Mutarotation isotope effect

The original experimental evidence for concerted acid-base catalysis of the mutarotation in benzene is now considered unsound133 134 and concerted acid-base catalysis has been difficult to prove for nonenzy-matic reactions in aqueous solution. However, measurements of kinetic isotope effects seem to support Swain and Brown s interpretation.135 Concerted acid-base catalysis by acetic acid and acetate ions may have been observed for the enolization of acetone136 and it may be employed by enzymes.1363... 

In addition to the reaction sequence shown, the substrate may have to mutarotate non-enzymically to the form favored for affording the first enzyme-substrate complex. In such multi-stage reactions, the observed isotope-effects for the overall reaction are usually low. The situation is further complicated by possible differences in Km between a deuterated substrate and the unlabeled substrate, so that, unless a full kinetic analysis of the reaction is performed, the values determined for the apparent isotope-effect may vary. 

Isbell, Horace S., and Pigman, Ward, Mutarotation of Sugars in Solution Part II, Catalytic Processes, Isotope Effects, Reaction Mechanisms, and Biochemical Aspects, 24, 13-65 Isbell, Horace S. See also, Pigman, Ward. 

Figure 1.18 Transition states for the acetate-catalysed, acetic acid-catalysed and water reactions in the mutarotation of tetramethyl glucose. The additional waters for the acetate and acetic acid reactions are drawn to indicate solvation, rather than a change in bonding that would alter fractionation factors. Isotope effects are taken from ref 34 and fractionation factors calculated from their data using 1.0, rather than 1.23, for the fractionation factor of the anomeric hydroxyl. The latter was based on an implausible, equilibrium isotope effect of 4.1 on the acid dissociation constant of tetramethyl glucose. ...

D2O has the further advantage that the solvent isotope effect on mutarotation is usually greater than that on glycosidase catalysis. The temperature coefficient of most enzymes, moreover, is less than that of mutarotation, so it is often advantageous to cool the sample. Finally, the more powerful the magnet, the fewer the FIDs and hence the shorter the time that is required to obtain a good spectrum. Tours de force of NMR-based glycosidase stereochemistry... 

IX. Isotope Effects in Mutarotation Reactions 1. Basis for Effects of Water and Deuterium Oxide... 

Differences in the rate of mutarotation of sugars in water and in deuterium oxide provide a valuable means for studying mutarotation reactions.135,224,233 237,238 The difference in rates arises from a combination of kinetic and solvent isotope-effects, and is usually expressed as a ratio,knlkD, called the isotope effect. Kinetic isotope-effects are caused by differences in the energy required for alteration of the normal and the isotopic bonds in the corresponding transition states solvent isotope-effects can exist when the isotopic compound is used both as a reactant and as a solvent. 

The high value of kHlkD (3.5 to 4.0) for the water-catalyzed reaction may arise from a substantial contribution to the overall mutarotation of the concerted mechanism proposed by Lowry. In the transition state for the concerted mechanism (see Fig. 9), an acid catalyst releases a proton to the ring-oxygen atom, and a base catalyst removes a proton from the anomeric hydroxyl group the isotope effect is high, because two proton-transfers occur in the transition state. 

Mutarotation of sugars in solution. Part II. Catalytic processes, isotope effects, reaction mechanisms, and biochemical aspects, H. S. Isbell and W. W. Pigman, Adv. Carbohydr. Chem., 24 (1969). 

---