Structural elucidation of hemicelluloses from Vetiver grass

Abstract

Structure of the hemicelluloses extracted from Vetiver grass (Vetiveria zizanioides Nash) leaves was studied. The monosaccharide compositions and the position of the linkages between monosaccharides in the hemicelluloses were defined by TFA hydrolysis and methylation analysis, respectively. ¹³C NMR and FT-IR spectroscopic methods gave details of the anomeric linkage configuration and confirmed the structure of the hemicelluloses. The proposed structure of the hemicelluloses from the Vetiver grass is an arabinoxylan mainly consisting of (1→4)-β-d-xylan backbone substituted in O-2 and/or O-3 by single α-l-arabinose residue, single α-d-glucuronic acid residue and/or short chains of sugar residues containing arabinose, xylose and galactose. In addition, the (1→4)-linked β-d-xylopyranosyl residues in the backbone may contain substituted phenolic acids (e.g. ferulic acid and p-coumaric acid). The substituted phenolic acids are esterified via their carboxyl groups to the C-5 hydroxyl of the α-l arabinofuranosyl residues in side chain.

Introduction

The majority of plant materials are constructed from three major polymeric components: cellulose, hemicelluloses and lignin. The hemicelluloses are estimated to account for one third of all components available in plants. Structures of the hemicelluloses vary significantly in different plants and have been topics of great academic interest. The hemicelluloses from cell wall of the Gramineae family plants such as cereal plants and grasses mainly consist of a chain backbone of (1→4)-linked β-d-xylopyranosyl residues to which α-l-arabinofuranose units are linked as side branches (Izydorczyk and Biloaderis, 1995, Xiao et al., 2001, Sun and Sun, 2002). The manner of attachment of arabinose units to the xylan backbone has been a matter of continuous research. The linkages of Ara_f to C(O)3 and to C(O)2,3 of xylose residues have been reported (Ebringerova et al., 1990). More recently, the presence of another linkage type, namely Ara_f linked to C(O)2 of Xyl_p residues, has been verified for arabinoxylans (Izydorczyk & Biloaderis, 1995). A small proportion of oligomeric side-chains consisting of two or more arabinosyl residues linked via 1→2, 1→3 and 1→5 linkages has been reported although most arabinofuranosyl residues in arabinoxylans are found as monomeric substituents. Terminal galactosyl and glucosyl residues can be present but are usually quantitatively minor and might originate from contaminant polysaccharides (Ebringerova et al., 1990, Izydorczyk and Biloaderis, 1995, Ebringerova and Heinze, 2000, Xiao et al., 2001). Glucuronopyranosyl (and 4-methyl ether) residues were found and attached directly to the C-2 position of xylose from barley straw (Sun & Sun, 2002), rice and rye straws (Izydorczyk and Biloaderis, 1995, Xiao et al., 2001).

Although arabinoxylans from various cereals and/or various plant tissues share the same basic chemical structure, they differ in the manner of substitution of the xylan backbone. The main differences are found in the ratio of arabinose to xylose (Ara/Xyl), in the relative proportions and sequence of the various linkages between these two sugars, and in the presence of other substituents. The ratio of Ara/Xyl in arabinoxylans from wheat endosperm may vary from 0.50 to 0.71 but it is usually lower than that found in bran (1.02–1.07). Similarly, rye bran endosperm arabinoxylans are less substituted (0.48–0.55) than rye bran counterparts (0.78) (Izydorczyk & Biloaderis, 1995). In general, arabinoxylans from rice (Shibuya & Iwasaki, 1985) and sorghum (Woolard, Rathbone, & Novellie, 1976) seem to consist of more highly branched xylan backbones than those from wheat, rye, and barley, and they may contain galactose and glucuronic acid substituents, in addition to the pentose sugars.

According to His Majesty's Initiative, the principal utilisation of the Vetiver grass in Thailand is for preventing soil erosion and conserving soil moisture due to its deep thick root system like on underground curtain. Similar to other undervalued agricultural residues, leaves of the Vetiver grass which has been normally cut every few months are not used as industrial raw materials and are burnt in field or on the side of the road. The main purpose of the work, being presented here just a part, is to assess the possibility of preparing wound dressing materials or other chemicals based on hemicelluloses extracted from the Vetiver grass. Therefore, elucidation of the molecular structure of the Vetiver grass hemicelluloses was of our particular interest.