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In this thesis, crude polysaccharides were extracted from the raw material of Durvillaea Antarctica using multistep extraction strategies including cellulase extraction, water extraction, hot water extraction and alkaline extraction. The physicochemical and structural properties of extracted polysaccharides were characterized. Seventeen kinds of Durvillaea antarctica polysaccharides including eight alginates, eight fucoidans and one mixture of alginate and fucoidan were successfully isolated by the traditional method as well as cellulose extraction. The yield of alginate extracted from Durvillaea Antarctica using more cellulose was getting lower while the total amount of polysaccharide product increased. Generally, more crude polysaccharides were obtained using cellulase process other than water extractions. The M/G rate of alginate extracted from cellulose-water solution was much higher than either alkaline solution or cool water, and the M/G rate increased along with the more enzyme involved in the degradation. The cellulase method was proved to have no significant differences in monosaccharide and functional group compositions in comparison with water extraction, hot water extraction and hot alkaline extraction method. However, the molecular weights of polysaccharides (fucoidan, alginate starch) obtained from the alcohol-precipitation of cellulase degradation were much higher than that from water or alkaline extractions.A water-soluble polysaccharide (DAC) was isolated and purified from the components of water extractions. According to its structural characterization and immune activity in vitro, the DAC was determined to possess 97.5% of total saccharide,2.0% of protein and 18.3kDa of molecular weight, respectively. After methylation and NMR analysis, it was concluded that DAC was mostly composed of →3)Glc(β1→as backbone, while small amount of glucan was attached as side chains in the form of →6)Glc(α/β1→. The phagocytic capacity of DAC towards macrophages achieved the maximum value at the concentration of 50 μg·mL-1 in the immune experiment in vitro.Carboxymethylation, sulfonation and sulfonation of carboxymethylated substrates from purified water-soluble glucan was applied on the preparation of three kinds of derivatives and evaluation of their immune activities in vitro. It was shown that the molecular weights of polysaccharides decreased after modification, but the derivatives associated with sulfate groups exhibited better activities even at low concentrations.The degradation of water-soluble glucan from Durvillaea antarctica was investigated by intestinal microorganism, and a special microbial cell from intestine was observed to be effective via microbiological degradation. The time of microbial degradation was optimized for 48 hours, and the sequences of oligosaccharides were determined to be di-, tri-, tetrasaccharide associated with 1,3-linked and 1,6-linked linkages.