Discovery of Unusual Highly Branched Galactomannan from Seeds of &amp;lt;i&amp;gt;Desmanthus illinoensis&amp;lt;/i&amp;gt;

Masakuni Tako,Takeshi Teruya,Yukihiro Tamaki

doi:10.4236/jbnb.2018.92009

Abstract

A galactomannan was isolated from seeds of a leguminous plant, Desmanthus illinoensis, which is grown in Okinawa, Japan. D-Galactose (molar ratio, 1.0) and D-mannose (0.82) were identified via High-performance Anion Exchange Chromatography Coupled with a Pulse Amperometric Detector. The molecular mass and specific rotation were estimated to be 1000 kDa and +53.8°, respectively. The infrared spectrum indicated that the galactomannan was involved in both α- and β-linkages, and two types of α-linkages were detected at 814 and 830 cm-1. The 1H- and 13C-NMR spectra indicated that the majority of the β-D-mannan main chain was substituted with mono α-D-galactose or α-D-galacto-disaccharide-side chains. Methylation analysis was used to identify 2,3,4,6-tetra-O-methyl-D-galactose (molar ratio, 3.3), 2,3,4-tri-O-methyl- D-galactose (1.0) and 2,3-di-O-methyl-D-mannose (3.1). Specifically, unique 2,3,4-tri-O-methyl D-galactose residue was identified from mass spectrum. The results suggested that the galactomannan was 1,4-linked-β-D-mannan substituted with α-D-galactose or 1,6-linked-α-D-galacto-disaccharide side chains at C-6 on the main chain. The galactomannan isolated from D. illinoensis was an unusual highly branched polysaccharide, and its chemical structure was proposed. This work is the first to report on the galactomannan involving 1,6-linked α-D-galacto-disaccharide side chains in addition to α-D-galactose mono side chains.

Highlights

Galactomannans are involved in the endosperms of leguminous plant seeds
A galactomannan was isolated from seeds of a leguminous plant, Desmanthus illinoensis, which is grown in Okinawa, Japan
The seeds of D. illinoensis were collected on the campus of the University of the Ryukyus, Nishihara, Okinawa, Japan

Summary

Introduction

Galactomannans are involved in the endosperms of leguminous plant seeds. Since the polysaccharides have high viscosity [1] and synergistic interactions with xanthan gum [2] [3] [4], κ-carrageenan [5] and agarose [6], they are often used as thickening, stabilizing and gelling agents in the food industry. The structure-function relationship of the synergistic interaction among galactomannans (locust bean gum [2] [7] [8] [9], guar gum [3], tara-bean gum [4], leucaena gum [10], and delonix gum [11]) and xanthan gum produced by a bacterium, Xanthomonas campestris [12] [13] are examined. The strength of the galactomannan-xanthan gelling interaction increased with decreasing content of D-galactose side chains of the former molecules. Synergistic interaction between Leucaena galactomannan (degree of side chain substitution, 66%) and xanthan did not occur at room temperature, but gelled after cooling [10]. The mixture of Delonix galactomannan (25%) with xanthan showed strong gel, even at 0.1% total concentration of the polysaccharides at room temperature [11]. The results supported proposed association sites between galactomannan and xanthan molecules [11] (Figure 1)

Methods

Results

Discussion

Conclusion