Chitosan and dialdehyde starch as macromolecular platforms for sulfa azo dye/selenium nanocomposites: Synthesis, molecular modelling, and dual DNA gyrase-DHFR inhibition.

Biobased polysaccharides offer versatile macromolecular platforms for designing antimicrobial nanobiomaterials with tunable structure and biological function. In this study, chitosan and dialdehyde starch were converted into Schiff base polymeric matrices through covalent conjugation with sulfonamide-based azo dyes, followed by controlled incorporation of elemental selenium nanoparticles (SeNPs) to form hybrid nanocomposites. Spectroscopic, thermal, microscopic, and textural analyses confirmed successful dye immobilization and uniform dispersion of crystalline SeNPs within the polymer networks. Among the developed systems, the dialdehyde starch-based formulation (SB3/SeNPs) exhibited the highest antimicrobial efficacy, showing broad-spectrum activity against Gram-negative and Gram-positive bacteria and Candida albicans, with minimum inhibitory concentrations as low as 1.25 mg mL-1. The formulation also demonstrated strong antibiofilm performance, achieving inhibition levels exceeding 85% against Escherichia coli and C. albicans, indicating effectiveness against both planktonic and biofilm-associated microorganisms. Enzyme inhibition studies identified bacterial DNA gyrase and dihydrofolate reductase as primary molecular targets, with potent dual inhibition (IC₅₀ = 7.29 ± 0.29 and 0.48 ± 0.019 μg mL-1, respectively). These findings were supported by molecular docking, molecular dynamics simulations, and MM/GBSA free-energy calculations, which confirmed stable, high-affinity binding within catalytically relevant enzyme regions. Preliminary biocompatibility testing using WI-38 human lung fibroblasts showed comparatively low cytotoxicity for SB3/SeNPs (IC₅₀ = 209.50 ± 6.8 μg mL-1), substantially lower than ciprofloxacin. Overall, this work highlights renewable polysaccharides as effective platforms for developing multifunctional antimicrobial nanobiomaterials.