Kinetics of oxygen exchange at the anomeric carbon atom of D-glucose and D-erythrose using the oxygen-18 isotope effect in carbon-13 nuclear magnetic resonance spectroscopy.

Abstract

The 18O isotope induced shift in 13C nuclear magnetic resonance (NMR) spectroscopy affords a new and convenient method for the study of oxygen exchange at the anomeric carbon atom of simple sugars. The efficacy of the technique was confirmed by a study of the oxygen exchange reaction of D-[1-13C] glucose. At pH 7.0 and 61 degrees C, the incorporation of 18O from solvent H2(18)O onto the C-1 carbon atom of the diastereomeric alpha- and beta-pyranose sugars was followed by 13C NMR spectroscopy in a continuous assay mode. The pseudo-first-order rate constant for exchange of both the alpha and the beta anomers was 9.5 X 10(-5) s-1, which is in agreement with a rate constant obtained in a previous study by a chemical conversion-mass spectrometry technique. The new technique was applied to a study of the oxygen exchange at the anomeric carbon atom of D-[1-13C]erythrose, a furanose sugar for which no experimental data were available. In unbuffered, aqueous solutions the incorporation of the 18O label from the medium (H2(18)O) onto the C-1 carbon atom of the alpha- and beta-D-[1-13C]erythrose and the D-[1-13C]erythrose hydrate forms was followed by 13C NMR at 10, 23, and 36 degrees C. From analysis of the data for the alpha and beta diastereomers, the pseudo-first-order rate constants for exchange were 1.4 X 10(-4) s-1 at 10 degrees C, 4.8 X 10(-4) s-1 at 23 degrees C, and 8 X 10(-4) s-1 at 36 degrees C, and the apparent energy of activation for the exchange reaction was 12.1 kcal/mol. Particularly in conjunction with the use of specifically 13C-enriched sugars, the new technique for studying oxygen exchange reactions of carbohydrates has many distinct advantages over earlier approaches, including the ability to follow simultaneously the exchange reactions of all of the sugar species for which a 13C NMR signal can be detected, the continuity of the assay, the avoidance of possible artifacts due to incomplete or selective derivatization reactions, and the simplicity of the data analysis.