The study, published in Discover Chemistry, employed a polypharmacological approach known as multiple-target-directed ligands, or MTDLs. The underlying logic is elegant: conventional antibiotics that inhibit a single bacterial enzyme impose intense selective pressure, allowing pathogens to acquire resistance through relatively simple genetic changes. By contrast, a compound that disrupts several essential pathways at once distributes that pressure across interconnected biological networks, theoretically making it far harder for the bacterium to evolve escape routes. The team selected five enzymes that are indispensable for S. aureus survival: MurB, which drives peptidoglycan biosynthesis for the cell wall; DNA gyrase B, required for DNA replication and transcription; dihydrofolate reductase and dihydropteroate synthase, both crucial for folate synthesis; and tyrosyl-tRNA synthetase, essential for protein synthesis.

To find compounds capable of engaging all five targets, the researchers turned to structure-based virtual screening, a technique that considers the entire three-dimensional architecture of an enzyme’s active site when searching for matching ligands. This contrasts with ligand-based screening, which relies on matching a limited number of pharmacophoric features, typically four or five, derived from known inhibitors. The team downloaded the MolMall database of 22,401 molecules and prepared each enzyme from high-quality crystal structures deposited in the Protein Data Bank, selecting entries with a resolution of 2.5 angstroms or better, no reported mutations, and a bound co-crystallized ligand. Using the Maestro molecular modeling suite from Schrödinger, the proteins were protonated, energy-minimized, and fitted with receptor grids centered on their native ligands: FAD for MurB, and the co-crystallized inhibitors 07N, Q27, HH2, and 662 for the remaining enzymes.

A critical step before any large-scale screening was validation of the docking protocol itself. The researchers extracted each co-crystallized ligand from its enzyme and re-docked it into the prepared binding site, then superimposed the docked pose on the original crystallographic orientation. The root-mean-square deviation of heavy atoms remained within acceptable limits, and the key interactions between ligands and active-site residues were consistent with reported binding features, confirming that the computational pipeline could reliably reproduce experimentally observed binding modes. Only then did the team unleash the full virtual screening workflow, passing the ligand library through successive high-throughput, standard-precision, and extra-precision docking stages with retention thresholds of 50, 40, and 30 percent respectively, progressively narrowing the pool to the most promising candidates.

The screening produced 191 compounds with favorable docking profiles across all five enzymes. After applying computational filters for predicted absorption, distribution, metabolism, and excretion properties, along with Lipinski’s rule of five, Jorgensen’s rule of three, and estimates of binding free energy using the molecular mechanics/generalized Born surface area method, fifteen multitarget hits were shortlisted. Six of these were physically procured through MolMall’s molecular exchange program and sent to the CSIR-Central Drug Research Institute in Lucknow for in vitro antibacterial testing against the ESKAPE pathogen panel, a collection of bacteria notorious for antibiotic resistance, using the broth microdilution method with levofloxacin as a reference agent.

The experimental results were strikingly selective. Four of the tested compounds, designated MolMall 643, 644, 10988, and 17782, exhibited inhibitory activity against S. aureus, while showing limited effect against the other ESKAPE pathogens. Two compounds, MolMall 643 and MolMall 17782, emerged as the most active, achieving minimum inhibitory concentrations of 4 micrograms per milliliter against the reference strain. Computationally, these two compounds also displayed their most favorable binding profiles against MurB, with docking scores of minus 8.884 and minus 6.781 kilocalories per mole and MM/GBSA binding free energies of minus 97.801 and minus 72.404 kilocalories per mole respectively, suggesting that the cell wall biosynthesis enzyme may be a principal contributor to their antibacterial effect.

To verify that the docking-predicted binding modes were physically plausible over time, the researchers subjected the MurB complexes of the two most active compounds to 100-nanosecond molecular dynamics simulations using the Desmond engine. The systems were solvated, neutralized, and brought to physiological salt concentrations before equilibration and production runs. Throughout the simulations, both protein-ligand complexes remained stable, with root-mean-square deviation values confined below 3 angstroms, and both ligands stayed within the MurB active site for the entire duration. MolMall 643 sustained a hydrogen bond with Gly81, while MolMall 17782 interacted with Val199 through a hydrogen bond and formed halogen bonds with Ser143 and Asn80. Notably, these residues also participate in binding the native FAD cofactor, indicating that the synthetic ligands occupy the same functional pocket as the enzyme’s natural partner.

The flexibility analysis revealed subtle but informative details about how the compounds adapt within the binding pocket. MolMall 643 showed localized fluctuations at the sulfur atom of its thiazolidinone moiety and at several positions on its piperidine ring, while MolMall 17782 displayed increased mobility at the meta- and para-chloro substitutions on its phenyl ring. These localized movements did not disrupt overall binding stability, and the authors suggest that such limited flexibility may actually facilitate adaptive fitting within the MurB active site, a phenomenon sometimes described as induced fitting that allows ligands to optimize their contacts without detaching.

The researchers are careful to delineate the boundaries of their findings. No target-specific enzyme inhibition assays were performed, meaning the proposed interactions with the five enzymes remain computationally predicted rather than experimentally confirmed. The compounds should therefore be regarded as experimentally validated antibacterial agents against S. aureus with computationally predicted multitarget activity. The molecular basis for the limited activity against Gram-negative organisms remains unclear, resistance frequency studies were not conducted, and any advantage of the multitarget strategy in delaying resistance remains theoretical at this stage. Lead optimization and comprehensive toxicity assessment were also beyond the scope of the current hit-identification campaign.

Nevertheless, the study demonstrates a complete and rational workflow that connects computational prediction with experimental validation, moving from a database of more than 22,000 molecules to two compounds with measurable antibacterial activity and dynamically stable binding modes. By targeting enzymes involved in cell wall biosynthesis, DNA replication, folate metabolism, and protein synthesis simultaneously, the approach explores pathway-level inhibition rather than dependence on a single molecular interaction. As multidrug-resistant S. aureus continues to complicate clinical management globally, such structure-based multitarget screening pipelines may offer a practical blueprint for discovering the next generation of antibacterial starting points, providing a foundation for subsequent structural optimization and mechanistic investigation.

Subject of Research: Structure-based identification of multitarget inhibitors of essential Staphylococcus aureus enzymes with in vitro antibacterial validation

Article Title: Structure-based identification of potential inhibitors of essential Staphylococcus aureus enzymes with in vitro antibacterial screening

Article References: Singhal, L., Kumar, N., Sharma, C. S., Akhter, M., & Shrivastava, A. (2026). Structure-based identification of potential inhibitors of essential Staphylococcus aureus enzymes with in vitro antibacterial screening. Discover Chemistry, 3(1), Article 461. https://doi.org/10.1007/s44371-026-00919-8

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00919-8

Keywords: Staphylococcus aureus, multiple-target-directed ligands, polypharmacology, antibacterial resistance, structure-based virtual screening, MurB, molecular docking, molecular dynamics simulations, MM/GBSA, antimicrobial drug discovery, ESKAPE pathogens, minimum inhibitory concentration

The synthetic route itself is elegantly simple. Substituted aromatic aldehydes, methyl cyanoacetate and barbituric acid are combined in a 1:1 mixture of ethanol and water at 85 degrees Celsius with just 0.03 mol percent of Bi(OTf)3. The catalyst orchestrates a domino Knoevenagel-Michael sequence: it coordinates to the ester oxygen of methyl cyanoacetate, sharpening the electrophilicity of the carbonyl carbon, while the aldehyde-derived enolate attacks to form a carbon-carbon bond. The resulting alpha,beta-unsaturated intermediate is then struck by the weakly nucleophilic nitrogen of barbituric acid, and intramolecular cyclization closes the fused pyridopyrimidine ring. Eight derivatives, bearing nitro, bromo, hydroxyl, methyl, methoxy or unsubstituted phenyl groups, were isolated in yields ranging from 76 to 91 percent within two to five hours, with minimal side products and no need to purify intermediates.

What makes the protocol genuinely green is the solvent system and the catalyst. Bismuth(III) salts have gained a reputation as environmentally benign alternatives to conventional Lewis acids because they are air- and moisture-stable, functionally tolerant, commercially available and far less corrosive than many metal halides. Compared with earlier methods, the advantages are clear. A DMAP-catalyzed protocol requires ultrasonic irradiation in toxic DMF, ZrO2 nanoparticles demand nanomaterial synthesis and raise agglomeration concerns, and the sulfonated SBA-15 catalyst involves a cumbersome multistep preparation. The bismuth route runs in a drinkable solvent mixture under ordinary reflux, and the catalyst can be recovered, although reusability tests showed a gradual decline: the model reaction yielded 79 percent in the first cycle, 74 percent in the second, 65 percent in the third and only 33 percent in a fourth run that stretched to nine hours. The authors suggest that immobilization strategies could extend catalyst lifetime in future work.

On the computational side, the team optimized all eight molecules at the B3LYP/6-31G(d,p) level of density functional theory, confirming that each geometry corresponds to a true energy minimum. Frontier molecular orbital analysis revealed that the nitro-substituted compounds 4a and 4b possess the smallest HOMO-LUMO gaps, at roughly 0.135 electron volts, making them the most electronically soft and reactive members of the series, primed for donor-acceptor interactions with biological macromolecules. In contrast, the bromo, methyl and unsubstituted derivatives 4c, 4f and 4g showed the largest gaps and the greatest hardness, indicating higher kinetic stability. Mulliken charge analysis and molecular electrostatic potential maps pinpointed the carbonyl oxygens and ring nitrogens as the electron-rich hotspots most likely to engage in hydrogen bonding with protein targets, while the N-H hydrogens carried the complementary positive charge.

The biological evaluation used the broth dilution method to determine minimum inhibitory concentrations against four bacterial strains, the Gram-negative Escherichia coli and Pseudomonas aeruginosa and the Gram-positive Staphylococcus aureus and Streptococcus pyogenes, benchmarked against ampicillin, and three fungal strains, Candida albicans, Aspergillus niger and Aspergillus clavatus, benchmarked against griseofulvin. The structure-activity trends were striking. The unsubstituted compound 4g and the methoxy-bearing 4h were the standout antibacterial agents, with MIC values as low as 65 and 62 micrograms per milliliter respectively against E. coli and S. aureus, figures that actually beat ampicillin on those strains. The methyl derivative 4f was the most potent antifungal, matching griseofulvin’s 100 micrograms per milliliter against A. niger. Electron-donating groups such as methyl and methoxy generally enhanced activity, likely by improving lipophilicity and membrane penetration, while nitro groups tended to raise MIC values.

To explain these observations mechanistically, the researchers docked all eight compounds into two clinically significant enzymes: bacterial beta-lactamase, the molecular engine of antibiotic resistance, and sterol 14-alpha demethylase CYP51, the fungal enzyme that builds ergosterol membranes. Using AutoDock 4.2 with a Lamarckian genetic algorithm, they found that compounds 4b and 4h bound beta-lactamase most favorably, with estimated free energies of binding of minus 9.30 and minus 9.43 kilocalories per mole, stabilized by networks of hydrogen bonds, pi-alkyl contacts and pi-cation interactions with residues such as ARG 661, PRO 298 and TYR 611. Against CYP51, compound 4b again led with minus 7.79 kilocalories per mole, and the methyl-substituted 4f formed six hydrogen bonds within the active site, consistent with its antifungal potency. Torsional free energies were nearly uniform across the series, indicating that differences in affinity stem from noncovalent interaction networks rather than ligand flexibility.

Molecular dynamics simulations then stress-tested the most promising complexes over 100-nanosecond production runs using the Maestro-Desmond package with the OPLS3e force field. The 4f-beta-lactamase complex held a backbone root mean square deviation averaging 1.60 angstroms, comfortably below the 2.0 angstrom threshold that validates docking reliability, and stabilized after an initial 30-nanosecond equilibration. The 4d-CYP51 complex averaged 1.40 angstroms, closely tracking the reference antifungal ligand VT1 at 1.35 angstroms. Root mean square fluctuation analysis showed that flexibility was confined to loop and terminal regions, leaving the binding pockets rigid. Contact histograms revealed that water-bridged hydrogen bonds, involving residues such as Ser64, Gln120, Thr122 and Tyr132, together with hydrophobic contacts, were the main glue holding the complexes together throughout the simulations.

The pharmacoinformatic layer of the study added yet another dimension. POM analysis, which classifies bioactivity based on dipolar interactions between electron-rich and electron-deficient centers, identified three distinct antitumor pharmacophore regions built from NH donor to carbonyl oxygen acceptor pairs, plus a single antibacterial pharmacophore involving an NH-CO motif. OSIRIS toxicity screening found no major mutagenic, tumorigenic, irritant or reproductive risks across the series, with compounds 4d and 4g earning the highest drug scores at 0.70 and 0.87. Molinspiration calculations confirmed that all eight molecules respect Lipinski’s criteria almost entirely: molecular weights below 500 daltons, five or fewer rotatable bonds, moderate polarity and acceptable lipophilicity, with only the heavily nitrated pair showing a single violation.

Taken together, the work demonstrates a rare degree of coherence between experiment and computation. The compounds with the best MIC values, notably 4f, 4g and 4h, also posted strong docking energies and stable dynamic behavior, while the electrostatic potential maps independently flagged the same carbonyl and ring nitrogen atoms that docking identified as interaction points. The authors conclude that pyrido[2,3-d]pyrimidines represent a promising scaffold for further antimicrobial development, and that targeted substitution of the aromatic ring is the key lever for tuning activity. With antimicrobial resistance declared one of the top global public health threats, a synthetic method that is cheap, green and high-yielding, paired with a validated computational pipeline for prioritizing candidates, offers exactly the kind of integrated approach the field has been calling for. The next step will be translating these laboratory and in silico hits into lead compounds capable of surviving the far harsher tests of animal models and, eventually, the clinic.

Subject of Research: Green synthesis and antimicrobial evaluation of pyrido[2,3-d]pyrimidine derivatives catalyzed by bismuth(III) triflate

Article Title: Bi(OTf)₃-catalyzed one-pot synthesis and antimicrobial evaluation of bioactive pyridopyrimidines supported by DFT, molecular dynamics, POM and pharmacophore analyses

Article References: Mehta, A. W., Abdel-Megid, M., Patil, R. C., Ahmed, S., Salem, M. E., Abu-Rayyan, A., Shtaiwi, A., Hajam, Y. A., Bhat, A. R., Eissa, M. E., Agisho, H. A., Mujahid, M. H., Yamari, I., Raza, N., & Zbancioc, A. M. (2026). Bi(OTf)₃-catalyzed one-pot synthesis and antimicrobial evaluation of bioactive pyridopyrimidines supported by DFT, molecular dynamics, POM and pharmacophore analyses. Results in Chemistry, 30, Article 103849. https://doi.org/10.1016/j.rechem.2026.103849

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103849

Keywords: pyridopyrimidines, bismuth(III) triflate, multicomponent reaction, green chemistry, antimicrobial activity, molecular docking, molecular dynamics, DFT, beta-lactamase, CYP51, drug discovery, pharmacophore