The Monosaccharide Units

Of the aldopentoses, only n-xylose and n-arabinose are well established. Although the presence of n-ribose in a polysaccharide preparation is most frequently an indication of nucleic acid as an impurity, this sugar has now been found occurring naturally apart from nucleic add in serologically specific polymers and elsewhere.  

Of the 6-deoxyaldohexoses, only L-rhamnose and L-fucose have been found, but 6-deoxyglucose could easily have been overlooked on paper chromatograms. Again, these sugars have not been isolated often enough for it to be assumed that only the l isomer of each occurs, especially since both 3,6-dideoxy-D- and L- galactose and 3,6-dideoxy-n- and L- man-nose have been found the discovery of the l isomer in the first pair mentioned stemmed from serological studies. 

The series of sugars to which the main additions have been made are considered separately below. 

The characterization of a new series of sugars followed the observations of Westphal and of Pon and Staub, who noted, in hydrolyzates of some specific polysaccharides of Salmonella, unknown components which traveled 

A color test described by Webb and Levy for 2-deoxy sugars was investigated further, and it was shown that, unlike 2-deoxypentoses and 2-deoxyhexoses, no reaction was obtained with a 2-deoxytetrose. However, the Webb-negative tyvelose and abequose, after periodate oxidation, gave a positive Webb test (as for 2-deoxypentoses), behavior which was consistent with their formulation as 3,6-dideoxyhexoses.  

It will be appropriate to consider first the building units of the carbohydrate residues and it will be immediately apparent that n-mannose and D-galactose with N-acetyl-amino sugars and hexuronic acids play a predominant role. It is proposed, therefore, to discuss briefly some of the more important discoveries connected with the two latter groups and with the carbohydrate sulfates. 

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Amino sugars, such as o-glucosamine, have an OH group leplaced by an -NH2. The N-acetyl amide derived from o-glucosamine is the monosaccharide unit from which chitm, the hard crust that protects insects and shellfish, is made. Still otheramino sugars are found in antibiotics such as streptomycin and gentamicin. 

Polysaccharides formed from a-glucose are called starches. A starch stores sugar until it is needed for energy production. Three important starches are glycogen, which animals produce in their livers, and amylose and amylopectin, produced by plants through photosynthesis. On average, plant starch is about 20% amylose and 80% amylopectin. Each of these polysaccharides contains glucose as its monomer, but they differ in how the monosaccharide units are linked. 

On the assumption that methylation of the degraded arabic acid was complete, the location of hydroxyl groups in the various cleavage fragments indicates the positions through which the monosaccharide units are involved in union with the other residues. Thus the isolation of three molecular proportions of 2,3,4-trimethyl-D-glucuronic acid (VIII) and one molecular proportion of 2,3,4,6-tetramethyl-D-galactose (XI)... 

A modification of the monosaccharide units of polysaccharides may obviously be effected at different stages of the biosynthesis of a polymer (a) prior to formation of the activated form of a monosaccharide, (b) at the level of glycosyl nucleotides, (c) at the stage of formation of oligosaccharide intermediates, and (d) after the synthesis of a polymeric chain. 

Chemical shifts (8) are given in p.p.m. downfield from sodium 4,4-dimethyl-4-silapentane-l-sulfonate the values in parentheses are coupling constants (Jia) in Hz. The numbering of the monosaccharide units in the reference compounds corresponds to that in the asialoglycan-Asn (see fundamental structure in Table II). Values from a convolution-difference spectrum. 

Question Why are the monosaccharide units in oligo- and polysaccharides called residues ... 

What is a disaccharide What monosaccharide units make up the dissacharide sucrose What is the bond called that forms between the monosaccharide units ... 

Figure 8 depicts all the monosaccharide units discussed in this article. 

The monosaccharide units can be present in both their furanose and pyranose forms, and the anomeric configuration can be both a and (3 in the case of nonreducing disaccharides. When both monosaccharides have the same configuration (e.g., D-glucose), there are several variations. When both monosaccharide units form furanose rings, three isomers — two symmetric... 

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