The team, led by Fatemeh Masoumi and Mohsen Golbon Haghighi, synthesized four platinum(II) complexes built around two classic cyclometalating frameworks: 2-phenylpyridinate (ppy) and benzo[h]quinolinate (bhq). These C^N ligands bind platinum through a strong platinum–carbon sigma bond, a feature thought to enhance the stability and biological activity of cycloplatinated drugs. To this core the researchers added either bis(diphenylphosphino)methane (dppm) or bis(diphenylphosphino)amine (dppa). In the mononuclear complexes C1 and C2, the diphosphine chelates one platinum atom, leaving a cationic species with a non-coordinated counterion. In the binuclear complexes D1 and D2, the same diphosphines instead act as bridges between two platinum centers, each carrying a chloride ligand. The result is a matched set of molecules in which coordination mode, aromatic surface area, and steric bulk can be varied almost independently.

DNA remains the canonical pharmacological target for platinum anticancer agents, so the first question was how each complex associates with calf thymus DNA. Transition metal complexes can bind DNA covalently, by displacing a labile ligand with a DNA nitrogen base such as guanine N7, or non-covalently, through intercalation between base pairs, electrostatic attraction to the anionic phosphate backbone, or minor- and major-groove binding. Ultraviolet–visible absorption titrations at pH 7.4 provided the first clues. As DNA was added, the absorption bands of C1, C2, and D1 showed hypochromism—a decrease in intensity often associated with intercalative stacking between aromatic chromophores and nucleobases—while D2 displayed hyperchromism, pointing to a weaker, non-intercalative association. Intrinsic binding constants derived from Benesi–Hildebrand analysis ranged from 1.07 × 10^4 M^-1 for C1 to 1.60 × 10^5 M^-1 for C2, placing the strongest complexes in the moderate-to-strong range but still orders of magnitude below classical intercalators such as ethidium bromide.

Because absorption spectroscopy alone cannot definitively assign a binding mode, the team turned to competitive fluorescence assays with two well-characterized probes. Ethidium bromide fluoresces strongly only when intercalated into duplex DNA, so a molecule that quenches the EB–DNA emission may itself be an intercalator. Hoechst 33258, by contrast, binds with high affinity to the minor groove of AT-rich sequences. All four complexes quenched the EB–DNA fluorescence in an incomplete manner—only about 15 percent displacement—suggesting that the platinum compounds occupy low-affinity sites rather than performing deep intercalation. Stern–Volmer constants followed the order D1 > C2 > C1 > D2. The Hoechst displacement assays produced a different ranking, D2 > C1 > D1 > C2, and slightly larger quenching constants overall, indicating that groove perturbation is the more responsive endpoint. Taken together, the two assays pointed toward a mixed binding picture in which minor-groove association dominates over intercalation.

Viscosity measurements delivered the most decisive verdict. Classical intercalation lengthens the DNA helix as base pairs are pried apart to admit the ligand, producing a pronounced rise in solution viscosity; ethidium bromide, the benchmark, generates a slope of roughly 1.4 in these experiments. The platinum complexes produced slopes of only 0.009 to 0.010—essentially negligible. Since groove binding bends or kinks the helix without materially changing its effective length, the near-flat viscosity profiles effectively rule out classical intercalation as the dominant interaction. The authors emphasize that hydrodynamic data of this kind must be read alongside spectroscopic results, but in this case the convergence was clear: these are groove-binding, electrostatically driven associates, not base-pair inserters.

Molecular docking with AutoDock4.2 against a rigid B-DNA dodecamer (PDB ID: 5BNA) added a structural rationale. The optimized geometries of all four complexes, computed at the B3LYP level with the LanL2DZ basis set for platinum, were docked using the Lamarckian Genetic Algorithm. Estimated binding free energies ranged from −6.53 to −7.68 kcal/mol, all favorable, with the mononuclear complexes C1 and C2 tying for the strongest scores. In the lowest-energy poses, the flat cyclometalated ppy and bhq groups orient toward the DNA and sit within the major groove, while the bulky diphosphine moieties—each bearing four phenyl rings—remain far from the base-pair gap, physically preventing deep insertion. For C1, the docking model resolved specific contacts: π–π stacking with the aromatic faces of DG4, DT20, and DA5 at distances of 4.39 to 5.98 Å, hydrogen bonds to DA17 and DA18 at 2.32 to 2.98 Å, and electrostatic and hydrophobic contacts at 3.18 and 4.39 Å. The binuclear complexes showed reduced steric hindrance, attributed to the opening of the chelating ring when the diphosphine bridges two metals.

The cytotoxicity data, obtained by MTT assay after 24 hours of exposure, revealed striking cell-line dependence. Against MCF-7 breast adenocarcinoma cells, the mononuclear complexes C1 and C2 were the clear winners, with IC50 values of 329.0 and 328.6 μM respectively—both more potent than cisplatin, which required 833.2 μM under the same conditions. Against A549 non-small cell lung carcinoma cells, the picture inverted: C2 led with an IC50 of 164.3 μM, followed by D2 at 207.1 μM, and notably D2 outperformed cisplatin roughly four-fold at the highest tested concentrations, while cisplatin itself was weakest at 833.2 μM. The binuclear D1 was the least active compound against both cancer lines. On the non-cancerous mouse fibroblast line L929, all complexes were toxic in a dose-dependent manner, and selectivity index calculations revealed values near or below one for most compounds—meaning little preferential killing of cancer cells over healthy ones, a weakness the authors acknowledge candidly.

Statistical analysis using one-way ANOVA with Tukey’s multiple comparison tests confirmed significant, dose-dependent viability reductions across all three cell lines (p < 0.05), with the magnitude of response shaped by both the identity of the complex and the cell type. The structure–activity relationships that emerge are consistent across the biophysical and biological datasets: the larger, more planar aromatic surface of bhq relative to ppy favors DNA association through π-stacking, while the steric bulk and coordination mode of the diphosphine dictate whether deep intercalation is geometrically possible at all. C2, bearing the dppa ligand in a chelating mononuclear configuration, emerged as the most interesting candidate, combining the strongest DNA binding, the highest cytotoxicity against both cancer lines, and the best solution stability, retaining more than 93 percent of its absorbance after 24 hours in phosphate-buffered saline.

What makes this study notable is its methodological honesty. The authors explicitly caution that UV-derived binding constants can overestimate affinity because ethidium bromide itself participates in multiple binding equilibria, and that small spectral perturbations are supportive rather than definitive evidence of intercalation. By triangulating absorption spectroscopy, dual-probe fluorescence quenching, viscosity, and docking, they build a plausible rather than proclaimed model: accessible groove binding and electrostatic attraction to the phosphate backbone dominate, with possible hydrogen bonding from the amine-bearing dppa ligand to adenine nitrogen or thymine oxygen atoms adding stability. The work also underscores how much remains unknown—future experiments with Annexin V/PI assays, caspase activity measurements, reactive oxygen species detection, and cell-cycle analysis will be needed to pin down the actual death mechanism. For now, the message for drug designers is precise: in cyclometalated platinum chemistry, the shape of the ligand is not a detail. It is the switch that determines whether a molecule slides between base pairs, hugs the groove, or kills the tumor at all.

Subject of Research: DNA interaction and cytotoxicity of cyclometalated platinum(II) complexes with chelating versus bridging bisphosphine ligands

Article Title: Influence of ligand coordination mode on DNA interaction and cytotoxic investigations of cyclometalated platinum(II) complexes with bisphosphine ligands

Article References: Masoumi, F., Haghighi, M. G., Noroozicharandabi, V., Kiadehi, S. R. B., & Tayefeh, A. R. (2026). Influence of ligand coordination mode on DNA interaction and cytotoxic investigations of cyclometalated platinum(II) complexes with bisphosphine ligands. Results in Chemistry, 31, Article 103914. https://doi.org/10.1016/j.rechem.2026.103914

Image Credits: AI Generated

DOI: Not provided

Keywords: platinum(II) complexes, cyclometalated ligands, bisphosphine, DNA binding, intercalation, groove binding, molecular docking, cytotoxicity, cisplatin, MCF-7, A549, medicinal inorganic chemistry