Mutarotation of Sugars in Aqueous Solution

According to the reaction-rate theory,256 the rates of interconversion of the anomers, as well as the rates of ring isomerization, should depend on the differences in free energy between the reactants in the ground state and in the transition states. One curve of Fig. 11 depicts a pseudo-acyclic intermediate on each side of the transition state (B) governing the a jS-pyranose interconversion. The a-pyranose may be converted into a furanose form through a pyranose-furanose transition state (D) with less activation energy than that required for the 

Equilibrium with other acyclic and ring modifications 

The susceptibility of the atoms of sugar molecules to attack by acid and base catalysts may be associated with electronic density, as well as with stereomeric factors. Zhdanov and coworkers259 have calculated the electronic charges of the atoms of various types of carbohydrate molecules by an adaptation of the molecular-orbital method. The charge distributions calculated for aldopentopyranose and aldopento-furanose molecules are as follows  

As indicated in Fig. 12, the reaction sequence starts with the rapid, reversible addition of the acid, HA, to the ring-oxygen atom. This addition is followed by a series of reactions that result in a pseudo-acyclic intermediate (HSaHA) which establishes equilibrium with 

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Numerous workers have examined the rate constants for the mutaro-tation of D-glucose in the presence of acetic acid and sodium acetate in an attempt to ascertain whether equation 9 or 10 applies. Although the two equations differ widely, it is not easy to distinguish experimentally between them. It is probable that the concerted mechanism is of no significance in the mutarotation of sugars in aqueous solution, with the possible exception of reactions catalyzed by the water molecule, as will be discussed later. In solvents of low dielectric constant, the formation of ionic intermediates becomes less favored, and the concerted mechanism may apply. Some of the methods that have been used for studying these reactiqns and for determining the catalytic coefficients will be considered next. 

Mutarotation. The change in optical rotation of a sugar that is observed immediately after it is dissolved in aqueous solution, as the result of the slow approach of equilibrium of a pyra-nose or a furanose in its a and /3 forms. 

Isbell and Pigman17 have shown that the rapid and anomalous mutarotation involves pyranose—furanose interconversion. On the basis that only D-fructofuranose (Ic) is fermented by yeast, Gottschalk18 has shown that the equilibrium mixture in aqueous solution at 0° contains 12% of D-fructofuranose. Gottschalk has calculated, from the kinetics of the mutarotation, that the aqueous solution at 20° contains about 20 % of the sugar in the furanose form. 

Maltose behaves as a normal reducing sugar, showing mutarotation and existing finally in aqueous solution as an equilibrated mixture of the a-form and the S-form Modern ideas about connections between optical rotation and structure are reported in the literature [105]. 

Unlike the other disaccharides that have been discussed, sucrose is not a reducing sugar and does not exhibit mutarotation because the glycosidic bond is between the anomeric carbon of glucose and the anomeric carbon of fructose. Sucrose, therefore, does not have a hemiacetal or hemiketal group, so it is not in equilibrium with the readily oxidized open-chain aldehyde or ketone form in aqueous solution. 

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