Synthesis and evaluation of antibacterial and anti-oxidant activity of small molecular chitosan–fucoidan conjugate nanoparticles

Low-molecular weight chitosan (LMWC) and low-molecular weight fucoidan (LMWF) have been reported to possess diverse biological activity. In this study, LMWC and LMWF of different molecular weights were prepared by a H₂O₂-involved oxidative depolymerization method. Covalently linked LMWC–LMWF conjugates were synthesized through the activation of carboxyl groups of LMWF and an amidation reaction between the activated LMWF and LMWC. Light transmittance assay and TEM analysis showed that the LMWC–LMWF conjugates gradually form colloidal nanoparticles during the amidation reaction. The LMWC–LMWF conjugates prepared from lower-molecular weight (depolymerized in a longer degradation time) LMWC and LMWF have larger particle sizes and lower zeta potentials. The LMWC–LMWF conjugates were investigated for their ability to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH), hydroxyl and superoxide anion radicals. The DPPH and superoxide anion radical scavenging activities of the LMWC–LMWF conjugates were higher than that of LMWC alone and generally increased with a decrease in degradation time during the preparation of LMWF and LMWC. LMWC has superior hydroxyl radical scavenging activity, thus the hydroxyl radical scavenging activity of LMWC–LMWF conjugates were higher than that of LMWF alone. Bacterial inhibition assay of the conjugates against Escherichia coli and Staphylococcus aureus indicated that the conjugation of LMWC with LMWF improved the antibacterial activity of LMWF.