Carbohydrates mutarotation

Predict which carbohydrates mutarotate, which reduce Tollens reagent, and which undergo epimerization and isomerization under basic conditions. (Those with free hemiacetals will, but glycosides with full acetals will not.) Problems 23-58, 62, 63, and 66... 

It shows the usual carbohydrate reactions and most resembles mannose in behaviour it exists in OL and fi forms which exhibit mutarotation. 

A particular carbohydrate can mterconvert between furanose and pyra nose forms and between the a and (3 configuration of each form The change from one form to an equilibrium mixture of all the possible hemi acetals causes a change m optical rotation called mutarotation... 

Mutarotation (Section 25 8) The change in optical rotation that occurs when a single form of a carbohydrate is allowed to equilibrate to a mixture of isomeric hemiacetals... 

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 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). 

The original publication by Sweeley and coworkers5 was concerned with the separation of a wide range of carbohydrates, from mono- to tetra-saccharides. Most of the subsequent publications have considered the quantitative analysis of mixtures of varied complexity, although two studies have demonstrated the separation of the protium from the deuterium fonns of monosaccharides.200,201 The study of mutarotational equilibria by gas-liquid chromatography has been discussed in Section IV (see p. 38). 

Some carbohydrates actively inhibit the crystallization of lactose, whereas others do not. Carbohydrates that are active possess either the /3-galactosyl or the 4-substituted-glucose group in common with lactose, so that adsorption can occur specifically at certain crystal faces (Van Krevald 1969). (3-Lactose, which is present in all lactose solutions [see Equilibrium in Solution (Mutarotation )], has been postulated to be principally responsible for the much slower crystallization of lactose compared with that of sucrose, which does not have an isomeric form to interfere with the crystallization process (Van Krevald 1969). Lactose solubility can be decreased substantially by the pres-... 

Examination of Figure 9.11 demonstrates an excellent separation of a-glucose from (3-glucose which points out the power of GLC for the separation of these materials as well as a significant complication in the study of carbohydrates. It is well known that solutions of some carbohydrates undergo anomerization and that an initially pure form of a sugar may result in an equilibrium mixture (mutarotation) as in Figure 9.12. 

The majority of the many methods used to study the composition of equilibrium solutions of carbohydrates examine the mixture without separating the individual components. With the discovery that the anomeric forms of sugars could be readily separated by gas chromatography of their tri-methylsilyl ethers, a new approach to the problem was found. A protocol was developed for the direct gas chromatographic analysis of the amount of each anomer present in an aqueous solution. The protocol can be used on the micro scale and can be used in enzyme assays such as that for mutarotase. The method has been made more effective by combining gas chromatography with mass spectrometry. It is shown how mass spectral intensity ratios can be used to discriminate anomers one from another. The application of these methods to the study of complex mutarotations is discussed. 

The question of the detailed composition of sugar solutions in various solvents together with the chemistry of the mutarotation reaction has continued to attract attention through the years. In their classical review, Mutarotation of Sugars in Solution, completed in 1969, Isbell and Pig-man have cited references to more than 320 papers and books (5, 6). The appearance of this volume justifies Isbells faith that carbohydrate chemistry continues to have a significant impact on the development of new concepts permeating all branches of chemistry and biochemistry (7). ... 

During mutarotation, an °=or (3 form of a carbohydrate is converted to an equilibrium mixture of the two. Mutarotation can be detected by observing the change in optical activity overtime as the two forms equilibrate... 

Polarimetry is extremely useful for monitoring reactions of optically active natural products such as carbohydrates which do not have a useful UV chromophore, and samples for study do not need to be enantiomerically pure. Nevertheless, compared with spectrophotometry, the technique has been applied to relatively few reactions. It was, however, the first technique used for monitoring a chemical reaction by measuring a physical property when Wilhemy investigated the mutarotation of sucrose in acidic solution and established the proportionality between the rate of reaction and the amount of remaining reactant [50]. The study of a similar process, the mutarotation of glucose, served to establish the well-known Bronsted relationship, a fundamental catalysis law in mechanistic organic chemistry. 

Carbohydrates in nature are optically active and polarimetry is widely used in establishing their structure. Measurement of the specific rotation gives information about the linkage type (a or (3 form) and is also used to follow mutarotation. Nuclear magnetic resonance spectroscopy (NMR) can be used to differentiate between the anomeric protons in the a- or /3-pyranose and furanose anomers and their proportions can be measured from the respective peak areas. 

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