Pyranose-furanose interconversions with reference to the mutarotations of galactose, levulose, lactulose,and turanose

Mutarotation measurements at 20 and 0 C are reported for lactulose and turanose in buffered solutions at pH 4.6 and for galactose, gl ucose, and levulose at numerous points between pH 1 and 7. The mutarotation of lev ulose and the rl1pid mutarotation of galactose are extremely sensitive to the catalytic action of acids and bases and vary with pH in like manner. The mutarotation reactions of levulose, lactulose, and turanose differ fundamentally from the alpha-beta pyranose interconversions and r esemble in all resp ects the rapid mutarotation reactions of sugars which contain the galactose, talose, and idosc structures. Since the mutarotation of levulose is like that of the fructofuranose set free from sucrose by the action of invertase, but in the opposite direction, the mutarotation of levulose must consist in a pyranose-furanose interconversion . The conclusion that the mutarotation of levulose results from a pyranose-furanose interconversion leads to the following important deductions: (1) That the mutarotation of lactulose is caused by a furanose-pyranose change and that crystalline lactulose is a furanose; (2) that the mutarotation of Hibbert's 1,3,4-trimethyl-fructofuranose results from a furanose-pyranose change and that the sugar is properly classified as a furanose; (3) that the mutarotation of turanose is due to a pyranosefuranose change and therefore the glucosido group is not united with the fifth or sixth carbon, but in all probability it is un ited with the third carbon; (4) that the rapid reactions characteristic of the complex mutarotations of galactose and other sugars are pyranose-furanose inter con versions and that the complex mutarotations arise from the establishment of an equilibrium betwecn the furanose and the normal alpha and beta pyranoses. CONTENTS Page I. The sugars in solution __________________________________________ 774 II. Comparisons of the mutarotations of levulose and galactose _________ 776 III. Classification of the m, utarotation reactions-__ _______ _____ ____ ____ 777 IV. E,:idence that. the mutarotation of levulose is a pyranose-furanose lnterconverslon _____ _________ ____ ___