Glycoside mutarotation

The most familiar of all the carbohydrates is sucrose—common table sugar. Sucrose is a disacchar ide in which D-glucose and D-fructose are joined at then anomeric carbons by a glycosidic bond (Figure 25.7). Its chemical composition is the same ine-spective of its source sucrose from cane and sucrose from sugar beets are chemically identical. Because sucrose does not have a free anomeric hydroxyl group, it does not undergo mutarotation. 

Glycosides are named by first citing the alkyl group and then replacing the -ose ending of the sugar with -oside. Like all acetals, glycosides are stable to neutral water. They aren t in equilibrium with an open-chain form, and they don t show mutarotation. They can, however, be converted back to the free monosaccharide by hydrolysis with aqueous acid (Section 19.10). 

Towards the end of the nineteenth century it was realized that the free sugars (not only the glycosides) existed as cyclic hemiacetals or hemiketals. Mutarotation, discovered in 1846 by Dubrunfaut, was now interpreted as being due to a change... 

When the anomeric hydroxyl group of one monosaccharide is bound glycosidically with one of the OH groups of another, a disaccharide is formed. As in all glycosides, the glyco-sidic bond does not allow mutarotation. Since this type of bond is formed stereospecifically by enzymes in natural disaccharides, they are only found in one of the possible configurations (a or P). 

The sections that follow describe the process of mutarotation in glucose and how glycoside formation can inhibit it. 

The presence of a glycoside, which involves the formation of an acetal (see Figure 16-6) or a hemiacetal, can block mutcirotation. Glycosides are different from the original carbohydrates in that they can t undergo mutarotation because the ring is locked (a locked ring can t reopen). 

Problem 22.14 Glycosides do not react with either Fehling s or Tollens reagents and do not mutarotate. Explain. M... 

Glycosides are acetals. They are stable in the basic Fehling s and Tollens solutions and in aqueous solutions used in mutarotation. Glycosides are nonreducing. 

Acetals and ketals are also called glycosides. Acetals and ketals (glycosides) are not in equilibrium with any open chain form. Only hemi-acetals and hemiketal s can exist in equilibrium with an open chain form. Acetals and ketals do not undergo mutarotation or show any of the reactions specific to the aldehyde or ketone groups. For example, they cannot be oxidized easily to form sugar acids. As an acetal, the carbonyl group is effectively protected. 

Similarly, in a sample containing two nonequivalent nuclei Ax and A2, the transverse magnetization results from two components due to two Larmor frequencies. In this case, the FID signal is modulated by the chemical shift difference of Larmor frequencies, Ay = Vj — v2. This modulation is illustrated in Fig. 2.6(a) by the FID signal of a sample of 1 -13C-n>-glucose after mutarotation. The product mixture of mutarotation contains a-and /i-glucopyranose with differently shielded glycosidic carbons separated in the 13C NMR spectrum by 87.5 Hz. 

No, glycosides cannot undergo mutarotation because the anomeric carbon is not free to interconvert between < = and P configurations via the open-chain aldehyde or ketone. 

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