Anomeric carbon atom

For each of the ring modifications of the sugars, two isomers can exist, because a new asymmetric carbon atom is created by ring closure at the reducing carbon atom. These isomers are known as a,j3-isomers or anomers. 

As noted previously, the existence of such isomers was one of the most important reasons for the formulation of ring structures. The isomeric a-and jS-glucoses have quite different solubilities, melting points, and rotations. The isomeric pentaacetates and methyl glucosides exhibit similar differences in properties. 

The periodic acid oxidation provides a means for correlating the configuration of the anomeric carbon atoms of the glycosides (see also p. 218). As shown in the accompanying formulas, representative of the hexosides, carbon 3 is removed in the process (as formic acid), and the asymmetry of carbons 2 and 4 is destroyed. 

In the dialdehyde (II) only two asymmetric carbon atoms remain, and these are derived from carbon atoms 1 and 5 of the original glucoside (I). Hence, all of the D-aldohexosides should yield the same dialdehyde (II) as the corresponding a- or jS-D-glucoside. The configuration of carbon 1 of each of the glycosidic derivatives of the hexoses may be correlated with those of the glucosides in this manner (31). 

Evidence for the configuration of carbon atom 1 of some phenyl glucosides has been obtained by the conversion of the jS-glucosides to 1,6-anhydro derivatives (see Chapter IV, Glycosans), and by the stability of the a-anomers to strong alkali (3 ). 

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Carb-16.2. Replacement of OH at a non-terminal, non-anomeric carbon atom... 

The situation in which the anomeric OH is retained but H is lost from the anomeric carbon atom is indicated by use of the ending -yl without locants in conjunction with the prefix 1-hydroxy- (not by the ending -1-C-yl ). N.B. In this case, the anomeric prefix a or P refers to the free valency, not the OH group. 

The ratio of the different isomeric products was found to vary with time, temperature, and initial concentration. This suggested that some kind of equilibration was occurring between isomers. I3C NMR spectroscopy of a reaction mixture showed, upon cooling, the reversible formation of a pair of signals in the anomeric region. These signals were ascribed to the anomeric carbon atoms of fructofuranosyl fluorides (10), which were presumed to be in equilibrium with the reactive fructofuranosyl cation, 11. 

The problems of anomeric equilibrium may be avoided by investigating 2-ketoses. Both a hydroxyl group and a hydroxymethyl group are attached to the anomeric carbon atom in such sugars, and the bulky hydroxymethyl group favors the equatorial position. These authors measured c.d. spectra for three ketoses, the 2-(hydroxymethyl) derivatives of a-L-xylose, a-D-xylose, and a-D-mannose, in aqueous solution. 

V-Acetylneuraminic acid is a common group in glycoproteins, and it contains both the amide and carboxyl chromophores. As shown in formula 11, this nine-carbon sugar derivative has an equatorial amido group on C-5 and both a hydroxyl group and a carboxyl group on the anomeric carbon atom. 

These observations have several implications for studies of glycoproteins (i) the resonances of anomeric carbon atoms that are involved in N-glycosylic linkages will be rather difficult to identify, because of their proximity to nonanomeric-carbon resonances and (ii) due to the proximity of the chemical shifts of C-l and C-5 (they are only 0.6 p.p.m. apart for / -d-G1cNAc — Asn), it may be difficult, but not impossible, to ascertain whether N-glycosylic linkages exist in the glycoprotein (see later). 

Chemical shift (p.p.m.) of model compounds in H20 relative to internal 1,4-dioxane (67.86 p.p.m.). Chemical shifts for these compounds are given at pH 5.5-7.5. Estimated precision for the chemical shifts is 0.05 p.p.m.h See Refs. 82 and 83. See Ref. 20.d These assignments may have to be interchanged. See Ref. 21 numbers in the brackets below the given chemical shift values refer to those published in Ref. 86. f See Ref. 19. The chemical shift for the -anomeric carbon atom was found to be 100.6 p.p.m. and was determined from an anomeric mixture of this compound. The existence of the a-Man — Ser unit was confirmed by the l]CH value (169 Hz) obtained for this compound. See Ref. 84. 

C-N.m.r. Chemical-shift Data for the Anomeric-Carbon Atoms and C, and Coupling Constants ( /ch) Measured for Selected Gland /S-D-Glycosyl-L-threonine Model Compounds19- 1,82-84... 

The anomeric-carbon region shows six peaks, numbered in Fig. 4. Peak 1 (at 103.6 p.p.m.), which arises from 25 5 carbon atoms, can be assigned to the anomeric-carbon atoms of a-D-mannopyranosyl units, having free hydroxyl groups at C-2, that are involved in glycosidic linkages to 0-2 or 0-3 of other a-D-mannopyranosyl residues.15 Peak 2 (at 102.5 p.p.m.) probably arises from 2-O-substituted a-D-mannopyrano-syl units involved in linkages to 0-2 or 0-3 of other a-D-mannopyranosyl units (internal carbohydrates)15 and also to the anomeric carbon atom of the a-D-mannopyranosyl residue linked to L-threonine.19 Peak 3 (101.8 p.p.m.) arises from the a-D-mannopyranosyl unit linked to L-ser-ine.19... 

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