Preparation, characterization and antioxidant activities of polymannuronic acid phosphate, H-phosphonate and sulfate

Three derivatives of polymannuronic acid (PM) were prepared by chemically polyanionic modification. PM was substituted with phosphate (OPO3(2-)), H-phosphonate (OPO2H(-)) and sulfate (OSO3(-)) groups with the degree of substitution (DS) of 0.41, 1.42 and 1.04, respectively. The structures of all PM derivatives were characterized by FT-IR, (31)P NMR and (13)C NMR spectroscopy. The weight average molecular weight (Mw) was determined by high performance gel permeation chromatography (HPGPC). The antioxidant activities of PM and its derivatives were evaluated in vitro. The results indicated that both phosphate and H-phosphonate groups improved the hydroxyl radical scavenging activity, while sulfate group enhanced the superoxide radical scavenging activity. However, all of the derivatives scavenged DPPH less effectively than PM. The mechanism for how the different anionic substituent groups influenced the antioxidant activities was discussed.