Isomerism of sugars

In the second method, which can be applied to compounds with an optically active center near the potentially tautomeric portion of the molecule, the effect of the isomerization on the optical activity is measured. In favorable cases both the rate of racemization and the equilibrium position can be determined. This method has been used extensively to study the isomerization of sugars and their derivatives (cf. reference 75). It has not been used much to study heteroaromatic compounds, although the very fact that certain compounds have been obtained optically active determines their tautomeric form. For example, oxazol-5-ones have thus been shown to exist in the CH form (see Volume 2, Section II,D,1, of article IV by Katritzky and Lagowski). 

Fig. 3.—The basic assumption for Fischer s research of the optical isomerism of sugars.

Note that harsher conditions may lead to further changes, e.g. epimerization at C-3 in fmctose, plus isomerization, or even reverse aldol reactions (see Section 10.3). In general, basic conditions must be employed with care if isomerizations are to be avoided. To preserve stereochemistry, it is usual to ensure that free carbonyl groups are converted to acetals or ketals (glycosides, see Section 12.4) before basic reagents are used. Isomerization of sugars via enediol intermediates features prominently in the glycolytic pathway of intermediary metabolism (see Box 10.1). 

In rearrangements (isomerizations, not shown), groups are shifted within one and the same molecule. Examples of this in biochemistry include the isomerization of sugar phosphates (see p.36) and of methylmalonyl-CoA to succinyl CoA (see p. 166). 

As is the case with other carbonyl compounds, the carbonyl group of a sugar causes any hydrogens on adjacent carbons to be weakly acidic. This provides a mechanism for the isomerization of sugars in basic solution. Thus, D-glucose is isomerized to D-mannose and D-fructose under basic conditions. 

There is yet another level of isomerism of sugars, because aldehydes (and ketones) form freely reversible adducts with alcohols (hemiacetals and hemiketals) in aqueous solutions. In the presence of... 

Isomerization of sugars sugar phosphates xylose (glucose) isomerase triosephosphate isomerase... 

Mendicino, J. F. (1960). Effect of borate on the alkali-catalyzed isomerization of sugars. Journal of the American Chemical Society, 52(18), 4975 979. 

Bermejo-Deval R, Assary RS, Nikolla E, Moliner M, Roman-Leshkov Y, Hwang S-J, Palsdottir A, Silverman D, Lobo RE, Cinliss LA, Davis ME (2012) Metalloenzyme-like catalyzed isomerizations of sugars by Lewis acid zeolites. Proc Natl Acad Sci U S A 109(25) 9727-9732... 

Aldose-l-phosphates synthesized by the reaction of aldoses with ATP or by isomerization of sugar-co-phosphates are the primary glycosides. They may act as glycosyl donors in the synthesis of other glycosides either directly or after transformation to nucleotide-diphosphate sugars (C 6). 

Germanium is an element in group 14 of the periodic table and many compounds of germanium similar to silicon and tin compounds have been synthesized [1]. However, germanium is a highly expensive metal compared with silicon or tin, and the amount used is small. Now, the research and development in the field of pharmaceuticals have been energetically conducted. Its other uses are as catalysts for isomerization of sugars, etc. 

The isomerization of glucose into fructose for the production of high-fructose com symps (HFCS), a mixture of glucose and fructose, which is 1.3 times sweeter than sucrose, is the largest biocatalytical process in the world. In the next decades, the isomerization of sugars probably will... 

Among sucrose glucose and fructose fructose is the sweetest Honey is sweeter than table sugar because It contains fructose formed by the isomerization of glucose as shown in the equation... 

Fructose—Dextrose Separation. Emctose—dextrose separation is an example of the appHcation of adsorption to nonhydrocarbon systems. An aqueous solution of the isomeric monosaccharide sugars, C H 2Dg, fmctose and dextrose (glucose), accompanied by minor quantities of polysaccharides, is produced commercially under the designation of "high" fmctose com symp by the enzymatic conversion of cornstarch. Because fmctose has about double the sweetness index of dextrose, the separation of fmctose from this mixture and the recycling of dextrose for further enzymatic conversion to fmctose is of commercial interest (see Sugar Sweeteners). 

Liquid sucrose and Hquid invert, generally made by redissolving white sugar and inverting with invertase enzyme, are refinery products in Europe and outside the United States. In the United States they have been almost completely replaced by cheaper com symps made by enzymatic hydrolysis of starch and isomerization of glucose. 

Glucose Isomeriza.tion, Enzymatic isomerization of glucose to fmctose provides a real alternative to sugar (sucrose) derived from sugarcane or sugarbeets. The commercial product obtained is known as high fmctose com symp (HECS). Two grades of the symp have become estabUshed on the world market, HECS-42 and HECS-55, which contain 42 and 55% fmctose on dry substance basis. These products account for over one-third of the caloric sweetener market ia the United States. 

All known eight-stranded a/p-barrel domains have enzymatic functions that include isomerization of small sugar molecules, oxidation by flavin coenzymes, phosphate transfer, and degradation of sugar polymers. In some of these enzymes the barrel domain comprises the whole subunit of the protein in others the polypeptide chain is longer and forms several additional domains. An enzymatic function in these multidomain subunits, however, is always associated with the barrel domain. 

The present work involves the study of methyl glycosides and O-isopropylidene ketals of various isomeric deoxy sugars by mass spectrometry. Several of the compounds selected for the present study have free hydroxyl groups, and interpretation of their mass spectra shows the scope of the study of these and related deoxy sugar derivatives by mass spectrometry without prior substitution of all hydroxyl groups. Some of the candidates (compounds 4, 7, 8 and 10) are structurally related to biologically-derived deoxy sugars. 

The isomerism of a- and jS-glucose is to be attributed to the spatially different arrangement of the H and OH-groups attached to the asymmetric carbon atom 1. This atom is asymmetric in the cyclic lactol formula (Tollens). The mutarotation of the sugars, i.e. the gradual change to the final stationary value of the optical rotation, is to be explained by an equilibrium occurring in solution between the various... 

The fundamental principles governing the spatial isomerism of the simple sugars and the theoretical and experimental work of Emil Fischer cannot be dealt with here. Those who are not already familiar with the subject should at once fill this gap in their knowledge they are emphatically advised to use space models. 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

---