Monosaccharide specific rotation

One mole of sucrose produces 1 mol each of glucose and fructose, and the specific rotation is one-half the sum of those of the two monosaccharides i.e. 

Anomeric forms of a monosaccharide are in equilibrium in aqueous solution. Mutarotation is the change in specific rotation that accompanies this equilibration ... 

The monosaccharide D-glucose, whose chemistry is representative of all aldoses containing four or more carbon atoms, exists predominantly in the two pyranosc forms 4 and 5. These are six-membered hemiacetals formed by the reversible cyclization of the acyclic polyhydroxy aldehyde 3 (Eq. 23.1). In the cyclic forms 4 and 5, the ring carbon that is derived from the carbonyl group is referred to as the anomeric carbon atom. The specific rotation, [a] (Sec. 7.5), of a-D-(+)-glucose (4) is +112 whereas that of the -anomer 5 is +19°. When crystals of either pure 4 or pure 5 are dissolved in water, the [a]p changes to an equilibrium value of +52.7°. This process is termed mutarotation. At equilibrium in water, the a- and p-forms are present in the ratio of 36 64 only about 0.03% of D-glucose is in the acyclic form 3. 

The specific rotations for a-D-mannose and p-D-mannose are -1-29.3° and -17.0°, respectively. When either of these monosaccharides is dissolved in water, the rotation reaches an equilibrium value of -I-14.2. Calculate the percentage of the a-anomer present at equilibrium. Show your calculations. 

A classical, and still most useful, physicochemical parameter for characterizing glycosaminoglycans is their specific optical rotation. For pure heparins from common sources, [a]80 = + 45 to + 53 ° (see Table II). The actual value depends on the relative proportions of monosaccharide constituents (that is, the L-idosyluronic acid residues make a less positive contribution than the n-glucosyluronic residues), and on the degree of sulfation of the preparation (that is, although sulfate groups may have little direct influence on the molecular rotation, they lower the value by acting as diluents ). 

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. 

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