Abstract:A polysaccharide with a homogeneous component was purified from a polysaccharide with an alcohol precipitation concentration of 65% and named Undaria pinnatifida polysaccharide（UPP）.The carboxymethylation modification process of UPP，as well as the physicochemical properties and hypoglycemic activity in vitro of the polysaccharides before and after modification were studied.The modified polysaccharide was obtained and named carboxymethylated UPP（CM-UPP）by the response surface optimization experiment based on a single factor test.Fourier transform infrared spectra，thermogravimetry，and X-ray diffraction were used to characterize the two polysaccharides.The results showed that the content of protein and polyphenol in UPP was lower，and that of total sugar was（93.94±0.47）%.The optimum condition for carboxymethylation modification was as follows：Chloroacetic acid concentration of 1.46 mol/L，reaction temperature of 65 ℃，NaOH concentration of 20%，and degree of substitution of 0.887.Both UPP and CM-UPP were amorphous structures，and CMUPP had stronger thermal stability.Activity tests in vitro showed that the inhibition of UPP and CM-UPP on αamylase was not dose-dependent，and the highest inhibitory rates of UPP and CM-UPP were 69.01% and 76.82%.The inhibition of α-glucosidase by the two polysaccharides showed a dose-effect relationship，and the IC50 values of UPP and CM-UPP were 3.467 mg/mL and 0.503 mg/mL.In summary，carboxymethylation modification can improve the hypoglycemic ability in vitro of UPP.It provides a theoretical foundation for the further development of UPP.