A-anomer, of glucose

Starch molecules contain multiple a anomers of glucose joined by a linkage between the anomeric carbon ( 1) of one ring and the fourth Ccirbon ( 4) of the next ring. This linkage is therefore known as an a 1 4 linkage. 

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. 

As an example, we can find all of the proton resonances (and determine their chemical shifts) for the a-anomer of glucose by starting at its anomeric proton resonance at 5.23 ppm. These correlations have been... 

The HMQC-TOCSY spectrum for lactose is given in Figure 5.28 with all of the proton and carbon resonances labeled. The overall appearance of this spectrum is reminiscent of an HMBC but the correlations are quite different. It is equally interesting and useful to start on the proton axis (F2) or the carbon axis (FI). If we start on the proton axis at 5.23 ppm, the anomeric proton for the a-anomer of glucose (ad), and proceed downward vertically, we find six correlations to the six carbons of this glucose residue. If we refer back to the simple HMQC spectrum for lactose, we find only one correlation for this proton. Likewise, the anomeric proton of the /3-anomer of glucose at 4.67 ppm also shows six correlations to the carbons of its respective glucose residue. 

The half-life for nonenzymic interconversion of the anomers is about 0.4 sec. Stopped-flow kinetic measurements, using the isomerase from yeast, indicate that both the a- and j8-anomers of glucose 6-phosphate are used directly by the enzyme to give the a- and )8-anomers of fructose 6-phosphate, although the a-anomer is consumed at least 20-fold faster than the )3-anomer. In addition, the enzyme is capable of catalyzing the interconversion (anomerization) of the a-and jS-anomers. With the a-anomer of glucose 6-phosphate, the anomerization reaction is approximately twice as fast as the isomerization reaction. 

FIGURE 16.22 The structure of starch is based on the a-anomer of glucose. The... 

Lactose is hydrolyzed by the membrane-bound enzyme lactase. The enzyme is also known as a -galactosidase because its action involves the cleavage of the P-anomer of galactose from its 1 4 bond with an a-anomer of glucose. Note this arrow nomenclature for the bond or linkage between the two monosaccharides. 

Note that there are two forms of starch amylase and amylopectin. In amylase, the glucose residues are joined end to end by 1—>4 glycosidic bonds between a-anomers of glucose. This produces a linear, unbranched polysaccharide. Hydrogen bonds between amylose molecules twist the chains into large hehcal rodhke stractures that stack to form sheets. These sheets impede digestion by amylase. 

If the a-anomer of glucose O-methylglycoside (or the P-anomer for that matter) is treated with an acetylating agent such as acetic anhydride, the corresponding tetraacetyl ester results. Treatment of the l-(0-methyl)tetraacetate with dilute aqueous acid then results in demethylation, that is, the conversion of an acetal to a... 

The a anomers of 2-acetamido-2-deoxy-D-glucose and 2-acetamido-2-deoxy-D-galactose have the same nir c.d. bands in 1 1 methanol-water at 0° as the anomeric mixtures have in aqueous solution. This indicates that the anomeric configuration has little influence on the nir c.d. band. 

The ratio of the a- to the j -anomeric D-glucosyl ester can be influenced by changing the reaction conditions. In DMF the a-anomer of the crocetin bis(D-glucosyl ester) was formed in about 70% yield.[196] Esterification of D-glucose with the imidazolides of benzoic acid or stearic acid in pyridine furnished a mixture of the a- and j -anomers of the C(l) glucosyl ester.[196]... 

By considering the reversibility of the acetalforming reactions, it is apparent that treatment of either of the two methyl pyranosides with acidic methanol will prodnce the same eqnilibrium mixture. A related equilibration occurs with the anomers of glucose, as seen earlier (see Box 7.1, mutarotation of glucose). 

Certain procedures make it possible to obtain the a and 3 anomers of glucose in pure form. A 1-molar solution of a-D-glucose has a rotation value [a]o of +112°, while a corresponding solution of p-D-glucose has a value of +19°. These values change spontaneously, however, and after a certain time reach the same end point of +52°. The reason for this is that, in solution, mutarotation leads to an equilibrium between the a and p forms in which, independently of the starting conditions, 62% of the molecules are present in the P form and 38% in the a form. 

We are not going to spend too much time discussing the 1-D spectra except to note some of the obvious features. The anomeric proton resonances can be found at 4.45, 4.67, and 5.23 ppm and the anomeric carbon resonances at 91.7, 95.6, and 102.8 ppm. The reason for three anomeric protons and carbons is that the a- and /3-anomers of glucose give two sets of resonances while the galactose residue, which exists only in the /3-form, gives a single set of resonances in both the proton and carbon spectra. The other portions of both spectra, especially the proton, are quite complicated and show considerable overlap. 

Figure 9.5 Haworth representation of glucose and fructose. (A) Both the a and /3 anomers of glucose. (B) For fructose, the /3 anomer of fructofuranose. In the a anomer, the -OH on carbon 2 of fructose would be pointing down and the -CH2OH would be up. (C) The /3 anomer of D-fructopyranose. (Reproduced by permission from Diem K, Lentner C. Scientific Tables. Basel Ciba-Geigy, 1971.)...

The anomers of glucose. The hydroxyl group on the anomeric (hemiacetal) carbon is down (axial) in the a anomer and up (equatorial) in the /3 anomer. The /3 anomer of glucose has all its substituents in equatorial positions. 

Equation I is a simplified one, as it does not take into account the interchange between the enzymically active a anomer of D-glucose and the /3 anomer. The same reservation applies to D-fructose, which exists in four anomeric forms in solution, and which of the four is the... 

Hydrolysis of maltose forms two molecules of glucose. The Cl - O bond is cleaved in this process, and a mixture of glucose anomers forms. The mechanism for this hydrolysis is exactly the same as the mechanism for glycoside hydrolysis in Section 27.7B. 

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