The starting material is built from 2-hydroxy-1,4-naphthoquinone, a quinone scaffold with a long history in medicinal chemistry, condensed with 2-amino-4-chlorophenol in refluxing ethanol for six hours. The condensation forms the azomethine linkage, the characteristic carbon-nitrogen double bond that defines Schiff bases and gives them their talent for grabbing metal ions. The resulting ligand, with the formula C16H10NO3Cl and a molecular weight of 299.70 grams per mole, emerged as a deep brown solid in a 66.5 percent yield after recrystallization from an ethanol-water mixture. The researchers then reacted half a gram of this ligand with weighed quantities of bivalent metal salts in a two-to-one ligand-to-metal molar ratio, producing the six homoleptic complexes in yields ranging from 53.0 percent for the copper compound to 72.7 percent for the iron analogue.

Characterization began with the most workhorse technique in coordination chemistry: infrared spectroscopy. The free ligand displayed an intense imine stretching band at 1645 wavenumbers, alongside a carbonyl stretch at 1667 wavenumbers, a phenolic O-H vibration, and a C-O stretch at 1304 wavenumbers. When the metals were introduced, that imine band slid down to the 1611 to 1632 wavenumber region, a shift that signals the azomethine nitrogen has donated its lone pair to the metal center. The C-O band also moved, implicating the phenolic oxygen as a second donor atom. Most convincing were entirely new bands appearing in the low-frequency region, between 508 and 587 wavenumbers for metal-nitrogen bonds and 475 to 493 wavenumbers for metal-oxygen bonds. Those vibrations simply do not exist in the free ligand, so their presence is direct evidence that new chemical bonds to the metal have formed.

Electronic spectroscopy filled in the picture of the geometry around each metal. The free ligand shows pi-to-pi-star transitions at 304 and 316 nanometers and n-to-pi-star transitions at 328, 334, and 394 nanometers, fingerprints of its aromatic rings and azomethine chromophore. Upon complexation these bands shifted, and the spectra acquired features diagnostic of octahedral coordination environments. The nickel, cobalt, iron, manganese, and copper complexes all displayed d-to-d transitions consistent with octahedral geometry, while the zinc complex, whose d10 configuration makes d-to-d transitions impossible, showed only ligand-centered bands. The team also observed ligand-to-metal charge transfer in the iron and copper compounds and metal-to-ligand charge transfer in the cobalt and nickel species, the latter reflecting the ligand’s ability to act as a pi-acceptor. Together with the infrared data, the spectra support proposed structures of the general form ML2(H2O)2.

Physical measurements added further confirmation. The free ligand melts at 216 to 218 degrees Celsius, but every complex melts higher, between 235 and 237 degrees and above 300 degrees in the case of the iron and cobalt compounds. Since a pure substance melts at a characteristic temperature, the fact that none of the complexes shared the ligand’s melting point rules out simple mixtures and points to genuinely new compounds with enhanced thermal stability, a consequence of the stable chelate rings and stronger metal-ligand interactions. Solubility testing told a complementary story: the ligand and all six complexes are insoluble in water but dissolve in coordinating organic solvents such as dimethylformamide and dimethyl sulfoxide. Chelation reduces polarity and increases lipophilicity, a shift that matters for biology because more lipophilic molecules cross microbial membranes more easily.

The antimicrobial screening is where the compounds revealed their most striking behavior. Using agar well diffusion on Mueller-Hinton and Sabouraud dextrose media, the team tested the ligand and all six complexes against six bacterial strains and three fungal species, with streptomycin and miconazole as positive controls. Against Staphylococcus aureus, the manganese complex produced a mean inhibition zone of 21.0 millimeters, edging out even streptomycin at 19.0 millimeters, while the copper complex matched the antibiotic at 19.5 millimeters. The cobalt complex was the top performer against Bacillus species at 18.0 millimeters, again matching streptomycin. On the fungal side, the iron complex led against Aspergillus niger with a 14.0 millimeter zone, followed by zinc at 12.5 millimeters, and the iron and zinc compounds also ranked highest against Aspergillus flavus. Notably, Pseudomonas aeruginosa resisted every complex, underscoring the selective, strain-dependent nature of the activity.

The pattern of activity does not map neatly onto the Irving-Williams series, the classical ordering of divalent first-row transition metal complex stability that peaks at copper. The researchers argue that stability alone cannot explain the results; kinetic lability, redox behavior, membrane permeability, and specific interactions with microbial biomolecules all contribute. Copper and iron complexes are known to engage in redox cycling and generate reactive oxygen species inside cells, a plausible mechanism for their strong antibacterial effects, while zinc may disrupt fungal metalloenzymes. Nickel complexes were the weakest across the board, consistent with earlier reports that nickel Schiff base complexes often suffer from poor cellular uptake. The pronounced activity against Gram-positive bacteria such as Staphylococcus and Bacillus fits the idea that lipophilic chelates penetrate the exposed peptidoglycan layer of Gram-positive cell walls more readily than the outer membrane of Gram-negative organisms.

The second act of the study is industrial rather than biomedical. Corrosion of mild steel in hydrochloric acid is a costly problem in industries that use acid pickling and cleaning, and organic molecules rich in heteroatoms and conjugated pi-systems are classic candidates for inhibiting it. Using the weight-loss method, the team immersed polished steel coupons in 1 molar hydrochloric acid with inhibitor concentrations of 100, 300, and 500 parts per million, measuring mass loss after one, three, six, twelve, and twenty-four hours at temperatures from 303 to 373 kelvin. Inhibition efficiency climbed with concentration, reaching a maximum of 69 percent at 500 parts per million after twelve hours at 323 kelvin, where the corrosion rate fell to 0.459. The mechanism is adsorption: the ligand’s nitrogen, oxygen, and chlorine atoms and its aromatic pi-electrons form donor-acceptor and electrostatic interactions with the steel surface, building a protective film that blocks both anodic metal dissolution and cathodic hydrogen evolution.

Time and temperature, however, impose limits. Efficiency dropped after twenty-four hours at 323 kelvin, falling to 56 percent at the highest concentration, which the authors attribute to partial desorption of inhibitor molecules and gradual weakening of the protective film under prolonged acid attack. At the highest temperature studied, 373 kelvin, efficiency remained concentration-dependent but more modest, rising from 25 percent at 100 parts per million to 54 percent at 500 parts per million after one hour. These trends suggest the ligand works best as a short-duration, high-concentration inhibitor, and that formulation strategies to strengthen film adhesion could extend its practical utility. The authors are candid about the study’s limitations and recommend follow-up work including elemental analysis, advanced spectroscopic characterization such as NMR and metal-content assays, mechanistic biological studies, electrochemical corrosion analysis, and computational modeling. Even so, the message is clear: a single naphthoquinone-based chelator, when paired with the right metal, can rival a frontline antibiotic against Staphylococcus aureus while simultaneously protecting steel in one of chemistry’s harshest environments, a reminder that the periodic table remains the most versatile design toolkit available to materials and drug developers alike.

Subject of Research: Synthesis and characterization of divalent metal complexes of a naphthoquinone-derived Schiff base ligand with antimicrobial and corrosion inhibition properties

Article Title: Synthesis, characterization, antimicrobial, and corrosion inhibition studies of divalent complexes of 2-((5-chloro-2-hydroxyphenyl) amino) naphthalene-1,4-dione chelator

Article References: Synthesis, characterization, antimicrobial, and corrosion inhibition studies of divalent complexes of 2-((5-chloro-2-hydroxyphenyl) amino) naphthalene-1,4-dione chelator. (n.d.). https://doi.org/10.1007/s44371-026-00836-w

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00836-w

Keywords: Schiff base, naphthoquinone, metal complexes, coordination chemistry, antimicrobial activity, antifungal, corrosion inhibition, mild steel, octahedral geometry, FTIR spectroscopy, copper complex, chelation

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