Mutarotation of aldoses

Computer Modeling of the Kinetics of Tautomerization (Mutarotation) of Aldoses Implications for the Mechanism of the Process... 

The term intramolecular catalysis introduced by Bender is also widely used to describe neighboring group effects, especially when analogous intermolecular catalysis is observed. Thus this term is commonly used when referring to reactions that are subject to acid, base, and/or nucleophilic catalysis such as hydrolysis of esters, amides, and acetals the mutarotation of aldoses or the enolization of ketones. It is rarely used when referring to nucleophilic substitution reactions at saturated carbon. 

Mutarotation of Various Free or Partially Substituted 2,5-Anhydro-aMefce/do-aldoses... 

A number of other reactions in this general area such as the mutarotation of D-(+)-glucose431,432 and aldose-ketose isomerizations433 are also subject to catalysis by metal ions. 

In the D-fructosides the convention is that the more dextro-rotatory anomer is the a form. Attempts have been made by Boeseken and Couvert,82 Verschuur83 and MacPherson and Percival84 to apply Boese-ken s boric acid method to determine the configuration of D-fructose at C2. The problem is much more complex than for aldoses because there are three hydroxyl groups near the reducing center, and also because the mutarotation of D-fructose involves the conversion of some pyranose to furanose form with the loss of a pair of cis hydroxyls (on C4-C5). This work has been discussed by Boeseken88 but no conclusion has been reached. 

The anomeric forms derived from equilibration of aldoses give rise to multiple peaks when trimethylsilylated and gas chromatographed [311]. A method of overcoming this problem, assuming that mutarotation itself is not under study, is to modify the aldose. It can be oxidised and lactonised to the aldonolactone, for example, and characterised as its TMS derivative [322]. Alternatively for the identification of aldoses and alditols, more use may be made in the future of the separations achievable on open tubular columns of the poly-0-acetylaldonic nitriles (18) produced from aldoses and the poly-acetyl esters from alditols [323]. Figure 1.18 shows the separation of 32 assorted polyols and aldoses. 

Mutarotases (For a treatment of enzymological terms and concepts see Chapter 5). Mutarotases ensure that the mutarotation of sugars in vivo is much faster than spontaneous, and thus couples various transport systems and metabolic pathways. The relatively non-specific aldose mutarotases isolated... 

Aldose-1-epimerase), quite widespread in animal tissue and bacteria, which catalyses mutarotation. The Escherichia coli enzyme has a maximum activity close to neutral pH. The activation energy 4G =11.9 kcal mol" is greatly lowered, as usual with respect to that of the non-enzymically catalysed reaction, close to 17 kcal mol". o-Glucose, D-galactose, and D-fucose are substrates but not D-mannose (Hucho and Wallenfels 1971). 

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