A novel chitosan-Schiff bases bearing a new quinoxaline moiety as an approach for potent antimicrobial agent: Synthesis, characterization and in vitro assessments.

This study aims to enhance the antimicrobial properties of chitosan through preparing novel chitosan Schiff bases via coupling with 4-formylphenyl 2,3-dioxo-1,2,3,4-tetrahydroquinoxaline-6-sulfonate (B5) where, different molar ratios of B5 were used to prepare various Schiff bases with chitosan, resulting in Schiff bases coded as d5, d6, d7, and d8, respectively. The modified chitosan samples (d5, d6, d7, and d8) showed reduced crystallinity and improved thermal stability. The crystallinity index of unmodified chitosan was 64 %, which decreased to 59, 55.7, 52 and 48 % for d8, d7, d6 and d5 samples, respectively. TGA analysis demonstrated improved thermal stability in Schiff base derivatives, with T50 values at 311 °C for chitosan, increasing to 520 °C for d5 sample, while d6, d7 and d8 samples were detected at 430, 425 and 419 °C, respectively. Degrees of substitution were determined from nitrogen content (N%) and were found to correlate with the B5-to-chitosan ratio. The d5 sample, having the highest B5 content, exhibited the maximum degree of substitution at 0.52. The d7 formulation demonstrating significant growth inhibition: 98.16 % for Staphylococcus aureus, 93.91 % for Candida parapsilosis, and 87.35 % for Klebsiella pneumoniae. These high inhibition rates indicate d7 strong potential as a robust antimicrobial agent, particularly suitable for applications targeting multidrug-resistant infections.