Monosaccharides epimers

Cagmat EB, Szczepanski J, Pearson WL, Powell DH, Eyler JR, Polfer NC (2010) Vibrational signatures of metal-chelated monosaccharide epimers gas-phase infrared spectroscopy of Rb -tagged glucuronic and iduronic acid. Phys Chem Chem Phys 12 3474-3479... 

The carbonyl group (and adjacent alcohol) oxidizes with excess phenyl hydrazine (PhNHNH2) to form an osazone (see Figure 16-15). Osazone formation is very important in determining the relationship between various monosaccharides. For example, both D-glucose and D-mannose produce the same osazone, so they re epimers. Epimers differ by only one chiral center, which osazone formation destroys. 

Synthesis. The synthases are present at the endomembrane system of the cell and have been isolated on membrane fractions prepared from the cells (5,6). The nucleoside diphosphate sugars which are used by the synthases are formed in the cytoplasm, and usually the epimerases and the other enzymes (e.g., dehydrogenases and decarboxylases) which interconvert them are also soluble and probably occur in the cytoplasm (14). Nevertheless some epimerases are membrane bound and this may be important for the regulation of the synthases which use the different epimers in a heteropolysaccharide. This is especially significant because the availability of the donor compounds at the site of the transglycosylases (the synthases) is of obvious importance for control of the synthesis. The synthases are located at the lumen side of the membrane and the nucleoside diphosphate sugars must therefore cross the membrane in order to take part in the reaction. Modulation of this transport mechanism is an obvious point for the control not only for the rate of synthesis but for the type of synthesis which occurs in the particular lumen of the membrane system. Obviously the synthase cannot function unless the donor molecule is transported to its active site and the transporters may only be present at certain regions within the endomembrane system. It has been observed that when intact cells are fed radioactive monosaccharides which will form and label polysaccharides, these cannot always be found at all the membrane sites within the cell where the synthase activities are known to occur (15). A possible reason for this difference may be the selection of precursors by the transport mechanism. 

Problem 22.24 What monosaccharide is obtained from a Ruff degradation of o-mannose (Problem 22 11), the C epimer of o-glucose (Problem 22.23)7 ... 

Monosaccharides generally contain several chiral carbons and therefore exist in a variety of stereochemical forms, which may be represented on paper as Fischer projections. Epimers are sugars that differ in configuration at only one carbon atom. 

Compounds that have the same chemical formula but have different structures are called isomers. For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C6H1206. If two monosaccharides differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon, see anomers below), they are defined as epimers of each other. (Of course, they are also isomers ) For example, glucose... 

Experimental evidence for activated forms of the sugars of this group is presented only in the case of 6-deoxytalose, which is an epimer, at C-4, of 6-deoxymannose. In the d series, the GDP derivative of the monosaccharide is formed, together with GDP-D-rhamnose, through nonspecific, enzymic... 

Some monosaccharides may also be classified as being epimers. Epimers are compounds that have identical configurations except for one carbon atom. For example, a-D-glucose and a-D-fructose are epimers. Epimers sometimes react with the same reagent to form the same product. For example, both a-D-glucose and a-D-fructose react with phenylhydrazine to form the same osazone. 

Classification of Carbohydrates 1102 23-3 Monosaccharides 1103 23-4 Erythro andThreo Diastereomers 1106 23-5 Epimers 1107... 

A non-specific bacterial acid phosphatase from Shigella flexneri (PhoN-Sf) has been screened for regioselective phosphorylation of primary alcohol(s) of more than 20 different cyclic and acyclic monosaccharides using pyrophosphate as the phosphate donor (O Scheme 61) [368]. These studies have shown that PhoN-Sf is capable of phosphorylating a range of hexoses (D-glucose epimers, glycosides, and C-2 derivatives), pentoses, heptoses, ketoses, and acyclic carbohydrates. 

D-Lyxose, an epimer of D-arabinose rarely found in Nature, has been identified as an important component of the glycolipid adhesion receptors for mycobacteriophages, and the derivatives of this monosaccharide are currently used to evaluate the precise stmctural requirements for the attachment of the mycobacteriophage to the cell wall of its host [32]. Derivatives of a-nucleotides of D- and L-lyxofuranosylbenzimidazoles and ribofuranosylbenzimidazoles have been evaluated as antiviral agents O Fig. 7 [33]. 

Terms in bold are defined glycoconjugate 238 monosaccharide 238 oligosaccharide 238 disaccharide 238 polysaccharide 238 aldose 239 ketose 239 Fischer projection formulas 240 epimers 240... 

It is worth to note that P-acetoxy-a-diazoesters were also readily accessible through the reaction of monosaccharides in their hemiacetal form. However, such a reaction required the use of an equimolecular amount of diethylzinc creating likely very reactive zinc diazointermediates which react smoothly with aldehydes [l 02]. Applying this procedure KO and its 3-epimer were prepared, starting from 2,3 5,6-di-0-isopropylidene-D-mannofuranose [101]. 

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