The Glycosidic Bond

All monosaccharides and their derivatives that possess aldehyde or ketone groups (that is, excepting derivatives such as alditols and aldonic acids) will have reducing properties. Moreover, those with the appropriate number of carbon atoms can form rings occurring in two forms (anomers) and in which the potential reducing carbon is called the anomeric carbon. 

Question-. How reactive is the anomeric hydroxyl group compared with the other hydroxyl groups in a monosaccharide  

The reactivity of the anomeric hydroxyl group is illustrated by the ease with which monosaccharides react with alcohols and with amines. The normal hydroxyl groups in the molecule do not react, but the anomeric hydroxyl does. The process is known as glycosylation (of the alcohol or amine), and the products as O-glycosides and N-glycosides. 

As with the parent monosaccharide, the anomeric carbon can have either the a or the /3 configuration since it remains a chiral center. 

Question In what respect does a glycoside ring differ from the ring structure of a monosaccharide  

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Maltose obtained by the hydrolysis of starch and cellobiose by the hydrolysis of cellulose are isomenc disaccharides In both maltose and cellobiose two d glucopyra nose units are joined by a glycosidic bond between C 1 of one unit and C 4 of the other The two are diastereomers differing only m the stereochemistry at the anomeric carbon of the glycoside bond maltose is an a glycoside cellobiose is a (3 glycoside... 

FIGURE 25 6 Molecu lar models of the disaccha rides maltose and cellobiose Two D glucopyranose units are connected by a glycoside linkage between C 1 and C 4 The glycosidic bond has the a orientation in maltose and IS p in cellobiose Mai tose and cellobiose are diastereomers... 

Lactose is a disacchande constituting 2-6% of milk and is known as milk sugar It differs from maltose and cellobiose m that only one of its monosaccharide units is D glucose The other monosaccharide unit the one that contributes its anomeric carbon to the glycoside bond is d galactose Like cellobiose lactose is a (3 glycoside... 

Sections Disaccharides are carbohydrates in which two monosaccharides are 25.14-25.15 joined by a glycoside bond. Polysaccharides have many monosaccharide units connected through glycosidic linkages. Complete hydrolysis of disaccharides and polysaccharides cleaves the glycoside bonds, yielding the free monosaccharide components. 

FIGURE 11.12 Rotation around the glycosidic bond is sterically hindered syn versus anti conformations in nucleosides are shown. 

RNA is relatively resistant to the effects of dilute acid, but gentle treatment of DNA with 1 mM HCl leads to hydrolysis of purine glycosidic bonds and the loss of purine bases from the DNA. The glycosidic bonds between pyrimidine bases and 2 -deoxyribose are not affected, and, in this case, the polynucleotide s sugar-phosphate backbone remains intact. The purine-free polynucleotide product is called apurinic acid. 

Apparently no phosphate migration was noticed by either group of authors, in spite of the acid conditions employed for the cleavage of the glycosidic bond. 

The glycosidic bond to an anomeric carbon can be either a or (3. Maltose, the disaccharide obtained by enzyme-catalyzed hydrolysis of starch, consists of two cv-D-glucopyranose units joined by a 1->4-o-glycoside bond. Cellobiose, the disaccharide obtained by partial hydrolysis of cellulose, consists of two /3-o-glucopyranose units joined by a 1—>4-/3-glycoside bond. 

The homology between compounds 1 and 2 is obvious. Paeoniflorin (1) can be formed in the event that a / -glycosidation of a suitably protected derivative of paeoniflorigenin (2) can be achieved. Retrosynthetic cleavage (see Scheme 1) of the glycosidic bond in 1 furnishes intermediate 3 as a potential precursor and key synthetic... 

At temperatures above Tm, chemical and enzymatic degradation of microbial exopolysaccharides is enhanced. The apparent enhanced stability of microbial exopolysaccharides in their ordered confirmation is thought to be due to the glycosidic bonds in the backbone of the polymer which raises the activation energy. This restricted movement would also restrict access of enzymes and chemicals to the backbone. 

Note. This prefix includes the oxygen of the glycosidic bond. An example is given in 2-Carb-31.2 more are given below. 

RNA is as suitable (if not more so) than DNA as a cleavage target [37]. In contrast to DNA, RNA is substantially less prone to oxidative cleavage [38] as a consequence of the higher stability of the glycosidic bond in ribonucleotides compared to that in deoxyribonucleotides. On the basis of the properties described in the introductory sections RNA is by contrast, much less stable to hydrolytic cleavage. For this reason the hydrolysis of the phosphate bond in this system can be successfully catalyzed not only by metal ions but also by ammonium ions. 

Fig. I.—Hypothetical Transition State for the Cleavage of the Glycosidic Bond of a (GlcNAc) Chito-oligosaccharide Chain at Sub-Site D of the Substrate Binding Cleft of Lysozyme (from Ref. 65, with Permission).

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