Mutarotation equilibrium

It is to be expected, on these grounds, that the predominating component of the mutarotation equilibrium of 3,6-anhydro-glucose will be 3,6-anhydro-glucofuranose whereas that of 3,6-anhydro-galactose will be the open-chain form. Neither will include pyranose forms. 

Acetamido-2-deoxy-D-glucose derivatives generally crystallize as the a anomer and mutarotate in polar solvents to a mixture of the two anomers, but the pure /3 anomer may be prepared by acylation of 2-amino-2-deoxy-(3-n-glucose at a low temperature with the acid anhydrides in A, A-dimeth-ylformamide solution.100 Mutarotation in this solvent is very slow, and the reaction product may be removed long before mutarotational equilibrium has been established. 

In aqueous solution, a mutarotational equilibrium of the two anomers is reached spontaneously - but not instantaneously - in which the ratio a/ 3 is 36 64 at a temperature of approximately 30 °C. In water, the rate is much lower than in buffer(ed medium) [354].The enzyme mutarotase accelerates mutarota-tion considerably. The rate at which the equilibrium is reached spontaneously depends greatly on pH, temperature and other solution components. 

The cover shows the 13C NMR spectrum of a- and P-D-xylopyranose at mutarotational equilibrium (35% a, 65% p. in deuterium oxide. 100 MHz, H broadband decoupling with the CC INADEQUATE contour plot. An interpretation of the plot according to principles described in Section 2.2.7 gives the CC bonds of the two isomers and confirms the assignment of the signals in Table 2.12. 

A particular problem is the demonstration of inversion with sialidases, since they act on ot-A-acetylneuraminides, but the mutarotational equilibrium is 10% ot 90% The chemical shifts of the two H3 protons have different chemical shifts in the ot- and p-forms of A-acetylneuraminic acid and its mutarotation is not inconveniently fast, so demonstrating retention is straightforward. However, to demonstrate inversion, it is necessary to match the rate of liberation of aglycone with that of a retaining sialidase and plot time courses for both enzymes, in order to distinguish inversion from uninformative conversion of substrate to the equilibrium mixture i.e. it has to be demonstrated that the chemical flux through the system is easily adequate to maintain non-equilibrium proportions of a- and p-A-acetylneuraminic acid. 

The mixtures obtained by allowing aqueous solutions of each of several sugars to attain mutarotational equilibrium have been analyzed by Tipson and Isbell the analyses were performed on the dry lyophilizates of these solutions and not on the solutions themselves. The results showed, amongst other things, that each equilibrium mixture contained some of the carbonyl form of the sugar. 

At mutarotational equilibrium in water, D-fructose (51) exists preponderantly as the j8-D-pyranose anomer in the 1C(d) conformation. A 1,2-alkylidene acetal (52) is formed in the same way as for L-sorbose, but this monoacetal has cts-disposed hydroxyl groups at C-4 and C-5 that react readily, forming a l,2 4,5-di-0-alkylidene-)8-D-fructopyranose (53). No evidence is available to indicate that the 1,2-alkylidene acetal might rearrange to a 1,3-alkylidene acetal, and it is to be expected that the activation energy for this isomerization would exceed that for formation of an acetal at 0-4 and 0-5. 

Visser, R.A., 1982, Supersaturation of a-Lactose in Aqueous Solutions in Mutarotation Equilibrium, Netherlands Milk and Dairy J. 36, 89. 

One can prepare crystals of the form or of the form of D-glucopyranose. The first will be obtained by crystallization from aqueous solution, and the second by crystallization from pyridine. The two forms are distinguishable by a number of properties, as, for example, their melting point and their specific rotation in a freshly made solution. In solution, one form changes into the other, so that the rotation changes until an equilibrium is reached (mutarotation equilibrium). In the formulae of Haworth, the keto and aldehyde groupings are not written as in the linear formula, they are replaced by a potential-aldehyde or a potential-ketone group. 

Fig. 4.5. Temperature dependence of the mutarotation equilibrium of D-fructose, (— p-D-fructo-pyranose, -— P-D-fructofuranose,-------a-D-fructofuranose) (ac-...

Fig. 19.6. Fructose mutarotation equilibrium as affected by temperature (according to Shallenberger, 1975)...

Calculate the equilibrium ratio of a- and j8-glucopyranose (which has been given in the text) from the specific rotations of the pure anoints and the observed specific rotation at mutarotational equilibrium. 

At this point of the investigations, we reasoned that yields could be further increased by enhancing the effective amount of reactive acyclic species within the mutarotation equilibrium. This can easily be accomplished by a selective protection of the 5-hydroxy group of starting pentoses. In this way, the thermodynamically favored pyranoid stmctures of the starting pentoses are no longer available to participate in the mutarotation equilibrium [33]. Hence, by removal of this structure from the mutarotation equilibrium, the effective amount of acyclic carbonyl species should be increased. 

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