Galactose mutarotation

Like glucose, galactose mutarotates when it dissolves in water. The specific rotation of a-D-galactopyranose is +150.7°, and that of the 13 anomer is +52.8°. When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.2°. Determine the percentages of the two anomers present at 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. 

In our first measurements with solutions of 0.5 M a-D-galactose in 0.5 M boric acid, the constants of the mutarotation and of the change in the conductivity agreed very well. 

Fig. 2.16. Phase corrected 22.63 MHz PFT 1 C NMR spectrum of mutarotated D-galactose 100 mg/mL deuterium oxide 30 C proton decoupled 512 accumulated pulse interferograms ...

Simulation of the mutarotation of arabinose closely followed the work on galactose, except that four components plus a central intermediate were included, and the / -pyranose was the starting anomer. The components were the a-pyranose (kt, k2), / -pyranose (k3, k4), a-furanose (k5, k6), and / -furanose (k7j kg). Table III shows sets of values for the k s which yield the experimentally observed half-times for the approach of the components to their equilibrium levels. 

Fig. 6.—Mutarotation of 2-Desoxy-D-galactose p-Toluidide (a) In Pyridine (c, 1.0) (6) As (a) with 1 N H2SO4 (one drop) added (e) In Methanol (c, 1.0) (d) As (c) with 0.1 N H2SO4 (one drop) added.

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