Monosaccharides hexoses

Fructose a monosaccharide (hexose) found in high concentrations in honey a constituent of sucrose, common table sugar. 

Neutral monosaccharides, uronic acids, hexosa-mines, and sialic (neurominic acids) are identified and determined by specific colorimetric reactions. The principle behind the techniques rests on the condensation of the degraded products of the neutral monosaccharides (hexose, pentose and methyl pentose) by sulfuric acid with anthrone, cysteine hydrochloride, orcinol, and phenol reagents. Uronic acids may be determined by colorimetric and manometric procedures. While sialic acids are determined after chemical/enzymatic hydrolysis, gravimetric and Van-Soest detergent based methods are used to determine cellulose, hemicellulose, and fiber. 

A variety of monosaccharides (hexoses or pentoses) can be fermented to produce 2,3-BD (Syu, 2001). In bacterial metabolism, monosaccharides must first be converted to pyruvate before generation of major products. From glucose, pyruvate is formed in a relatively simple manner via the Embden-Meyerhof pathway (glycolysis). In contrast, the production of pyruvate from pentoses must proceed via a combination of the pentose phosphate and Embden-Meyerhof pathways (Jansen and Tsao, 1983). In addition to 2,3-BD, the pyruvate produced from the monosaccharides is then channeled into a mixture of acetate, lactate, formate, succinate, acetoin, and ethanol, through the mixed acid-2,3-BD fermentation pathway (Ji et al., 2011a). 

The simple sugars or monosaccharides are polyhydroxy aldehydes or ketones, and belong to Solubility Group II. They are termed tetroses, pentoses, hexoses. etc. according to the number of carbon atoms in the long chain constituting the molecule, and aldoses or ketoses if they are aldehydes or ketones. Most of the monosaccharides that occur in nature are pentoses and hexoses. 

Enantiomerically pure tetroses, pentoses, and hexoses have been synthesized by the following reaction sequence (A.W.M. Lee, 1982 S.Y. Ko, 1983), which is useful as a repetitive two-carbon hotnologi-.ation in total syntheses of higher monosaccharides and other polyhydroxy compounds (1) Wittig reaction of a protected hydroxy aldehyde with (triphenylphosphor-... 

Note 2. Since all aldoses up to the hexoses have trivial names that are preferred, the systematic names apply only to the higher aldoses. However, the configurational prefixes are also used to name ketoses (see below) and other monosaccharides. 

GalNAc, Gal, GlcNAc, Hex and HexNAc are the abbreviations used when discussing sequences of monosaccharides and represent A-acetylgalactosamine, galactose, A-acetylglucosamine, hexose, i.e. a monosaccharide with six carbon atoms, and A-acetylhexosamine, respectively. 

Monosaccharides have the formula (CH2 0) , where n is between 3 and 6. Of the 70 or so monosaccharides that are known, 20 occur in nature. The most important naturally occurring monosaccharides contain five carbons (pentoses) or six carbons (hexoses). Structures of ribose, an important pentose, and a-glucose, a hexose that is the most common monosaccharide, are shown in Figure 13-14. As shown in the figure, it is customary to number the carbon atoms in a monosaccharide, beginning with the HCOH group adjacent to the ether linkage. 

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. 

Fractose is a hexose monosaccharide that forms a five-membered ring stmcture. In the ball-and-stick model, the ring is highlighted in blue. 

Supplement 1941 1-161 Derivatives. Methyl alcohol, 273. Ethyl alcohol, 292. Ethyl ether, 314. Glycerol, 502. Carbonyl Compounds Aldehydes, Ketones, Ketencs and Derivatives. Formaldehyde, 558. Acetaldehyde, 635. Acetone, 635. Ketene, 724. Hydroxy-Carbonyl Compounds Aldehyde-Alcohols, Ketone-Alcohols, Monosaccharides and Derivatives. Glycolaldehyde, 817. Aldol, 824. Pentoses, 858. Hexoses, 878. 

There are three possible classes of sugar acids which may be produced by the oxidation of monosaccharides (Figure 9.11). The aldonic acids are produced from aldoses when the aldehyde group at carbon 1 is oxidised to a carboxylic acid. If, however, the aldehyde group remains intact and only a primary alcohol group (usually at carbon 6 in the case of hexoses) is oxidised then a uronic acid is formed. Both aldonic and uronic acids occur in nature as intermediates in... 

Figure 9.12 Deoxy derivatives. These contain one less oxygen atom than the monosaccharide from which they are derived. 2-Deoxyribose is a most important deoxy pentose and is a major constituent of deoxyribonucleic acid (DNA). Deoxy hexoses are widely distributed among plants, animals and microorganisms especially as components of complex polysaccharides. Examples are rhamnose (6-deoxymannose), a component of bacterial cell walls, and fucose (6-deoxygalactose), which is often found in glycoproteins and is an important constituent of human blood group substances.

Two other commonly occurring hexoses which are usually found as components of polysaccharides or combined with other molecules in complex structures are galactose and fructose and, in a similar manner to other monosaccharides, enzymic methods are available for their measurement. An enzymic method for the measurement of fructose using hexokinase was described earlier, together with the method for mannose and glucose (Figure 9.22). 

Historically, techniques such as the formation of osazones and the demonstration of fermentation have contributed significantly to the separation and identification of carbohydrates. Observation of the characteristic crystalline structure and melting point of the osazone derivative, prepared by reaction of the monosaccharide with phenylhydrazine, was used in identification. This method is not completely specific, however, because the reaction involves both carbon atoms 1 and 2 with the result that the three hexoses, glucose, fructose and mannose (Figure 9.19), will yield identical osazones owing to their common enediol form. 

---