial strains, the manganese complex produced the largest inhibition zone of the study, 22.5 millimeters against Klebsiella pneumoniae, exceeding the streptomycin control. The free ligand HN-MPD outperformed the standard drug against Staphylococcus aureus with a 21-millimeter zone, and LQ-CFNA beat the control against Escherichia coli with 17 millimeters. Against fungi, the iron complex delivered a 19.5-millimeter zone against Aspergillus flavus, surpassing the miconazole standard, while the free HN-MPD ligand reached 24.5 millimeters against Fusarium species. The team attributes the enhanced activity of many complexes to chelation, which disperses the positive charge on the metal ion, delocalizes electrons over the ligand rings and increases lipophilicity, allowing the compounds to penetrate microbial membranes more effectively.

DNA cleavage experiments using agarose gel electrophoresis added a therapeutic dimension. HN-MPD showed clear cleavage activity, with the disappearance of the supercoiled plasmid DNA band and the appearance of nicked and linear forms, while LQ-CFNA showed no cleavage effect. The authors suggest the ligand’s conjugated pi-system and donor atoms bring it into close contact with the DNA backbone, promoting strand scission through hydrolytic pathways or, potentially, reactive oxygen species generated by redox cycling. Antioxidant testing by the FRAP assay showed that complexation generally boosted radical-scavenging power: the copper and iron complexes reached estimated IC50 values of about 5 milligrams per milliliter, comparable to gallic acid, while the manganese complex was the weakest at 25 milligrams per milliliter. The redox-active nature of copper and iron, which readily changes oxidation state to stabilize radical intermediates, explains their superior performance.

The corrosion work may prove equally consequential for industry. Using weight-loss measurements on mild steel coupons immersed in 1 molar hydrochloric acid, the team found that inhibition efficiency rose with inhibitor concentration and depended strongly on temperature. At 303 kelvin, HN-MPD was the champion, reaching 91.49 percent efficiency at 62.5 parts per million, followed by LQ-CFNA at 74.69 percent and the zinc complex at 49.29 percent. At 373 kelvin the ranking flipped, with LQ-CFNA achieving the best performance at 74.69 percent. The ligands adsorb onto the steel surface through donor-acceptor interactions between lone electron pairs and the metal, forming a protective film that slows dissolution in the aggressive acid.

Density functional theory calculations using the B3LYP functional with 6-31G(d,p) basis sets, and LANL2DZ for the metal centers, tied the experimental observations to electronic structure. The free ligands showed large HOMO-LUMO gaps of 3.48 and 3.55 electronvolts, marking them as hard, stable molecules, but complexation shrank the gap dramatically, to just 1.04 electronvolts for the manganese complex, 1.72 for iron, 1.69 for zinc and 1.81 for copper. Lower gaps, reduced chemical hardness, increased softness and higher electrophilicity all correlated with the enhanced biological activity of the complexes, since softer, more electrophilic molecules interact more readily with nucleophilic sites in DNA bases and amino acid residues. The manganese complex emerged as the softest and most electrophilic species, matching its standout antibacterial result. The authors caution that the relationship between quantum descriptors and activity is not strictly linear, and that geometry, substituents and solubility also matter, but the overall agreement between theory and experiment is striking.

By combining two electronically distinct Schiff bases in a single coordination sphere, and by evaluating antimicrobial, antioxidant, DNA-interaction and corrosion properties side by side, the study offers a template for designing multifunctional metal complexes rather than single-purpose ones. The researchers, C. Wodi and C. Festus of Ignatius Ajuru University of Education in Port Harcourt, note that single-crystal X-ray diffraction could not be performed because suitable crystals could not be grown, but argue that the combined spectroscopic, magnetic, conductance and computational evidence strongly supports their proposed structures. If the LCH framework’s performance holds up in further testing, these naphthoquinone-based complexes could find roles ranging from antimicrobial agents to green corrosion inhibitors for the oil and gas and metal-processing industries.

Beyond the specific results, the study illustrates a broader trend in coordination chemistry: the deliberate pairing of ligands with complementary electronic roles. The naphthoquinone fragment contributes low-lying π* orbitals that can accept electron density during redox cycling, while the piperidinone-derived Schiff base supplies flexible donor geometry. Together they give the metal center access to both storage and transfer of electrons, which is precisely what antioxidant and DNA-cleavage chemistry demands.

The corrosion findings also fit established adsorption theory. In acidic media, protonated heteroatoms and π-electrons of aromatic rings can bind to positively charged steel surfaces, and the temperature dependence observed here suggests a shift between physisorption at lower temperatures and chemisorption as thermal energy increases. Such plant-free, synthetic organic inhibitors are of interest because they can be effective at parts-per-million loadings, reducing the volume of chemical discharged into the environment.

Methodologically, the work shows how quantum descriptors can serve as screening tools. A small HOMO-LUMO gap and high electrophilicity index, computed cheaply before any synthesis, correlated with the strongest antimicrobial performers, hinting that computational pre-screening could prioritize which metal complexes to prepare. The authors acknowledge limitations, including the absence of crystallographic confirmation and the need for minimum inhibitory concentration measurements to complement diffusion zones, which remain a standard caveat in antimicrobial screening. As an open-access contribution, the study makes its full dataset available for groups interested in extending the LCH platform toward drug leads or industrial formulations.

Subject of Research: Synthesis, characterization and multifunctional evaluation of mixed-ligand metal(II) complexes derived from naphthoquinone-based Schiff base ligands

Article Title: Experimental and theoretical investigation of mixed ligand metal(II) complexes derived from LCH for multifunctional applications

Article References: Wodi, C., & Festus, C. (2026). Experimental and theoretical investigation of mixed ligand metal(II) complexes derived from LCH for multifunctional applications. Discover Chemistry, 3(1), Article 493. https://doi.org/10.1007/s44371-026-00950-9

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00950-9

Keywords: Schiff base, mixed-ligand complexes, naphthoquinone, metal(II) complexes, antimicrobial activity, DNA cleavage, corrosion inhibition, DFT calculations, antioxidant, coordination chemistry, mild steel, spectroscopy