Sugars equilibrium compositions

Hultiple products are frequently observed for the separation of TMS-sugar derivatives. At equilibrium reducing sugars can exist in more than one isomeric form known as anomers. Formation of their THS derivatives followed by gas chromatography will result in multiple peaks corresponding in composition to the equilibriue anomeric mixture [436]. 

Sugar Glycoside Original rate constant Equilibrium composition, % Solution time, hours... 

Extrapolation of rates back to zero time also showed a small but definite formation of pyranosides from the free sugar, in addition to conversion from the furanosides. The reversibility of the conversion of furanosides into pyranosides was definitely demonstrated by obtaining the final equilibrium composition from two directions, namely, from the a-D-pyranoside and from the free sugar. 

No substantial changes in the equilibrium composition or in the NMR spectra of D-glucose, D-mannose, and D-arabinose were observed on the addition of calcium chloride. These sugars lack the required ax-eq-ax sequence of hydroxy groups. 

Table 1.1 gives the equilibrium compositions of the common sugars, in some cases at several temperatures. The following trends are apparent ... 

Table 1.1 Equilibrium composition of solutions of reducing sugars. 

The factors that control the equilibrium composition of sugars in solution are complex. Although the well-established preference for substituents in six-membered rings to be equatorial rather than axial is important, it is not always the overriding factor. The next section introduces a new structural feature that plays a significant part in determining carbohydrate conformations and a/p anomeric ratios. 

Although most of these forms of sugars have never been isolated, they can be detected in the n.m.r. spectra of the sugars, and their proportion in the equilibrium mixture can be measured. Before the advent of n.m.r. spectroscopy, only a rudimentary knowledge of the composition of sugars in solution was available, but, since 1961, an extensive collection of data has been built up, mainly by the use of n.m.r. spectroscopy. These data are the subject matter of this article. The composition of sugars in solution was reviewed3 in 1969, but since then, much new information has been accumulated. 

Knowledge of the composition of sugars in solution is fundamental to carbohydrate chemistry. The physical and chemical properties of the sugars in solution depend on the proportions of their various forms and their biological properties may also show such dependence. Enzymes that utilize these sugars as substrates may not be able to use each of the forms. Where only a single form is utilized, the other forms may either be converted into the reactive form or may function as inhibitors. The latter is especially important if the reactive form is present in very low proportion at equilibrium. It is possible that the substrate form is utilized faster than it is generated from the other forms the observed rate of the reaction is then that of the tautomeric interconversion.4... 

Dilute acids or bases have no noticeable effect on the composition of sugars in aqueous solution at high concentration, however, they alter it considerably. What is then observed is, in fact, not the equilibrium of the sugars themselves but that of their cations and anions, respectively, sugars being weak bases and weak acids. Thus, the a yj-pyranose ratio for D-glucose in 2,5 M DC1 was found90 to be 45 55 the same ratio was obtained when the solvent was formic acid. 

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. 

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