Structure of Fructan Prepared from Onion Bulbs (Allium cepa L.)

Abstract

Fructan was prepared from an extract of onion bulbs by preparative paper and gel permeation chromatographies. Fructan was a mixture of saccharides with degree of polymerization (DP) and MW range of approximately 7‒13 and 1,152‒2,124, respectively, as determined by matrix-assisted laser desorption ionization/time of flight mass spectrometry. It yielded fructose and glucose upon hydrolysis with acid or yeast β-fructofuranosidase. The ratio of D-fructose to D-glucose in the enzyme hydrolysate was 8.7:1, determined by high performance anion-exchange chromatography analysis. Structural determinations were made, based on ¹³C-nuclear magnetic resonance spectroscopy. Numerous signals corresponding to carbon-1 (C-1), C-2, C-3, C-4, C-5, and C-6 of the D-fructose residues of fructan were observed at δ 61.04‒61.81, 103.70‒104.41, 77.46‒78.10, 74.50‒75.15, 81.77, and 62.85‒62.96, respectively. These chemical shifts were similar to those of inulin. Moreover, weaker signals were detected at δ 92.90, 71.75, 73.10, 69.76 and 72.22 due to C-1, C-2, C-3, C-4, and C-5, respectively of the D-glucose residue. The chemical shifts are almost identical to those of the D-glucose carbons of neokestose, 6^G(1-β-D-fructofuranosyl)₂ sucrose, and 1^F, 6^G-di-β-D-fructofuranosyl sucrose. These findings were supported by analysis of the methanolysate from permethylated fructan using gas-liquid chromatography. The fructan from onion bulbs was composed of saccharides possessing approximately 6‒12 D-fructose residues, linked by β-2,1 bonds, and a non-terminal D-glucose residue bound with D-fructose residues at the C-1 and C-6 positions.