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