(4-O-Methyl-α-d-glucurono)-d-xylan from Rudbeckia fulgida, var. sullivantii (Boynton et Beadle)

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Abstract

From the medicinal plant Rudbeckia fulgida, var. sullivantii (Boynton et Beadle) a low-molecular-mass (4-O-methyl-α-d-glucurono)-d-xylan was isolated by alkaline extraction, followed by ethanol precipitation, ion-exchange chromatography, and gel filtration. The results of compositional and linkage analyses, supported by those of 1H and 13C NMR measurements of oligomers generated on partial acid hydrolysis, showed the (1→4)-linked β-d-xylopyranosyl backbone with about 18% of 4-O-methyl-d-glucuronic acid attached to O-2 of the xylose residues. From the mean distance of adjacent carboxyl groups, obtained from experimentally determined single-ion activity coefficients of calcium counterions, it followed that the uronic acid units are separated and distributed regularly along the xylan chain, i.e. approximately each sixth d-xylose unit bears a 4-O-methyl-d-glucuronic acid residue.

Introduction

Isolation of compounds from medicinal plants which might be the active principle of a drug is not an easy task. In order to find an explanation for the medicinal use of various extracts from herbs, a great number of chemical and pharmacological studies have been undertaken.

Our continuing effort is to isolate homogeneous polysaccharides from active, crude polysaccharide mixtures of plant origin and characterize their structures as exactly as possible. In previous work[1]we found that the water-extractable polysaccharide complex from the aerial parts of the medicinal plant Rudbeckia fulgida, var. sullivantii (Boynton et Beadle) showed high antitussive activity, exceeding even the activity of some synthetic non-narcotic drugs used in clinical practice to treat coughing. The hemicellulose fraction obtained from the same source also showed high cough-suppressing activity in our preliminary studies. Therefore, we started a thorough investigation of this active fraction and in the present paper we report on structure analysis of its dominant polysaccharide component, a (4-O-methyl-α-d-glucurono)-d-xylan.

Section snippets

Materials

The medicinal plant was collected in 1991 in the garden of the Faculty of Pharmacy, Comenius University, Bratislava, Slovak Republic. DEAE-Sephadex A-50 was obtained from Pharmacia (Sweden), Bio-Gel P-4 and P-2 from Bio-Rad (USA), and Sep-pak C18 cartridges from Waters Associates (USA).

General

Concentrations were performed under diminished pressure at a bath temperature not exceeding 45 °C. Optical rotation (1 mL cells) was measured at 20±1 °C with a Perkin-Elmer Model 141 polarimeter. Free-boundary

Results and discussion

From the methanol-pretreated and water-extracted plant material a crude hemicellulose fraction was obtained in 4.5% yield by alkaline extraction, followed by ethanol precipitation. The product was freed from the accompanying UV-absorbing compounds (most probably lignins) by washing with acidified aqueous ethanol. The non-dialyzable portion showed, after hydrolysis, xylose as the major spot on PC, smaller amounts of uronic acid, and four other neutral sugars, suggesting heterogeneity of the

Acknowledgements

This investigation was supported by Grant Nos 2/4148 and 2/1234 from the Slovak Scientific Grant Agency (VEGA).

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