Glycosyl unit

Every polysaccharide contains glycosyl units with unsubstituted hydroxyl groups available for esterification or etherification. Polysaccharide derivatives are described by their degree of substitution (DS), which is the average number of substituent groups per glycosyl unit. Because each monomeric unit of cellulose molecules has free hydroxyl groups at C-2, C-3, and C-6, the maximum DS for cellulose, and all polysaccharides composed exclusively of neutral hexosyl units, the majority of polysaccharides, is 3.0. 

A disaccharide in which one glycosyl unit has replaced the hydrogen atom of an alcoholic hydroxy group of the other is named as a glycosylglycose. The locants of the glycosidic linkage and the anomeric descriptor(s) must be given in the full name. 

The oligosaccharide-alditols in PG-lc-4 were analyzed by GC-MS as permethylated oligosaccharide-alditols. The results indicated that PG-lc-4 contained di- to tetrasaccharide-alditols possessing glycosyl units such as Hex-(l- 4)-6-deoxyHex-ol, Pen->Pen->deoxyHex-ol, Pen- Hex->deoxyHex-ol, Hex->Hex- Hex-ol,... 

An elegant synthesis of a monomeric porphyrin intermediate bearing one pyridyl and three glycosyl units, precursor of cationic dimers, has also been described (Scheme 18).153 Thus pyrrole, p-formylphenyl 2,3,4,6-tetra-<9-acetyl-/i-D-... 

Supports activated with divinyl sulfone, like those activated with bisoxirane, can be coupled to underivatized carbohydrates.21 The supports activated with divinyl sulfone couple to the hemiacetal hydroxyl group of the carbohydrate by the vinyl group. Accordingly affinity adsorbents prepared by this method will contain glycosyl units. The structure of the reaction product of L-fucose with agarose activated with divinyl sulfone is shown in 5. 

Structural features met in some cellulases include an a,a barrel111 similar to that of glucoamylase (Fig. 2-29) and, in a cellobiohydrolase,101 a 5-nm-long tunnel into which the cellulose chains must enter. Ten well-defined subsites for glycosyl units are present in the tunnel.101 A feature associated with this tunnel is processive action, movement of the enzyme along the chain without dissociation,105 a phenomenon observed long ago for amylases (see Section 9) and often observed for enzymes acting on nucleic acids. 

Figure 20-1 Some routes of interconversion of monosaccharides and of polymerization of the activated glycosyl units.

The addition of a Lewis acid, i.e., ZnC significantly decreases the production of tar and enhances the production of char due to the enhanced dehydration reactions. At higher temperatures the glycosyl units and the random condensation products are further degraded to a variety of volatile products, as shown in Table V (9). Comparison of this table with the high temperature pyrolysis products listed for cellulose in Table III shows that the products of both fractions are basically similar. The significant increase in the yields of 2-furaldehyde, water and char and decrease in the yield of tar by the addition of ZnCl verifies the enhanced dehydration and is similar to observed effects in cellulose pyrolysis. 

Studies of the immunoreactivities of NGPs with homologous antibodies (Abs) have been mainly directed to qualitative conclusions, some of which have already been mentioned. Table VI shows the structures of the outer regions of the hapten moieties of the M. avium serocomplex, followed by the structures of the glycosyl units in the synthetic NGPs, and the reactivity or lack thereof with the relevant specific antibodies. Antibodies were of two types, polyclonal rabbit antisera and... 

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