The structure of monosaccharides

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. 

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Hexose monosaccharides can form both five- and six-membered rings. In most cases, the six-membered ring structure is more stable, but fructose is an important example of a hexose that is more stable as a five-membered ring. The structure ofy6-fhictose is shown in Figure 13-17. Notice that there are — CH2 OH fragments bonded to two positions of this five-membered ring. Examples and explore the structures of monosaccharides in more detail. 

The structure of monosaccharides is often written in the acycUc form although only very minor amounts of it ever occur in that form. Because the interconversions are rapid, the carbonyl groups of sugars can and do react both as if they are free and as if they are in a hemiacetal ring form. 

William Mills described a similar convention to depict the structures of monosaccharides. While the ring atoms of the Haworth projections are oriented perpendicular to the paper, Mills chose to depict the carbon skeleton in the plane of the paper (Fig. 1.5). Although Fischer, Haworth, and Mills projections are useful tools for depicting the structures of carbohydrates, the planar nature of these representations does not provide an accurate picture of the actual geometry of the molecules. In order to understand carbohydrate function and reactivity, recognition of each distinct conformation and the properties associated with it is required [15]. 

Compare the structures of monosaccharides, disaccharides, and polysaccharides. Which has the largest molecular mass The smallest ... 

Figure 1. Some examples of the structures of monosaccharides occurring in mammalian glycoconjugates (glycoproteins, glycolipids, and proteoglycans).

Later in this chapter we shall see how the great carbohydrate chemist Emil Fischer was able to establish the stereochemical configuration of the aldohexose D-(+)-glucose, the most abundant monosaccharide. In the meantime, however, we can use D-(+)-glucose as an example illustrating the various ways of representing the structures of monosaccharides. 

In the next section, we will describe the structures of monosaccharides. Later, we will see how these units are linked to form oligosaccharides and polysaccharides. 

Now that we have described the structures of monosaccharides, let us examine some of their common reactions. 

The building blocks of nucleic acids are themselves composed of certain organic bases linked to either ribose or deoxyribose, both of which are sugar molecules. In this short subsection, we will briefly discuss the structure of monosaccharides, or sugars, before we launch into the structure of nucleic acids. 

The use of C n.m.r. spectroscopy in elucidating the structures of monosaccharides, polysaccharides, nucleosides, and nucleotides has been reviewed. A study of the tautomerism of formycin and related compounds using C n.m.r. spectroscopy is noted in Chapter 21. ... 

The structure of monosaccharide sugars facilitates the calculation of such three-dimensional solvent densities. Transitions between ring conformations for the carbohydrates are... 

The intermolecular interactions stabilise the helices and greatly influence the properties of exopolysaccharides in solution, ie solubility, viscosity and gel-formation. A strong interaction or good-fit between molecules will lead to insolubility, whereas poor interaction will lead to solubility of exopolysaccharides. The interactions between molecules is influenced by the presence of side-chains. For example, cellulose is insoluble but introduction of a three monosaccharide side-chain into the cellulose chain gives the soluble xanthan. Small changes in the structure of the side-chains can alter the molecular interactions and thus properties of the exopolysaccharide. 

The basis for the name is the structure of the parent monosaccharide in the acyclic form. Charts I and IV (2-Carb-10) give trivial names for parent aldoses and ketoses with up to six carbon atoms. 2-Carb-8.2 and 2-Carb-10.3 describe systematic naming procedures. 

The structures of the a-D-aldo-pento and -hexo-pyranose monosaccharides are shown in Pig. 1. In all cases, these sugars will be studied as the six-membered-ring tautomer, as shown. 

C13-0015. Describe the differences in the structures of a-glucose and a-idose, a rare naturally occurring monosaccharide. 

Although this technique has been extensively used in the past for structural studies of monosaccharides and small oligosaccharides, its usefulness for study of the structure of oligosaccharides of glycoproteins has only begun. 

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