Thiol-Functionalized Succinoglycan via Cysteine Grafting: Enhanced Rheological Properties and Antioxidant Activity

Cysteine-modified succinoglycan (SG-Cys) was synthesized via EDC/NHS-mediated amidation by grafting cysteine onto succinoglycan isolated from Sinorhizobium meliloti. The successful introduction of cysteine moieties was confirmed by 1H NMR and FTIR analyses, while the degree of substitution was quantitatively determined using Ellman's assay. The incorporation of cysteine significantly influenced the physicochemical and rheological properties of the polymer. In particular, SG-Cys exhibited up to a 1.8-fold increase in viscosity compared with native succinoglycan. The viscoelastic behavior of SG-Cys was systematically evaluated under various environmental conditions, including different pH, ionic strengths, temperatures, and polymer concentrations, revealing enhanced responsiveness to external stimuli. Radical scavenging assays demonstrated that SG-Cys displayed up to a 2.5-fold increase in antioxidant capacity compared with unmodified SG, as determined by DPPH and ABTS assays. Cytotoxicity evaluation using HEK-293 cells confirmed that the modified polymer exhibited no significant cytotoxic effects. Overall, the results demonstrate that thiol functionalization of succinoglycan effectively improves both rheological performance and antioxidant activity, suggesting that SG-Cys is a promising multifunctional bioactive polymer for potential applications in biomaterials, drug delivery, and bioengineering systems.