Saccharides mutarotation

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). 

The chemistry of carba sugars is quite similar to that of cyclitols both groups lack the latent carbonyl groups of their saccharide counterparts, and therefore fail to exhibit many of the characteristic properties of monosaccharides. Thus carba sugars and cyclitols do not form hydrazones or osazones, nor do they mutarotate, or reduce heavy-metal salts in base, in contrast to their oxidation products, the inososes. 

The hydrazones of sugars are capable of existing in various cyclic forms, whose presence is apparent from their nuclear magnetic resonance (NMR) spectra and from the complex mutarotation curves they exhibit110,111117118 (which seldom follow first-order kinetics). The principal structures encountered in saccharide hydrazones are the acyclic, Schiff base-type true hydrazones and the cyclic hydrazino forms, namely glycopyranosyl- and gly-cofuranosylhydrazines. For example, three isomeric forms of D-glucose phenylhydrazone have been isolated.119 The Schiff base derivatives can be... 

Hemiacetals in five- and six-membered rings are considerably more stable than acyclic hemiacetals. They constitute the form in which many saccharides exist under biological conditions. Glucose exists primarily in its pyranose (six-membered ring) form, although furanose (five-membered ring) forms are present also. Both the pyranose and furanose forms have anomeric isomers, known as a and p. These are shown below for the pyranose form. The open chain form is present in neutral water at ambient temperature only to an extent of approximately 0.003% of the total equilibrium. In the context of saccharides, interconversion between the hemiacetals and the open chain forms occurs in a reaction called mutarotation (see below). This reac-... 

In the crystalline state, reducing monosaccharides exist exclusively in cyclic structures (a- or P-anomers). In solutions, a balance between the a- and P-anomers and acyclic forms is established as the individual forms interconvert over time. This process, schematically shown in Figure 4.2, is called mutarotation. The mechanism of mutarotation assumes cleavage of the saccharide cyclic forms... 

Rotation of Polarised Light.— By use of the polarimeter, all carbohydrates can be divided into a dextro-rotatory class and a Isevo-rotatoiy class. The sugar D-fructose is the commonest Isevo-rotatoiy saccharide, and for this reason is termed Icevtdose. Each piu% saccharide has a specific rotatory constant, which is determined under equilibrium conditions so as to avoid confusion due to mutarotation. 

Mutarotation The change in optical rotation associated with the equilibration, in solution, of the anomeric and ring forms of a saccharide beginning with a single crystalline form. Thus, dissolving a-D-glucopyranose in water allows formation of the /3-anomer as well as the furanose forms. Since all have different optical rotations, the initial value changes until the equilibrium mixture is achieved. 

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