Ultrasound Modifies the Properties of Sodium Alginate: Correlation between Biological Activities and Molecular Characteristics

Sodium alginate (SA) is a kind of seaweed-derived bioactive polysaccharides with broad application prospect, but the poor solubility and high viscosity associated with high molecular weight astricts its biological activities, low molecularization is an effective strategy for overcoming the defects. This work depolymerized SA using ultrasound and assessed the effect of reducing molecular weight on viscosity, antioxidation and anticoagulation of SA. Results indicate that, after ultrasound treatment at 50 ℃, 450 W for 1.5 h, 2.5 h and 4 h, weight-average molecular weight (Mw) of SA significantly declined from original 118.41 kDa to 52.37 kDa, 37.54 kDa and 32.14 kDa, respectively, conforming to a second-order kinetic model, which corresponded to the continuous decrease of viscosity of degradation products, but no alteration of spectral characteristics and monosaccharide composition was observed. Differently, there was no consistent bioactive change with the viscosity. Among SA and its three low molecular weight (LMW) forms, 37.54 kDa of LMW-SA presented the highest anticoagulant property demonstrated by the values of activated partial thromboplastin time (APTT) and prothrombin time (PT), especially APTT. Interestingly, discrepant scavenging capacities of SA and the three LMW-SAs on different radicals were observed, along with Mw. decrease of SA, scavenging rates of DPPH and hydroxide radical were continuous reduction, but the ability of scavenging superoxide anion was increase and the platform value reached at the treatment for 2.5 h. These findings provide an important theoretical basis for the low-molecular-weight modification of SA, potentially improving biological properties and advancing application in biomedical and industrial fields.