Interconversion of Ketose and Aldose Sugars in Dilute Aqueous Solution

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The present article contains a study of certain changes produced when dilute aqueous solutions of various sugars are boiled. Interest in the subject was aroused because it was observed that in a solution of glucose so treated fructose was apparently produced. Although the interconversion of glucose, fructose, and mannose in alkaline solution has been repeatedly studied since Lobry de Bruyn and Van Eckenstein first observed the phenomenon, results of quantitative studies similar to those reported do not seem to be available. The technical procedure was as follows: Pure glucose from the Bureau of Standards was dried over calcium chloride and dissolved in distilled water. Seventy-five cc. of a solution so prepared were boiled on an electric hot plate under a reflux condenser. At intervals the flask containing the solution was rapidly cooled in ice water and a sample removed for analysis. The solution was then reheated rapidly and the boiling continued. The changes in the concentration of reducing substances were determined by the copper reduction method of Benedict and variations in the intensity of the resorcinol reaction by the technique described by Roe for the determination of fructose in blood. The purest glucose and fructose obtainable were used as standards. The reaction values were estimated colorimetrically at room temperature. Pure commercial mannose and galactose prepared for the bacteriological laboratory were used in some experiments. Water solutions of glucose were studied first. It soon became evident that several conditions affected the composition of the boiled mixture. One of these was the concentration of glucose initially present. When the solution was very dilute the maximum concentration of “fructose” was generally reached after boiling for 4 hours, but in stronger ones such a value was not reached in very much longer periods. Results of analyses made after 6 hours are given in Table I. It can be seen that the concentration of “fructose” present at this time varied with the concentration of glucose initially present, but was not parallel to it. Since there was very little difference between the percentage composition of the products obtained by boiling 0.02 and 0.1 % glucose in a neutral buffer mixture (Table) it seems possible that variations in acidity may have been one of the causes of this lack of parallelism.