A C-13 NMR spectrometric method for the determination of intramolecular delta C-13 values in fructose from plant sucrose samples

Recent developments in C-13 NMR spectrometry have allowed the determination of intramolecular C-13/C-12 ratios with high precision. However, the analysis of carbohydrates requires their derivatization to constrain the anomeric carbon. Fructose has proved to be particularly problematic because of a byproduct occurring during derivatization and the complexity of the NMR spectrum of the derivative. Here, we describe a method to determine the intramolecular C-13/C-12 ratios in fructose by C-13 NMR analysis of the acetyl-isopropylidene derivative. We have applied this method to measure the intramolecular C-13/C-12 distribution in the fructosyl moiety of sucrose and have compared this with that in the glucosyl moiety. Three prominent features stand out. First, in sucrose from both C-3 and C-4 plants, the C-1 and C-2 positions of the glucosyl and fructosyl moieties are markedly different. Second, these positions in C-3 and C-4 plants show a similar profile. Third, the glucosyl and fructosyl moieties of sucrose from Crassulacean acid metabolism (CAM) metabolism have a different profile. These contrasting values can be interpreted as a result of the isotopic selectivity of enzymes that break or make covalent bonds in glucose metabolism, whereas the distinctive C-13 pattern in CAM sucrose probably indicates a substantial contribution of gluconeogenesis to glucose synthesis.