Amino Acid Schiff Base Metal Complexes Show Potent Power Against Deadly Biofilms

A team of chemists and microbiologists in India has synthesized two metal complexes built from an amino acid and an indole aldehyde that can tear apart the protective biofilms of Pseudomonas aeruginosa, one of the most stubborn and dangerous antibiotic-resistant pathogens in hospitals worldwide. The copper(II) complex blocked nearly 98 percent of biofilm formation at its highest tested concentration, while the zinc(II) complex achieved more than 84 percent inhibition, according to a study published in the journal Discover Chemistry. The work offers a fresh line of attack against antimicrobial resistance, a crisis responsible for millions of deaths each year and one that conventional antibiotics are steadily losing.

The compounds belong to a family known as Schiff base metal complexes. Schiff bases are formed through a simple condensation reaction in which a primary amine meets an aldehyde or ketone, producing an imine functional group characterized by a carbon-nitrogen double bond. These ligands are prized in coordination chemistry because they are easy to make, chemically versatile, and readily wrap around metal ions to form stable complexes with well-defined three-dimensional geometries such as tetrahedral, square planar, and octahedral arrangements. When a biologically active ligand is combined with a metal ion that has its own antimicrobial pedigree, the resulting complex often outperforms either component alone, a phenomenon researchers attribute to the chelation effect, which increases the molecule’s lipophilicity and helps it slip through microbial cell membranes.

For this study, the researchers chose two precursors with strong pharmaceutical credentials. The first was L-tyrosine, an amino acid whose phenolic hydroxyl group and carboxylate arm make it a multidentate chelator capable of stabilizing metals in various oxidation states, and which is itself associated with antimicrobial, anti-inflammatory, and DNA-binding activity. The second was indole-3-carboxaldehyde, an indole derivative widely studied for anticancer, antibacterial, and antidepressant effects. The team dissolved L-tyrosine in methanol with sodium hydroxide at 60 to 70 degrees Celsius, added the aldehyde, and refluxed the mixture for five hours. A yellow Schiff base ligand precipitated out with a yield of 68 percent. The ligand was then combined in methanol with copper(II) chloride dihydrate or zinc(II) acetate dihydrate at a one-to-two metal-to-ligand ratio, with the pH held near neutral, yielding a green copper complex at 85 percent and a white zinc complex at 78 percent.

Confirming the structures required a battery of spectroscopic techniques. Infrared spectroscopy revealed the telltale signature of successful coordination: the azomethine stretching vibration of the free ligand appeared at 1596 wavenumbers, but shifted to 1577 in the copper complex and 1510 in the zinc complex, indicating that the imine nitrogen had bonded directly to each metal ion. Bands assigned to asymmetric and symmetric carboxylate stretching, along with weak low-frequency vibrations for metal-oxygen and metal-nitrogen bonds, showed that the ligand acts as a bidentate donor, gripping each metal through the azomethine nitrogen and a carboxylate oxygen. Notably, the free indole nitrogen-hydrogen stretch remained intact in the 3100 to 3500 wavenumber region, proving that the indole nitrogen stayed out of coordination.

Electronic spectroscopy pinned down the geometry of each complex. The copper(II) complex displayed a broad absorption band centered at 620 nanometers, attributed to a d-orbital transition characteristic of a four-coordinated square planar geometry. The zinc(II) complex, by contrast, showed no d-d transitions at all, which is expected because zinc has a filled d10 electronic configuration; instead it exhibited a charge-transfer band at 304 nanometers consistent with a four-coordinated tetrahedral geometry. Proton NMR spectroscopy of the ligand showed the azomethine proton as a singlet at 8.07 parts per million, confirming Schiff base formation, and in the zinc complex that signal vanished entirely, further evidence of strong coordination through the imine nitrogen. Molar conductivity measurements of 12 and 10.8 units for the copper and zinc complexes respectively confirmed that both are non-electrolytic, neutral species with no dissociable counterions.

With the structures established, the team turned to biological testing against a panel of reference microorganisms including Pseudomonas aeruginosa, Bacillus cereus, Streptococcus mutans, Escherichia coli, and Candida albicans, following Clinical and Laboratory Standards Institute guidelines. In disc diffusion assays, the zinc complex showed the highest activity against S. mutans and B. cereus, while the copper complex was moderately active against P. aeruginosa and E. coli. Minimum inhibitory concentration testing revealed that the zinc complex reached an MIC of 2 milligrams per milliliter against P. aeruginosa and S. mutans, whereas the copper complex showed MIC values above that threshold. These results confirmed that both complexes act as broad-spectrum antimicrobial agents, with the enhanced activity likely stemming from chelation-driven lipophilicity that eases penetration through microbial membranes.

The most striking results came from the antibiofilm experiments. Biofilms are communities of bacteria encased in a self-produced matrix of polymeric substances that cling to living tissues such as wounds and to medical devices such as catheters and stents. Encased in this matrix, bacteria can be dramatically more resistant to antibiotics and immune defenses than their free-floating counterparts, which is why biofilm infections often force clinicians to remove implants or resort to invasive treatments. Using a crystal violet assay in 96-well plates, the researchers found that the zinc complex inhibited P. aeruginosa biofilm formation in a concentration-dependent manner, achieving 84.65, 68.8, and 59.14 percent inhibition at 4, 2, and 1 milligrams per milliliter. The copper complex outperformed it at every dose, reaching 97.56, 82.42, and 67.74 percent inhibition at the same concentrations. Two-way ANOVA confirmed that both complex type and concentration significantly affected biofilm formation, and fluorescence microscopy of mature 24-hour biofilms stained with acridine orange visually confirmed the disruption of biofilm architecture.

To understand how the complexes might work at the molecular level, the team performed molecular docking against eight P. aeruginosa proteins retrieved from the Protein Data Bank, including PelA, LpxC, AlgL, ExoS, PslG, MreC, flagellin FliC, and the flagellar capping protein FliD. These targets span the biofilm’s life cycle: PelA is essential for producing Pel polysaccharides, a vital structural component of the biofilm; LpxC builds lipid A, the molecule that shields the outer membrane from antibiotics; AlgL degrades alginate, promoting dispersal and colonization; and the flagellar proteins drive surface motility and early attachment. Using the CDOCKER algorithm with the CHARMm force field, both complexes showed strong binding affinities. The zinc complex docked to PelA with an interaction energy of minus 87.03 kilocalories per mole, the strongest score in the study, stabilized by two strong conventional hydrogen bonds to residues GLU101 and TRP224. The copper complex formed three hydrogen bonds with the same protein through SER219, TRP224, and ARG200. The zinc complex also bound LpxC with four robust hydrogen bonds and AlgL with four more, while the copper ion itself formed short metal-acceptor interactions with ASP315 of AlgL and TYR235 of the ExoS toxin, contacts that enhance the specificity and therapeutic potential of metal complexes.

The computational pharmacokinetic profiling added an important layer of caution and promise. Both complexes recorded zero violations of the Lipinski and Veber rules, the classical criteria for oral drug-likeness, and both showed good predicted intestinal absorption with low plasma protein binding. Neither was predicted to inhibit the CYP2D6 enzyme or to be a substrate of P-glycoprotein, suggesting favorable cellular retention. However, both complexes were predicted to be hepatotoxic, and the TOPKAT toxicity module flagged a positive carcinogenicity signal for the copper complex in the male mouse NTP model, while the zinc complex was non-carcinogenic across all tested models. Both were predicted non-mutagenic in the Ames test and non-irritating to skin. The authors emphasize that these in silico predictions require experimental validation through in vitro and in vivo toxicology before any therapeutic application can be considered.

The study’s implications reach beyond the laboratory bench. Because biofilms are a leading cause of implant and device-associated infections, the strong antibiofilm effects of these complexes suggest potential applications as antimicrobial coatings for medical devices, preventing microbial colonization before it starts. The researchers also note a natural division of labor between the two metals: the copper complex, with its superior biofilm disruption, may be suited for aggressive therapeutic strategies against multidrug-resistant pathogens, while the zinc complex, benefiting from zinc’s role as an essential trace element with comparatively low systemic toxicity, holds promise for safer biomedical applications. Differences in their activity may trace back to distinct molecular interactions, with zinc likely targeting polysaccharide biosynthesis and virulence regulators such as PelA, LpxC, AlgL, and ExoS, while copper may affect PelA, PslG, and FliC, which govern matrix degradation and motility. Further studies on mechanism of action, cytotoxicity, and in vivo efficacy are needed, but the work strengthens a growing body of evidence that rationally designed transition metal complexes can serve as effective weapons in the escalating war against antimicrobial resistance and the biofilms that shield resistant bacteria from harm.

Subject of Research: Synthesis and antimicrobial, antibiofilm, and molecular docking evaluation of Cu(II) and Zn(II) Schiff base complexes derived from L-tyrosine and indole-3-carboxaldehyde

Article Title: Synthesis, characterization, antimicrobial, antibiofilm, and molecular docking studies of Cu(II) and Zn(II) Schiff base complexes derived from L-tyrosine and indole-3-carboxaldehyde

Article References: Mohan, A., Joy, A., Rajan, P. P., Daisylet, B. S., Raphael, S. J., Alex, S., Mohanan, A. G., & Kumar, P. (2026). Synthesis, characterization, antimicrobial, antibiofilm, and molecular docking studies of Cu(II) and Zn(II) Schiff base complexes derived from L-tyrosine and indole-3-carboxaldehyde. Discover Chemistry, 3(1), Article 475. https://doi.org/10.1007/s44371-026-00918-9

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00918-9

Keywords: Schiff base, metal complexes, antimicrobial resistance, biofilms, Pseudomonas aeruginosa, copper(II) complex, zinc(II) complex, L-tyrosine, indole-3-carboxaldehyde, molecular docking, coordination chemistry, ADMET prediction