Mutarotation Anomeric ratio

The majority of the many methods used to study the composition of equilibrium solutions of carbohydrates examine the mixture without separating the individual components. With the discovery that the anomeric forms of sugars could be readily separated by gas chromatography of their tri-methylsilyl ethers, a new approach to the problem was found. A protocol was developed for the direct gas chromatographic analysis of the amount of each anomer present in an aqueous solution. The protocol can be used on the micro scale and can be used in enzyme assays such as that for mutarotase. The method has been made more effective by combining gas chromatography with mass spectrometry. It is shown how mass spectral intensity ratios can be used to discriminate anomers one from another. The application of these methods to the study of complex mutarotations is discussed. 

Fig. 1 Properties of SXA-catalyzed hydrolysis of 4-nitrophenyl- 3-D-xylopyranoside (4NPX). a Stereochemistry. SXA catalyzes hydrolysis of 4NPX with inversion of anomeric configuration. Mutarotation, off the enzyme, converts a-D-xylose to its equilibrium mixture (a P ratio of 1 2.5) with a half-life of 1 h [3]. R = 4-nitrophenyl. b Diprotic model. pX,s 5 and 7, assigned to catalytic base (D14) and catalytic acid (El 86), respectively, govern catalysis and binding of ligands [3]...

Mutarotation in Solution. In solution, equilibrium exists between the linear conformation and the two anomeric forms of pyranose and fiiranose cyclic hemiacetal forms (Fig. 9) the percentage of each form as well as the ratio a/p is characteristic of the monosaccharide considered. 

The monosaccharide D-glucose, whose chemistry is representative of all aldoses containing four or more carbon atoms, exists predominantly in the two pyranosc forms 4 and 5. These are six-membered hemiacetals formed by the reversible cyclization of the acyclic polyhydroxy aldehyde 3 (Eq. 23.1). In the cyclic forms 4 and 5, the ring carbon that is derived from the carbonyl group is referred to as the anomeric carbon atom. The specific rotation, [a] (Sec. 7.5), of a-D-(+)-glucose (4) is +112 whereas that of the -anomer 5 is +19°. When crystals of either pure 4 or pure 5 are dissolved in water, the [a]p changes to an equilibrium value of +52.7°. This process is termed mutarotation. At equilibrium in water, the a- and p-forms are present in the ratio of 36 64 only about 0.03% of D-glucose is in the acyclic form 3. 

---