Mutarotation coefficients

The mutarotation coefficient ( i + k2) can be determined by the change in optical rotation with time ... 

As a solution of one anomer was allowed to mutarotate, an increasing amount of the second anomer became evident until finally the usual equilibrium mixture was obtained (see Figure 2). By determining the peak areas, the percent of each anomer present at any given time interval could be calculated. Since the equation for the mutarotation coefficient, k, can be expressed in the form of Equation 1, the coefficient can be determined... 

Figure 3. Determination of mutarotation coefficients by the GLC-TMSi method. The conditions were those described previously (26).

Straight line plots are obtained by this method furthermore, the mutarotation coefficients for several sugars, determined under the same conditions as used in polarimetric work, give values identical to those obtained optically (see Table I). 

The mutarotation coefficient, fci + 2, should be the same for the a- and jS-isomers of each sugar. Hudson (48) demonstrated that the a- and P-iso-mers of lactose and of some other sugars give identical values for ki + k2 and that the mutarotations follow the first-order equation. Table III lists the mutarotation coefficients for several sugars (49). 

Mutarotation Coefficients and Activation Energies for Some Sugars... 

The catalysis of mutarotation of glucosamine hydrochloride involves an intramolecular mechanism, and so the catalytic coefficient of glucosamine. fcc,iNH2> must have the dimension, min.-1, instead of liters/mole min. The total rate is then... 

The rate of the interconversion may also be followed by measuring the change in volume or in refractive index. Such measurements give rate coefficients identical with those obtained by the polarimetric method. In Table XVIII, rate coefficients for the mutarotation of a number of sugars are listed. The rates of mutarotation of several sugars (for example, D-ri-bose, D-galactose, and all the ketoses) do not obey the first-order law. Their complex mutarotations result from the presence in solution, in appreciable concentrations, of more than two species. In addition to pyranoses, there must be present either furanoses or acyclic forms, or both. 

Although no mutarotation was observed with the first small sample of D-altrose, its [a]D value of +32.6° in water was in agreement with the equilibrium rotation —32.3° recorded by Austin and Humoller for L-altrose. When a larger amount of the sugar became available, D-altrose was found to exhibit a complex mutarotation. From calculations of the velocity coefficients it would appear that the mutarotation consists of a very rapid interconversion of furanose and pyranose modifications, followed by a slower interconversion of o and /3 pyranose modifications. 

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... 

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. 

To evaluate the catalytic coefficients for a specific reaction, the rate expression, equation 9, is set up for the known catalysts. Then, a series of measurements is made under such conditions that certain terms in the equation are negligible and the contribution of the catalyst under study becomes important. Under these conditions, the effect of the concentration of the catalyst on the rate constant may be determined. For example, to determine the catalytic coefficients for the mutarotation of D-glucose in an aqueous sodium acetate-acetic acid solution, the following equation is set up ... 

The coefficient for the water molecule acting as a proton donor (that is, as an acid catalyst) may be calculated from equation 12 by using data for the mutarotation of D-glucose at 18°, with acetic acid as the reference point. From equation 15,... 

In these experiments, the contributions of the D-glucosate ion to the total rate-constant ranged from 18 to 41 %, depending on the concentration of the sugar. In a subsequent investigation,230 the study was extended, and catalytic coefficients and heats of activation were determined for catalysis of the mutarotation of D-glucose in water by each of three acids and four bases. 

The interpretation of these rate coefficients involves a complex line of reasoning based on several plausible assumptions. Briefly, the authors conclusions were as follows cis-trans isomerization and mutarotation occur at appreciable rates only in the presence of ionic chloride. Racemization may occur very slowly in the absence of ionic chloride, although the data do not permit a definite decision on this point. The most important reaction involves simultaneous cis-trans isomerization (with reversal of sign of optical rotation) and exchange with ionic chloride, probably by an 8 2 process. Since most events leading to racemization do not result in cis-trans isomerization, substi-... 

EXAMPLE The mutarotation of glucose is a case of general acid-base catalysis, with the rate coefficient for the reaction in water having the form... 

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