O-Glycosidic linkages, formation

III. 3-6>-Glycopeptides of l-Serine or l-Threonine 1. Formation of the 3-O-Glycosidic Linkage... 

For glycoproteins, the formation of anhydro rings from hexosamine residues is important in chromogen formation, especially in relation to the Ehrlich reaction and its modifications (3). / -Elimination is involved in the removal of oligosaccharide or polysaccharide chains from the protein core of glycoproteins when they are attached by O-glycosidic linkages to serine or threonine residues. A subsequent -elimination may also... 

Acetonitrile is frequently used as the solvent of choice to enhance the formation of 1,2-trans-glycosidic linkages, by the intermediacy of a kinetically formed a-glycdsyl nitrilium ion [25], Accordingly, coupling of la with 4 in CHjCN gave a reversed stereoselectivity (a/p, 1 3) compared with toe reaction carried out in Et O or CHjClj (Scheme 6). 

The first report on DFA formation in higher plants dates back to 1933, when Schlubach and Knoop [23] isolated a compound tentatively identified as a-D-fructofuranose p-o-fmctofuranose l,2 2,l -dianhydride (10, also known as DFA I) from Jerusalem artichoke. Alliuminoside, a difructofuranose 2,6 6,2 -dianhydride for which configuration at the glycosidic linkages was not determined, was isolated from tubers of Allium sewertzowi [24], However, the fact that these results have not been further confirmed throws some doubt onto whether the DFAs were actually from plant origin or were formed by the presence of microorganisms. The enzymic formation of a-o-fructofuranose p-o-fructofuranose l,2 2,3 -dianhydride (1, DFA HI) in sterilized homogenates of the roots of Lycoris radiata, a plant use in China as a traditional folk medicine, unequivocally demonstrated the capacity of this plant to produce this particular DFA [25]. The compound was further extracted from the intact bulbs by supercritical carbon dioxide and its structure unequivocally established by NMR [26]. 

Cellulose is the principal structural component of vegetable matter. Wood is 30-40% cellulose, cotton over 90%. Photosynthesis in plants is responsible for the formation of 10 tons per year of cellulose. Structurally, cellulose is a polysaccharide composed of several thousand o-glucose units joined by 3(l,4)-glycosidic linkages (Figure 25.8). Complete hydrolysis of all the glycosidic bonds of cellulose yields o-glucose. The disaccharide fraction that results from partial hydrolysis is cellobiose. 

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