NMR analysis of tri- and tetrasaccharides from asparagus

Abstract

Detailed analysis of the ¹H and ¹³C NMR spectra of five fructo-oligosaccharides (1–5) isolated from Asparagus officinalis L. (Liliaceae) was carried out. Tetrasaccharides 1, 2 and 3 consist of one unit of glucose and three units of fructose, whereas trisaccharides 4 and 5 consist of one unit of glucose and two units of fructose. The ¹H NMR signals of these tetrasaccharides were severely overlapped around δ_H 3.6–4.0 ppm and the ¹³C NMR signals of the corresponding signals of three fructose residues were close to each other. Therefore, it was difficult to make assignments of those signals using conventional NMR methods. Overcoming these signal-overlapping problems required the application of some special NMR techniques. The spectral analysis was started from the anomeric proton of the glucose, since each compound had only one glucose residue and its ¹H NMR spectrum showed a separate characteristic anomeric proton signal. After the ¹H and ¹³C signals of the glucose had been assigned, the fructose residues attached to the C-1 and/or C-6 positions of glucose were determined from the HMBC spectrum, which also revealed further fructosyl fructose linkages. The assignments of each signal of C-5 and C-6 of the fructose residues overlapped in 2D spectra were achieved by the 1D INAPT method. Unresolved correlation peaks of methylene protons in HMQC could be separated by the CH₂-selected HSQC method, which gave sufficient ¹³C resolution to separate each CH₂ signal, resulting in the determination of the chemical shift of the methylene protons. Copyright © 2000 John Wiley & Sons, Ltd.