Accurate Quantitative Isotopic 13C NMR Spectroscopy for the Determination of the Intramolecular Distribution of 13C in Glucose at Natural Abundance

In order to understand C-13 isotope distributions in glucose and its metabolites, it is necessary to measure the internal C-13 distribution at natural abundance. These data, however, are not directly accessible, even by quantitative isotopic C-13 NMR spectrometry, due to anomerization at the C-1 position. A strategy has been developed that overcomes this difficulty by converting glucose via a three-step synthesis into 3,5,6-triacetyl-1,2-O-isopropylidene-alpha-D-glucofuranose (TAMAGF). This compound provides a satisfactory molecular probe to measure the site-specific C-13/C-12 ratios in glucose by C-13 NMR. It is shown that the isotopic C-13 NMR signal gives sufficient precision (repeatability standard deviation 0.8 parts per thousand) for routine use for the determination of the C-13 abundance of each carbon atom position in glucose. Thus, it can be seen that the internal C-13 distribution of glucose biosynthesized by the C3 and C4 metabolic pathways differs markedly. Furthermore, the method is suitable for determining the isotope ratio in the glucose moiety of sucrose and, possibly, in free fructose and the fructose moiety of sucrose.