Optical rotation of sugars

J. PeckaandM. Cerny, Syntheses with anhydro sugars. 15. Syntheses of l,6-anhydro-2,3,4-trideoxy-jS-D-glycero-hexopyranose and its unsaturated derivatives as model substances for studying the optical rotation of sugars, Collect. Czech. Chem. Commun., 38 (1973) 132-142. 

Table 4. Optical rotations of sugars in water and in 0.6M borate buffer (33).

The relation of ring conformations to the optical rotations of sugar derivatives offers promise in the interpretation of fine features of the structures 121). 

Polarimetry. Polarimetry, or polarization, is defined as the measure of the optical rotation of the plane of polarized light as it passes through a solution. Specific rotation [ a] is expressed as [cr] = OcjIc where (X is the direct or observed rotation, /is the length in dm of the tube containing the solution, and c is the concentration in g/mL. Specific rotation depends on temperature and wavelength of measurement, and is a characteristic of each sugar it may be used for identification (7). 

Polarization is the most common method for the determination of sugar in sugar-containing commodities as well as many foodstuffs. Polarimetry is apphed in sugar analysis based on the fact that the optical rotation of pure sucrose solutions is a linear function of the sucrose concentration of the solution. Saccharimeters are polarimeters in which the scales have been modified to read directiy in percent sucrose based on the normal sugar solution reading 100%. 

Sucrose, in contrast, is a disaccharide of almost universal appeal and tolerance. Produced by many higher plants and commonly known as table sugar, it is one of the products of photosynthesis and is composed of fructose and glucose. Sucrose has a specific optical rotation, of +66.5°, but an... 

The phosphate of ethylene glycol must derive from the ribitol phosphate moiety, which consequently is phosphorylated at a primary position, assumed to be 0-5 of (pro-D)-ribitol for biosynthetic reasons. In the proposed structure for the S10A repeating-unit (14), the anomeric natures of the sugar residues were not determined. The optical rotations of S10A and the hexasaccharide, [a]D +12° and +11°, respectively, indicate that they contain both a- and /3-D-linked sugar residues. 

Aldonamides are readily prepared by reaction of lactones with liquid ammonia (86,99,100), with ammonium hydroxide (101,102), or by bubbling ammonia gas into alcoholic solutions of the sugar lactones (103-104). Aldonamides of the tetronic adds are stable in aqueous solution (105), but penton- or hexon-amides are hydrolyzed, as shown by the change of the optical rotation of the amide solutions (106). The hydrolysis is catalyzed by acids and bases, and the product was the ammonium salt of the aldonic acid. 

Acid Hydrolysis. Lactose is resistant to acid hydrolysis compared to other disaccharides such as sucrose. In fact, organic acids, such as citric acid, that easily hydrolyze sucrose are unable to hydrolyze lactose under the same conditions. This is useful in analyzing a mixture of these two sugars, because the quantity of sucrose can be measured by the extent of these changes in the optical rotation of reducing power as a result of mild acid hydrolysis. The speed of hydrolysis of lactose varies with time, temperature, and concentration of the reactant, as shown in Table 6.8. 

The specific optical rotation of many sugars and sugar derivatives is altered by the presence of metal salts, the alteration being the greater, the greater the concentration of the salt. This phenomenon, which is now generally attributed to the formation of adducts, will be considered later in more detail. 

Vavrinecz has shown that, the optical rotation of sucrose depends upon the concentration of both the sugar and the salt. Empirical relationships for the chlorides, bromides, iodides, and acetates of sodium and potassium were determined. 

Although the crystalline forms of a- and /3-D-glucose are quite stable, in solution each form slowly changes into an equilibrium mixture of both. The process can be observed as a decrease in the optical rotation of the a anomer (+112°) or an increase for the /3 anomer (+18.7°) to the equilibrium value of 52.5°. The phenomenon is known as mutarotation and commonly is observed for reducing sugars. Both acids and bases catalyze mutarotation the mechanism, Equation 20-1, is virtually the same as described for acid- and base-catalyzed hemiacetal and hemiketal equilibria of aldehydes and ketones (see Section 15-4E) ... 

Mutarotation. The change in optical rotation of a sugar that is observed immediately after it is dissolved in aqueous solution, as the result of the slow approach of equilibrium of a pyra-nose or a furanose in its a and /3 forms. 

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