13C-labeled aldopentoses: detection and quantitation of cyclic and acyclic forms by heteronuclear 1D and 2D NMR spectroscopy

1H-Decoupled 13C NMR spectra (150 MHz) of the simple aldopentoses (M solutions in 2H2O, 28 °C) selectively labeled with 13C at C-1 (d-(1-13C)arabinose 1, d-(1-13C)lyxose 2, d-(1-13C)ribose 3, d-(1-13C)xylose 4) contain six enriched C-1 signals that were attributed to four cyclic (α- and β-furanoses and pyranoses) and two acyclic (aldehyde, hydrate) forms. Spectral data were collected and processed in a fashion to permit accurate quantitation of the cyclic and acyclic forms. Percentages of forms varied with pentose structure: α-furanose (0.8–7.4%), β-furanose (0.6–13.2%), α-pyranose (20.2–70.8%), β-pyranose (26.9–62.0%), hydrate (0.063–0.095%), aldehyde (0.009–0.042%). Aldehyde was least abundant in solutions of d-xylose and most abundant in solutions of d-ribose, and the hydrate/aldehyde ratio was higher for d-arabinose, d-lyxose, and d-xylose (6.3–7.8) than for d-ribose (2.1). Heteronuclear 2D HMQC–TOCSY and HCCH–TOCSY spectra were also obtained on several selectively and uniformly 13C-labeled model saccharides, respectively, to evaluate the advantages and limitations of these isotope-aided methods to detect 1H signals of specific forms in solution. These methodologies can be extended to studies of suitably 13C-labeled oligosaccharides and oligonucleotides.