Tautomeric composition of D-fructose phosphates in solution by fourier transform carbon-13 nuclear magnetic resonance

The Fourier transform 13C magnetic resonance spectra of D-fructose 6-phosphate (F6P) and D-fructose 1,6-diphosphate (FDP) were obtained. The signal assignments made on the basis of 13C chemical shifts and 13C-31P spin-spin couplings indicate that the earlier assignments of the C-4 and C-5 resonances of α- and β-fructofuranose in oligosaccharides and D-fructose [Allerhand, A. and Doddrell, D., J. Amer. Chem. Soc., 93, 2777, 2779 (1971)] should be reversed. Integration of signal intensities yields the following equilibrium composition at 35°C: F6P, α-anomer 19±2% and β-anomer 81±2%, FDP, α-anomer 23±4% and β-anomer 77±4%. Less than 1.5% keto or hydrated keto form is present in solutions of either fructose phosphate. The bearing of these findings on the tautomeric specificity of phosphofructokinase is discussed.