Anomeric Carbons Mutarotation

Changes in optical rotation due to interconversion of anomers in solution are called mutarotation. 

The a and f3 forms of D-glucose have identical configurations at every stereogenic center except at C-1, the anomeric carbon. 

How do we know that monosaccharides exist mainly as cyclic hemiacetals There is direct physical evidence. For example, if D-glucose is crystallized from methanol, the pure a form is obtained. On the other hand, crystallization from acetic acid gives the f3 form. The a and )3 forms of D-glucose are diastereomers. Being diastereomers, they have different physical properties, as shown under their structures in eq. 16.3 note that they have different melting points and different specific optical rotations. 

At equilibrium, an aqueous solution of D-glucose contains 35.5% of the a form and 64.5% of the f3 form. There is only about 0.003% of the open-chain aldehyde form present. 

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The OH unit derived from the carbonyl group, formed by cycliza-tion to a furanose or a pyranose, is attached to the so-called anomeric carbon. An aldose will cyclize to form a stable hemi-acetal structure a furanose or a pyranose. A ketose will cyclize to form a stable hemiketal structure a furanose or a pyranose. The Haworth projection is an older representation for carbohydrates that is based on a planar pyranose or furanose ring. Cyclization of an aldose or a ketose to a furanose or pyranose is accompanied by mutarotation at the anomeric carbon. Mutarotation is the change in optical rotation of a pure furanose or pyranose derivative to that resulting from a mixture of isomers at the anomeric carbon. 

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. 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

Mutarotation produces two types of cyclic forms called anomers (a and P), which differ in their arrangement about the anomeric carbon atom (originally the carbonyl carbon atom). If the -OH on the anomeric carbon atom is down, then the structure represents the a anomer if it s up, the structure represents the p anomer. Due to the equilibrium present, one anomer rapidly converts to the other. 

In the case of glucose, the mutarotation gives 36 percent a, 64 percent p, and negligible strciight chain. The unequal distribution of the two anomers is due to the fact that the -OH on the anomeric carbon of the p form is equatorial, which for a chair conformer is more stable. The -OH on the anomeric carbon in the a anomer is axial, which means this anomer is slightly less stable. 

Monosaccharides commonly form internal hemiacetals or hemiketals, in which the aldehyde or ketone group joins with a hydroxyl group of the same molecule, creating a cyclic structure this can be represented as a Haworth perspective formula. The carbon atom originally found in the aldehyde or ketone group (the anomeric carbon) can assume either of two configurations, a and /3, which are interconvertible by mutarotation. In the linear form, which is in equilibrium with the cyclized forms, the anomeric carbon is easily oxidized. 

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. 

Answer Lactose (Gal061— 4)Glc) has a free anomeric carbon (on the glucose residue). In sucrose (Glc(al<- 2 8)Fru), the anomeric carbons of both monosaccharide units are involved in the glycosidic bond, and the disaccharide has no free anomeric carbon to undergo mutarotation. 

Figure 9.4 Chair forms of glucose anomers. Note that the -OH group on the anomeric carbon (carbon 1) is axial (less stable) in a-D-glucopyranose, whereas it is equatorial in (3-D-glucopyranose. Mutarotation therefore favors the latter.

Yes. Although it is a glycoside, the second glucose unit possesses an anomeric carbon atom and its ring can open to give an aldehyde. For the same reason, solutions of maltose display mutarotation. 

A. It contains a free anomeric carbon and therefore cannot mutarotate. 

Maltose and cellobiose are both reducing sugars because the anomeric carbons on the right-hand glucopyranose units have hemiacetal groups and are in equilibrium with aldehyde forms. For a similar reason, both maltose and cellobiose e.xhibit mutarotation of a and p anomers of the glucopyranose unit on the right. 

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