A 13C NMR spectrometric method for the determination of intramolecular δ13C values in fructose from plant sucrose samples
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- •Recent developments in 13C NMR spectrometry have allowed the determination of intramolecular 13C/12C 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 13C/12C ratios in fructose by 13C NMR analysis of the acetyl-isopropylidene derivative.
- •We have applied this method to measure the intramolecular 13C/12C 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 C3 and C4 plants, the C-1 and C-2 positions of the glucosyl and fructosyl moieties are markedly different. Second, these positions in C3 and C4 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 13C pattern in CAM sucrose probably indicates a substantial contribution of gluconeogenesis to glucose synthesis.