Anomers, of sugars

A new NMR method for the determination of the anomeric configuration in mono- and disaccharides has been described.18 The protocol is based on the different cross-correlated relaxation between proton chemical shift anisotropy (CSA) and dipolar relaxation for the a and (3 anomers of sugars. Only the ot-anomers show the presence of CSA (HI or Hl )-proton dipole (H1-H2 or Hl -H2 ) in the longitudinal relaxation of the anomeric protons. The method is of special interest for cases in which vicinal coupling constants between HI and H2 in both anomers a and (3 are similar and small, such as D-mannose, and the non-ambiguous description of the anomeric configuration needs additional measurements. 

The p anomers of sugars are generally oxidized more rapidly than the a anomers (see later), a similar pattern is seen in the faster oxidation of /1-glycosides. The different rates of oxidation of /1-D-glucopyranosides and their a-D anomers have been attributed to the equatorial orientation of the anomeric hydroxyl group in the 4Ci conformation of the former. Other compounds have shown a similar behavior.10 Thus, the relative rate of oxidation of cw-2-tert-butylcyclohexanol (HO-axial) with respect to the trans isomer (HO-equatorial) is approximately 5 1. 

Efforts to improve the separation of closely related compounds are a frequent reason for using derivatives, and their application often makes it possible to separate compounds that otherwise cannot be separated. Chapter 5 gives various examples of this type, of which the separation of enantiomers of alcohols (p.90), carboxylic acids (p.125), amino acids (p.146) etc., are the most illustrative. The separation of sterols that differ in the position of the hydroxyl group may serve as another example. Isomers with a hydroxyl group in the a-position are not separated from 0-isomers on non-polar columns. However, if the hydroxyl group is converted into a suitable derivative, the two isomers can be separated well even on non-polar columns. The anomers of sugars can also be separated after their conversion into derivatives. 

Maltose and cellobiose are both reducing sugars because the anomeric carbons on the right-hand glucopyranose units have hemiacetal groups and are in equilibrium with aldehyde forms. For a similar reason, both maltose and cellobiose exhibit mutaiotation of a and /3 anomers of the glucopyranose unit on the right. 

Anomers (Section 25.5) Cyclic stereoisomers of sugars that differ only in their configuration at the hemiacetal (anoineric) carbon. 

Mutarotation (Section 25.5) The change in optical rotation observed when a pure anomer of a sugar is dissolved in water. Mutarotation is caused by the reversible opening and closing of the acetal linkage, which yields an equilibrium mixture of anomcrs. 

In solution, most simple sugars and many of their derivatives occur as equilibrium mixtures of tautomers. The presence of a mixture of two anomers of the same ring size may be indicated in the name by the notation a,P-, e.g. a,P-D-gIucose. In formulae, the same situation can be expressed by separating the representation of the ligands at the anomeric centre from the a and P bonds [see examples (a) and (c)], or by use of a wavy line [(b) and (d)] (particularly if hydrogen atoms are omitted). 

The carboxyl chromophore is axial for the a anomer and equatorial for the p anomer. The sugar was studied as the carboxylate anion as it has a (low) piC of 2.6, and the compound is degraded in acidic solution. The c.d. spectrum of this compound contains contributions from the carboxylate n-jr at 217 nm, the amide n-tr at 210 nm, and the amide 7T7r at 190 nm. Apparently, all of these bands are positive, giving rise to a c.d. spectrum (see Fig. 29) having " a maximum at 199 nm and a shoulder at 210 nm. The c.d. spectra of a number of derivatives confirmed these assignments. 

The study of monosaccharides subjected to conditions of methanolysis is considered for two reasons. First, the decomposition of monosaccharides is indicative of the decomposition of monosaccharides liberated during methanolysis second, the ratio of methyl glycosides of a particular standard monosaccharide is the same for the same monosaccharide released during methanolysis (for a particular set of methanolysis conditions), provided that the concentration of sugars is relatively low. Up to four methyl glycosides (the a and )8 anomers of the pyranoside and furan-oside forms) of a particular monosaccharide may be formed the acyclic dimethyl acetal is an additional possibility. The ratios of methyl glycosides of 10 monosaccharides subjected to methanolysis with M hydrogen chloride for 24 h at 80° has been reported. Similar information is also available in another study."... 

Solution studies have shown (26) that D-Gal can bind to ABP with an affinity comparable to that of L-Ara. D-Fuc and D-Xyl, on the other hand, have been found to act only as moderate inhibitors (26,22) To explain these observations the CCEM method was used to generate three-dimensional complexes of ABP with the a and /9-anomers of D-Gal, D-Fuc and D-Xyl. We report here the results obtained with the a-anomers only, since complexes with the /8-anomers of these sugars have been found to be energetically less favoured. 

The hydrolysis of cellulose, a polysaccharide, sometimes yields the disaccharide cellobiose. When cellobiose, a reducing sugar, is hydrolyzed, two glucose molecules result. Unlike maltose, which we describe in the preceding section, the linkage involves the (3 anomer of the left glucose (see Figure 16-24). 

The term mutarotation means the variation of optical rotation with time, ohserved in a solution of sugar on standing. Let us have a look at this phenomenon in a glucose solution. The pure a anomer of glucose has an m.p. of 146 °C and a specific rotation [a]o +112.2°, and the specific rotation on standing is +52.6°, while pure (3 anomer has an m.p. of 148-155 °C and a specific rotation [a]D + 18.7°, and the specific rotation on standing is + 52.6°. When a sample of either pure anomer is dissolved in water, its optical rotation slowly changes and ultimately reaches a constant value of + 52.6°. Both anomers, in solution, reach an equilibrium with fixed amounts of a (35 per cent), (3 (64 per cent) and open chain ( 1 per cent) forms. 

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