Calorimetric Studies on the Mutarotation of D-Galactose and D-Mannose*

1. Calorimetric measurements were made on the heat change accompanying the mutarotation on D-galactose and D-mannose to evaluate quantitatively the anomeric stability of the two sugars in aqueous solution. 2. It was found that in D-galactose the β-anomer is 1,300±50 J mol−1*** energetically more stable than the α-anomer, while in D-mannose the α-anomer is 1,900±80 J mol−1 more stable than the β-anomer at 25°C. 3. From stereochemical considerations regarding D-mannose, D-galactose, D-glucose, and D-xylose, it was assumed that the hydroxyl group on C2 in the pyranose ring plays a major role in determining the preferred form of the anomeric pairs. 4. By combining the data with those reported for the isomerization of eq-D-glucose to eq-D-mannose, the energies required for the conversion of a hydroxyl group on C2 of a-D-glucose and β-D-glucose in chair-1 form from equatorial to axial were estimated to be 7, 950 J mol−1 and 10, 880 J mol−1, respectively.